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Physics Condensed Matter

cond-mat Watch Condensed Matter

Wednesday 18 March

Strongly Correlated Electrons

Non-Fermi-liquid behaviour of electrons coupled to gauge phonons

Rutvij Gholap, Alexey Ermakov, Alexander Kazantsev et al. (+3 more)

We identify overdamped gauge phonons as a new microscopic route to non-Fermi-liquid behaviour in Dirac materials. These phonons couple to electronic currents rather than densities, thereby realising a lattice analogue of transverse gauge-field mechanisms without requiring proximity to a quantum critical point. By computing the electronic self-energy with a phonon propagator dressed by electron-phonon interactions, we show that the low-energy behaviour is controlled by the orbital susceptibility chi and a dimensionless damping parameter alpha. In the overdamped regime, alpha >> 1, quasiparticles display strong deviations from Fermi-liquid theory. For chi > 0, Fermi-liquid behaviour persists only in a parametrically narrow infrared window before crossing over to non-Fermi-liquid scaling. For chi < 0, the Fermi-liquid regime is replaced by marginal-Fermi-liquid behaviour at the lowest energies, followed by a crossover to non-Fermi-liquid scaling. These results establish strain-induced gauge phonons as a promising source of anomalous metallic behaviour in systems such as twisted bilayer graphene.
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All authors Rutvij Gholap, Alexey Ermakov, Alexander Kazantsev, Mohammad Saeed Bahramy, Marco Polini, Alessandro Principi

Superconductivity

Quantum-Material Josephson Junctions: Unconventional Barriers, Emerging Functionality

Kathryn A. Pitton, Michiel P. Dubbelman, Trent M. Kyrk et al. (+5 more)

Josephson junctions translate quantum phase coherence into an electrical response and underpin superconducting sensors and quantum circuits. In conventional junctions, the barrier acts primarily as a passive weak link, however, when the barrier is a quantum material with its own internal degrees of freedom like magnetism, strong correlations, or switchable polarization, the Josephson effect becomes a sensitive probe of symmetry and many-body physics in the interlayer. Here we review progress in quantum-material Josephson junctions, (QMJJ) focusing on three rapidly advancing barrier families: 1. magnetic barriers, where exchange, noncollinearity, and spin-active scattering enable 0-{\pi}-{\phi} ground states, singlet-triplet conversion, and nonreciprocal transport, 2. correlated barriers, where proximity effects acquire many-body character and recent van der Waals Kagome Mott interlayers exhibit field-free Josephson diode behavior, and 3. ferroelectric and multiferroic barriers, where nonvolatile polarization provides an internal control knob and can produce superconducting memory and memristive dynamics.
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All authors Kathryn A. Pitton, Michiel P. Dubbelman, Trent M. Kyrk, Houssam El Mrabet Haje, Yaozu Tang, Roald J.H. van der Kolk, Yaroslav M. Blanter, Mazhar N Ali

Superconductivity

Thermodynamic Discovery of Tetracriticality and Emergent Multicomponent Superconductivity in UTe$_2$

Sahas Kamat, Jared Dans, Shanta Saha et al. (+4 more)

The candidate topological superconductor UTe$_2$ exhibits a complex phase diagram with multiple superconducting states, yet the nature of their coexistence has remained a central mystery. In particular, the apparent intersection of two second-order phase boundaries at a “triple point” in the pressure-temperature phase diagram is thermodynamically forbidden, suggesting either hidden phase transitions or a fundamental misunderstanding of the superconductivity in UTe$_2$. Here, we use pulse-echo ultrasound to resolve this puzzle by discovering a new phase boundary that is characterized by a unique “upward jump" in the sound velocity – direct thermodynamic evidence for a phase transition where superconducting order is lost upon cooling. Our results establish $\left(P^{\star},T^{\star}\right)$ as a tetracritical point, beyond which the ambient and pressure-induced superconducting order parameters form a multi-component state. We use the measured phase diagram to demonstrate that strong competition between the two superconducting order parameters drives the loss of order on cooling, and leads to phase locking that suppresses superconducting fluctuations. These findings provide the definitive magnetic field-temperature-pressure phase diagram of UTe$_2$, and establish a thermodynamic foundation for multi-component – and potentially topological – superconductivity.
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All authors Sahas Kamat, Jared Dans, Shanta Saha, Artem D. Kokovin, Johnpierre Paglione, Jörg Schmalian, B. J. Ramshaw

Materials Science

Mechanistic Insights into Enhanced Alkaline Oxygen Evolution on Zn-Al Alloy Electrodes

Abdul Ahad Mamun, Rokon Uddin Mahmud, Shahin Aziz et al. (+3 more)

Electrochemical water electrolysis, which produces clean energy carriers to mitigate carbon emissions, lacks suitable, low-cost electrodes for efficient oxygen evolution reaction (OER) in alkaline water splitting. To address this challenge, we developed Zn-Al alloy electrodes with varying Al contents up to 20 wt.% via powder metallurgy method and conducted electrochemical measurements of the OER in alkaline solution to investigate their catalytic performance. We also performed first-principles calculations to examine their thermodynamic phase stability and electronic structures. Both theoretical and experimental results indicated that incorporating $\geq 20$ wt.% Al into Zn led to thermodynamic phase instability and secondary-phase segregation in Al-rich regions, limiting reaction kinetics and reducing catalytic efficiency. Although the Al content of 5 wt.% into Zn exhibited favorable thermodynamic and electronic characteristics, but its electrochemical performance was inefficient and poor due to inadequate reaction active sites on the surface. In contrast, the 10 wt.% and 15 wt.% Al into Zn showed approximately three- and two-fold increases in anodic exchange current density relative to pure Zn, respectively. Additionally, the anodic overpotential losses ($\eta_{0,a}$) measured at a current density of 12 mAcm$^{-2}$ were 0.240 V for Zn$_{0.9}$Al$_{0.1}$ and 0.5603 V for Zn$_{0.85}$Al$_{0.15}$, significantly lower than that of pure Zn ($\eta_{0,a} = 1.086$ V). While Zn$_{0.9}$Al$_{0.1}$ and Zn$_{0.85}$Al$_{0.15}$ showed similar charge transfer resistance ($R_{\rm CT}$), Zn$_{0.9}$Al$_{0.1}$ demonstrated superior reaction kinetics and lower $\eta_{0,a}$ across all samples tested. Furthermore, the improved kinetics and reduced overpotential of the Zn-Al alloys favorably compare with those of other transition-metal-based catalysts, including Fe-Co-Ni-Mo alloys and Fe-doped CuO.
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All authors Abdul Ahad Mamun, Rokon Uddin Mahmud, Shahin Aziz, Muhammad Shahriar Bashar, Ahmed Sharif, Muhammad Anisuzzaman Talukder

Soft Condensed Matter

Feedback control and delayed interactions in active matter

Viktor Holubec and Frank Cichos

Feedback control plays a central role in active matter, yet it is inevitably accompanied by noise and finite perception–action delays. This Perspective reviews recent advances on active systems with delayed interactions, showing how time delay can induce activity, chirality, transport, and collective pattern formation, and can act as an effective control parameter for switching between dynamical states. We discuss representative single-particle and many-body systems, highlight key experimental realizations, and argue that time delay constitutes an underexplored dimension of morphological intelligence–where intrinsic response dynamics, rather than explicit sensors or computation, enable functional behavior in active matter.
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Statistical Mechanics · Cross-listed from hep-ph

Universal scaling of transport coefficients near the liquid-gas critical point

Johannes V. Roth, Yunxin Ye, Sören Schlichting et al. (+1 more)

We employ a novel real-time formulation of the functional renormalization group (FRG) to compute universal scaling functions of the thermal diffusivity and the shear viscosity in the vicinity of the liquid-gas critical point, i.e., for the dynamic universality class of Model H from the Halperin-Hohenberg classification. We map out the universal dependence of the transport coefficients on temperature, external magnetic field, and wavenumber, and provide a detailed comparison with the Kawasaki approximation, which is here obtained from a perturbative one-loop approximation to our real-time FRG flow. In contrast to the Kawasaki approximation, the non-perturbative scaling functions from the full real-time FRG flow show a mild dependence on the thermodynamic path towards the critical point. We further compare our FRG results for the universal wavenumber and temperature dependence of the thermal diffusivity with experimental data from critical fluids.
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All authors Johannes V. Roth, Yunxin Ye, Sören Schlichting, Lorenz von Smekal

Statistical Mechanics

Two stroke Pumping Technique for Many-Body Systems

Serge Galam

I introduce a new analytical framework for estimating critical temperatures in interacting many-body systems, focusing on the Ising model. Combining the Bethe cluster setting, the Metropolis update, and the Galam Majority Model developed in sociophysics, I build a two stroke pumping technique (TSP). Applied to the Ising model in dimensions d=2, 3, 4, TSP yields values of T_c which are all at an excess of +0.03 from exact estimates. At d=1 the exact value T_c=0 is obtained. In addition, TSP indicates analytically the practical impossibility to reach full symmetry breaking at T=0. The results are thus found in good agreement with numerical findings while requiring significantly fewer computational resources than Monte Carlo sampling. Calculations are computationally efficient and transparent. The framework is general and can be extended to a broad class of discrete spin models. This positions TSP as an intermediate yet scalable tool for studying cooperative behavior in many body interacting systems.
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Soft Condensed Matter

Entropy maximization underlies topology and mechanical properties in dynamic covalent hydrogels

Lucien Cousin, Pietro Miotti, Bruno Marco-Dufort et al. (+2 more)

Adding dynamic bonds in polymer networks enables reprocessing and recycling; however the full impact of reversible bonds on dynamic network mechanics remains unclear. We build model dynamic networks and observe substantial deviations from classic theory. We rationalize these findings by considering that bond exchange enables the networks to rearrange and adopt a topology with a higher entropy. This allows us to accurately predict the gel point and elasticity of the dynamic networks. Further, we show by controlling bond exchange that network rearrangement can dramatically alter the mechanical properties, even without loss of bonds.
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All authors Lucien Cousin, Pietro Miotti, Bruno Marco-Dufort, Igor V. Pivkin, Mark W. Tibbitt

Materials Science

Substrate-controlled nucleation and growth kinetics in ultrathin Bi$_2$Te$_3$ films

Damian Brzozowski, Sander R. Hønnås, Egil Y. Tokle et al. (+2 more)

Metal chalcogenides are promising layered topological materials, yet their electronic performance is often limited by parasitic bulk conduction arising from defects that introduce excess carriers and shift the Fermi level out of the topological regime. Controlling early-stage growth and defect formation is therefore essential for suppressing bulk transport and enhancing surface-state conduction. Here we investigate ultrathin Bi2Te3 films grown by pulsed laser deposition on substrates spanning van der Waals, lattice-matched, and amorphous regimes to determine how substrate-dependent nucleation pathways influence defect formation and electronic transport. Phase-pure, c-axis-oriented Bi2Te3 forms on all substrates, but the growth morphology varies strongly. Layered growth with well-defined quintuple-layer terraces is governed primarily by substrate roughness rather than lattice match: atomically smooth mica and step-terraced SrTiO3 yield continuous terraces, whereas rougher BaF2 and amorphous Si3N4 produce island-structured films. Between the two smooth substrates, the higher surface energy of SrTiO3 enhances adatom adsorption and nucleation density, promoting rapid vertical growth and early Te depletion. Transport measurements reveal n-type conduction with carrier densities of 10e19-10e20 cm-3. The highest carrier density occurs for films on SrTiO3, consistent with defect formation during high-density nucleation, whereas mobility correlates with structural coherence and terrace formation. Weak anti-localization signatures confirm phase-coherent transport in films on mica and SrTiO3. These results show that substrate roughness and nucleation density provide key levers for controlling defect formation and strengthening topological surface transport in Bi2Te3 thin films.
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All authors Damian Brzozowski, Sander R. Hønnås, Egil Y. Tokle, Jørgen A. Arnesen, Ingrid G. Hallsteinsen

Strongly Correlated Electrons

Toward bootstrapping tensor-network contractions

Seishiro Ono, Yanbai Zhang and Hoi Chun Po

Accurate contraction of tensor networks beyond one dimension is essential in various fields including quantum many-body physics. Existing approaches typically rely on approximate contraction schemes and do not provide certified error bars. We introduce a numerical bootstrap framework which casts the problem of tensor-network contractions into a convex optimization problem, thereby yielding certified lower and upper bounds on expectation values of physical observables. As a proof-of-principle, we construct such constraints explicitly for translationally invariant matrix product states and demonstrate that, assuming a canonical form, second-order-cone relaxation can provide tight bounds on the contraction result. We further demonstrate that when the requirement on canonical form is lifted, a more general semidefinite-programming approach could yield similar tight bounds at higher but still polynomial computational cost. Our work suggests numerical bootstrap could be a possible way forward for the rigorous contractions of tensor networks.
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Mesoscale and Nanoscale Physics

Strongly entangled Quantum Spin Rings driven by Hückel rule

Manish Kumar, Deng-Yuan Li, Zhangyu Yuan et al. (+11 more)

Quantum spin rings represent an intriguing platform for studying unconventional magnetic order and exotic quantum phases, and they are also promising materials for emerging quantum technologies. Conventional spin systems consist of a set of weakly interacting localized spins that are well described by the Heisenberg spin models. Here, we demonstrate that strong interactions between radical centers in macrocycles of different sizes lead to fluctuations in the total number of unpaired electrons and to non-trivial antiferromagnetic order that extends beyond the Heisenberg picture. We demonstrate that the electronic structure of these spin rings is governed by the concept of 4n/4n+2 Hückel (anti)aromaticity for even-membered rings, whereas odd-membered rings possess a highly degenerate frustrated magnetic ground state. The strongly coupled spin rings are experimentally realized through the on-surface synthesis of {\pi}-magnetic carbon-based macrocycles, which consist of [2]triangulene units. The close correlation between the electronic structure and the Hückel aromaticity rule is revealed by scanning tunneling spectroscopy and multireference calculations. This work establishes a novel design principle employing the concept of Hückel aromaticity for quantum spin macrocycles.
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All authors Manish Kumar, Deng-Yuan Li, Zhangyu Yuan, Ying Wang, Diego Soler-Polo, Enzo Monino, Libor Veis, Yi-Jun Wang, Xin-Yu Zhang, Can Li, Jinfeng Jia, Pei-Nian Liu, Pavel Jelinek, Shiyong Wang

Superconductivity

Pressure-induced Superconductivity in AgSbTe2

Sudaice Kazibwe, Bishnu Karki, Wencheng Lu et al. (+8 more)

AgSbTe2 is a well-known thermoelectric material with a high Seebeck coefficient and intrinsically low thermal conductivity, but its behavior under pressure remains largely unexplored. Here we report a systematic investigation of the structural, electronic, and transport properties of non-stoichiometric AgSbTe2 under high pressure. At ambient pressure, the material can be described as having a cubic crystal structure that remains stable up to 21.7 GPa beyond which it loses long-range structural order, while its crystal system fully recovers upon decompression. Remarkably, superconductivity emerges at a very low pressure of 0.38 GPa with an onset superconducting critical temperature (Tc) of 3.2 K. Tc increases with increasing pressure, reaching 6.9 K at 31.9 GPa, and peaks at 7.4 K during decompression. Magnetic-field-dependent transport measurements and electronic structure calculations reveal an evolution of the superconducting state driven by an enhanced electronic density of states at the Fermi level under compression. Our findings uncover pressure-induced superconductivity in AgSbTe2 and demonstrate that pressure can effectively tune the electronic ground state of thermoelectric materials, extending their functionality beyond thermoelectric energy conversion.
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All authors Sudaice Kazibwe, Bishnu Karki, Wencheng Lu, Zhongxin Liang, Minghong Sui, Melissa Gooch, Zhifeng Ren, Pavan Hosur, Timothy A. Strobel, Ching-Wu Chu, Liangzi Deng

Statistical Mechanics

Quasi-local Edge Mode in XXX Spin Chain/Circuit with Interaction Boundary Defect

Tomaž Prosen

We study the Heisenberg spin-1/2 model on a semi-infinite chain - or, equivalently, a trotterized unitary SU(2) symmetric six-vertex quantum circuit - with a boundary defect where the interaction between the two spins nearest the edge differs from that in the bulk. For sufficiently strong boundary interaction we explicitly construct a conserved operator quasi-localized near the boundary using a matrix-product ansatz. This quasi-local edge mode leads to non-decaying boundary correlation functions, corresponding to a nonzero boundary Drude weight. The correlation length of the edge mode diverges at a finite critical value of the boundary interaction, signaling a transition to ergodic boundary dynamics for subcritical interactions.
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Strongly Correlated Electrons

Magnetism, electronic transport, and disorder in strongly correlated systems

Joel Bobadilla

This thesis investigates the magnetic, spectral, and transport properties of strongly correlated electronic systems, with a primary focus on the Hubbard model and its extensions relevant for real materials. Within the dynamical mean-field theory (DMFT) framework, different regimes of interaction strength, temperature, doping, and magnetic field are explored, highlighting the central role of local electronic correlations in shaping spectral reconstruction and nontrivial transport responses. For the antiferromagnetic Hubbard model under a Zeeman field, magnetoresistance and local metamagnetism are characterized, revealing the coexistence of distinct energy scales associated with charge and spin degrees of freedom. A minimal, purely correlation-driven mechanism for generating spin-polarized charge transport in structurally conventional collinear antiferromagnets is identified, controlled by the simultaneous breaking of particle–hole symmetry and antiferromagnetic sublattice equivalence. Finally, these concepts are applied to correlated materials with strong spin–orbit coupling, such as Sr$_2$IrO$_4$ and Sr$_3$Ir$_2$O$_7$, and to nanoparticle solids dominated by Coulomb blockade and disorder. The results show how ideas developed in correlated lattice models provide a unified interpretation of metal–insulator transitions and spectral reconstruction in complex systems.
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Strongly Correlated Electrons

Emergent superconformal symmetry in the phase diagram of a 1D $\mathbb{Z}_{2}$ lattice gauge theory

Bachana Beradze, Mikheil Tsitsishvili and Sergej Moroz

We investigate the phase diagram and critical properties of a one-dimensional $\mathbb{Z}_{2}$ lattice gauge theory describing an orthogonal metal, where spinless fermions and Ising spins are minimally coupled to a deconfined $\mathbb{Z}_{2}$ gauge field. Working at half-filling of fermions, we derive an exact gauge-invariant formulation that maps the model onto decoupled XXZ and transverse-field Ising chains. This mapping enables a controlled low-energy field-theory description in terms of a perturbed Luttinger liquid and Ising conformal field theories. Combining analytical arguments with numerical simulations, we determine the full phase diagram and identify various critical and multi-critical regimes. Along a specific multi-critical line, where the fermionic and bosonic velocities coincide, we find strong evidence for an emergent superconformal symmetry. Our results establish a minimal lattice realization of emergent superconformal criticalities in a gauge-matter system and provide a route toward its exploration in quantum simulators.
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Materials Science

Simulating the influence of stoichiometry on the spectral emissivity of Mo$_x$Si$_y$ thin films

Zahra Golsanamlou, Arseniy Baskakov, Robbert van de Kruijs et al. (+4 more)

In this work, we simulate the spectral emissivity of various stoichiometric crystal phases of Mo$_x$Si$_y$ compounds using density functional perturbation theory. The dielectric function, including electronic and ionic contributions, is calculated for each phase. We use the bulk properties obtained to simulate the optical absorption spectrum originating from the compound in thin film ($\sim$20 nm) form. We find that most thin films of Mo$_x$Si$_y$ are metallic, however, our results indicate that their emissivity is not simply correlated with the Mo content. For hot metallic films at around 900 K, we predict a maximal emissivity between 5-10 nm thickness. Our results are in good qualitative agreement with experiments, confirming that the emissivity of hexagonal MoSi$_2$ is much lower than in the tetragonal phase. This is related to the small band gap (hexagonal MoSi$_2$) and low density of states at the Fermi level (tetragonal MoSi$_2$). Furthermore, test calculations on defected MoSi$_2$ demonstrate that the infrared emissivity of MoSi$_2$ thin films can be substantially increased by introducing defects.
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All authors Zahra Golsanamlou, Arseniy Baskakov, Robbert van de Kruijs, Silvester Houweling, Giorgio Colombi, Marcelo Ackermann, Menno Bokdam

Soft Condensed Matter

Optimal transport and control of an active particle near a plane wall

Utkarsh Maurya, Kavya Swaminathan, Ejaz Ashraf et al. (+1 more)

The control of active colloidal particles via optical traps is a cornerstone for research of matter at the micron and nanometer scale. A central challenge in this domain is the derivation of optimal transport protocols that minimize the mean work required to move a particle over a finite-time interval. Here, we present the Ritz method in which open-loop protocols are constructed from a global basis of Chebyshev polynomials. The protocols are optimized using either a genetic algorithm or a gradient-based method. We apply the method to study optimal transport of an active particle, which is modeled as a force-dipole (or a stresslet) near a no-slip wall. The methodology is validated in the limits of zero activity and infinite wall separation, where it successfully recovers the known analytical protocols and the theoretical minimum work. Crucially, we demonstrate that the presence of the activity breaks the time-reversal symmetry of the optimal protocol found. This symmetry breaking is shown to be a complex function of the transport direction and the particle's intrinsic activity. Because the presented approach requires only the capability to simulate stochastic trajectories, it offers a robust, principled framework for optimizing transport protocols in complex fluid environments that remain inaccessible to exact analytical treatment.
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All authors Utkarsh Maurya, Kavya Swaminathan, Ejaz Ashraf, Rajesh Singh

Strongly Correlated Electrons

Site-selective renormalization and competing magnetic instabilities in paramagnet Y$_{3}$Cu$_{2}$Sb$_{3}$O$_{14}$

Yanpeng Zhou and Gang Li

Quantum spin liquids (QSLs) are exotic phases of matter characterized by long-range entanglement and the absence of magnetic order even at zero temperature. Here, we present a comprehensive theoretical study of the frustrated magnet Y$_3$Cu$_2$Sb$_3$O$_{14}$ to elucidate its electronic and magnetic properties. We uncover completely opposite crystal-field splittings of the two inequivalent Cu sites owing to their fundamentally distinct oxygen coordination - trigonal distorted octahedral CuO$_6$ and axially compressed CuO$_8$. This inversion places the unpaired hole in the $d_{z^2}$ orbital at the Cu-2 site, while Cu-1 maintains conventional $d_{x^2-y^2}/d_{xy}$ character, which results in a selective band-renormalization of orbitals from the two Cu ions. We further find multiple magnetic instabilities competing with nearly equal strength in this system: the spin susceptibility lacks dominant peaks, and the leading eigenvalues approach unity simultaneously across all wavevectors with increasing interactions. This competitive interplay, originating from the distinct local environments and geometric frustration on the triangular lattice, agrees well with the absence of long-range magnetic order in experiment. Our results support Y$_3$Cu$_2$Sb$_3$O$_{14}$ as a promising QSL candidate where the unique combination of disparate crystal-field environments, strong correlations, and competing exchange interactions conspire to stabilize an exotic quantum ground state.
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Soft Condensed Matter · Cross-listed from hep-th

Hydrodynamics of dilation and spin currents

Zhong-Hua Zhang, Xi-Hu Lv and Xu-Guang Huang

We formulate a relativistic hydrodynamic theory for fluids with spin and intrinsic dilation charges. Using an entropy-current analysis, we derive constitutive relations featuring a bulk viscosity and a dilation conductivity governing the relaxation and diffusion of dilation charge. Linear mode analysis reveals a gapped dilation excitation and the freeze-out of long-wavelength sound modes, similar to the superhorizon modes in cosmology. In the nonrelativistic limit, the theory reduces to that of microstretch fluids. Upon coupling to electromagnetic field, we show that the scale anomaly permits additional contributions in the electric current, dilation current, and energy-momentum tensor. Our theory naturally applies to nearly conformal fluids undergoing rapid expansion or contraction.
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Soft Condensed Matter

Shear and bulk viscosities of water up to 1.6 GPa and anomaly in the structural relaxation time

Jan Eichler, Johannes Stefanski, José Martin Roca et al. (+4 more)

Deep in the Earth's crust, pressure exceeds one thousand times the atmospheric pressure. Water still flows under these conditions, but experiences dramatic changes in structure and fluidity. Using combined dynamic and inelastic light scattering techniques, we simultaneously measure the shear and bulk viscosities of water as a function of pressure. The former increases faster than the latter, so that their ratio shows a two-fold decrease from 0 to 1.6 GPa; we confirm this trend with simulations. We analyze our results in terms of the structural relaxation time $\tau$. Contrary to other liquids, pressure initially accelerates relaxation in water. Our measurements reveal that $\tau$ reaches a minimum close to 1 ps around 0.5 GPa. We interpret $\tau$ as a the equilibration time of hydrogen bonds, and propose that the minimum in $\tau$ arises from a structural anomaly which allows fastest interconversion between local structures in water, and generates a cascade of thermodynamic and dynamic anomalies.
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All authors Jan Eichler, Johannes Stefanski, José Martin Roca, Isabelle Daniel, Bruno Issenmann, Chantal Valeriani, Frédéric Caupin

Strongly Correlated Electrons

Hamiltonian Monte Carlo enhanced by Exact Diagonalization

Finn L. Temmen, Martina Gisti, David J. Luitz et al. (+2 more)

Strongly correlated fermionic systems are of great interest in condensed matter physics and numerical methods are indispensable tools for their study. However, existing approaches such as exact diagonalization (ED) and stochastic quantum Monte Carlo methods each suffer from fundamental limitations: ED is hindered by exponential scaling in system size, while Monte Carlo methods are plagued by sign problems and long autocorrelation times. These limitations restrict the accessible parameter space and developing algorithms that efficiently alleviate them remains a central challenge in computational physics. In this work, we propose a hybrid algorithm that combines ED and Hamiltonian Monte Carlo (HMC) to simulate 2D arrays of coupled quantum wires, modeled as interacting fermionic Hubbard chains. We demonstrate how our hybrid implementation of HMC, which we dub H$^2$MC, outperforms either method alone across several key simulation facets. When compared to pure ED, H$^2$MC has a much more favorable computational scaling, which allows us to push simulations to much larger 2D arrays. H$^2$MC also greatly alleviates the sign problem and reduces autocorrelation times when compared to pure HMC formulations utilizing either real or imaginary auxiliary fields. Our formalism demonstrates how complementary strengths of seemingly disparate methods can be leveraged to enable feasible simulations in an extended parameter space.
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All authors Finn L. Temmen, Martina Gisti, David J. Luitz, Thomas Luu, Johann Ostmeyer

Superconductivity

Reaching Quantum Critical Point by Adding Non-magnetic Disorder in Single Crystals of Superconductor $(\text{Ca}_x\text{Sr}_{1-x})_3\text{Rh}_4\text{Sn}_{13}$

Elizabeth H. Krenkel, Makariy A. Tanatar, Romain Grasset et al. (+5 more)

The Remeika series superconductor, $(\text{Ca}_x\text{Sr}_{1-x})_3\text{Rh}_4\text{Sn}_{13}$, shows a rare nonmagnetic quantum critical point (QCP) associated with the continuous charge-density wave (CDW) and structural transition under the “dome” of superconductivity achieved by tuning composition and applying pressure. Here we use a nonmagnetic point-like disorder induced by 2.5 MeV electron irradiation to suppress the CDW and drive the system to and even beyond the QCP. This conclusion is based on a clear evolution of temperature-dependent resistivity, $\rho\left(T\right)$, from the Fermi liquid to the non-Fermi liquid regime with increasing amount of disorder. Starting on the CDW side, below the suggested QCP concentration of $x_c=0.9$, added disorder resulted in a progressively larger linear term and a reduced quadratic term in $\rho\left(T\right)$. Nearly perfect $T-$linear dependence is observed at the dose at which long-range CDW order is suppressed to $T=$0, consistent with the expectations. We refine the QCP location in this system and place it in the interval between $x=$0.75 and 0.85. Our results strongly support the concept that the disorder can tune the system to the quantum critical regime and even beyond. It follows from the argument by Imry and Ma that any ordered phase is unstable toward quenched disorder. Introduced in a controlled way, this disorder becomes a novel non-thermal tuning parameter likely applicable to a variety of different systems.
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All authors Elizabeth H. Krenkel, Makariy A. Tanatar, Romain Grasset, Marcin Kończykowski, Shuzhang Chen, Cedomir Petrovic, Alex Levchenko, Ruslan Prozorov

Mesoscale and Nanoscale Physics

Magneto-rotation coupling dominates surface acoustic wave driven ferromagnetic resonance in the longitudinal geometry

Gyuyoung Park, OukJae Lee and Jintao Shuai

We present a phonon-magnon extension for the mumax+ micromagnetic framework that implements three surface acoustic wave (SAW) coupling mechanisms: magnetoelastic strain coupling, magneto-rotation coupling arising from the antisymmetric displacement gradient, and spin-rotation (Barnett) coupling from the lattice angular velocity. Six benchmark simulations validate the implementation through SAW-driven domain-wall motion, magnetization switching, magneto-rotation and Barnett field validation, nonreciprocal SAW-magnon absorption from Rayleigh-wave chirality, and spatially resolved coupling in a standing SAW cavity. For the longitudinal geometry (m_0 parallel to k_SAW), we show that the magnetoelastic coupling produces zero transverse torque despite generating a 50 times larger effective field; the magneto-rotation channel provides the sole driving mechanism. The crossover angle below which MR dominates is theta_c approximately 1.1 degrees for YIG parameters. Treating the magneto-rotation coupling constant K_mr as a tunable parameter, we map out the cooperativity phase diagram and show that MR alone can achieve strong coupling (C = 257 for K_mr = 1 MJ/m^3) with an avoided-crossing splitting of 13.6 MHz.
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Mesoscale and Nanoscale Physics

Strain-driven spin mixing and dark-exciton recombination in a neutral Ni2+ doped quantum dot

K. E. Polczynska, S. Karouaz, W. Pacuski et al. (+1 more)

We investigate the optical properties of neutral excitons in CdTe/ZnTe quantum dots containing a single Ni2+ ion. We show that the photoluminescence spectra provide a direct spectroscopic signature of strain induced mixing of the Ni2+ spin states. A misalignment between the principal axis of the local strain tensor and the quantum dot growth direction reorients the spin quantization axis of the magnetic ion, reducing the hole Ni2+ exchange interaction at low magnetic field and giving rise to photoluminescence replicas around the partially linearly polarized bright-exciton transitions. A longitudinal magnetic field restores the circularly polarized optical selection rules, allowing the three spin projections S_z = 0, +-1 of the Ni2+ ion to be spectrally resolved. Dark exciton emission appears on the low energy side of the spectra and is dominated at low field by transitions involving spin flips of the magnetic ion. An effective spin Hamiltonian including strain orientation and valence band mixing reproduces the magnetic field evolution of both bright and dark exciton spectra. These results highlight the key role of the local strain environment in determining the spin exciton coupling of transition metal dopants in semiconductor quantum dots.
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All authors K. E. Polczynska, S. Karouaz, W. Pacuski, L. Besombes

Quantum Gases

Bosonic quantum mixtures with competing interactions: quantum liquid droplets and supersolids

Sarah Hirthe and Leticia Tarruell

These lecture notes contain an introduction to quantum simulation of bosonic systems in the continuum, focusing on weakly interacting Bose-Bose mixtures with competing mean-field interactions. When the values of such interactions are fine-tuned to almost completely cancel the mean-field energy, quantum fluctuations become apparent and dominate the behavior of the system, stabilizing an ultradilute quantum liquid phase. An analogous situation appears in single-component dipolar quantum gases. We review the mechanism that gives rise to this exotic quantum liquid, which can form droplets that are self-bound in the absence of any external confinement, and discuss their properties and dynamics in both the mixture and the dipolar cases. In dipolar gases, arrays of dipolar droplets stabilized by quantum fluctuations can establish global phase coherence and form supersolids. In bosonic mixtures, supersolidity can emerge already at the mean-field level through spin-orbit coupling. We discuss the properties of such spin-orbit-coupled supersolids, comparing them to their dipolar counterparts. Specifically, we focus on their periodic density modulation, phase coherence, and peculiar excitation spectrum, which hosts both superfluid and crystal excitations. Finally, we conclude by discussing open research directions in the areas of quantum liquid droplets and spin-orbit-coupled supersolids, in particular at the interface of the two research topics.
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Statistical Mechanics

Critical Scaling of Finite-Size Fluctuations around Marginal Stability in Long-Range Hamiltonian Systems

Yoshiyuki Y. Yamaguchi and Julien Barré

Finite size fluctuations are a crucial ingredient in kinetic theory of long-range interacting collisionless systems. In this Letter, we introduce a phenomenological theory which predicts an anomalous scaling close to marginal stability for these fluctuations. It also pinpoints the critical window inside which the fluctuations are anomalous, and outside which they are Gaussian. Shrinking very slowly as $N^{-1/5}$, this critical window encompasses a wide region around marginal stability. We confirm our predictions through extended numerical simulations on two different simplified models.
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Superconductivity

Electron-Hole Scattering Dichotomy and Anisotropic Warping in Quasi-Two-Dimensional Fermi Surfaces of UTe2

Motoi Kimata, Jun Ishizuka, Freya Husstedt et al. (+13 more)

We present a combined experimental and theoretical study of the detailed Fermi-surface (FS) geometry of UTe2, a heavy-fermion superconductor that has recently attracted considerable attention as a promising candidate for spin-triplet pairing. Using angle-dependent magnetoresistance oscillations, a bulk- and low-energy-sensitive transport probe for quasi-two-dimensional (Q2D) electronic structures, we directly determine the in-plane FS geometry. We found that the Q2D FS exhibits a rectangular cross-sectional shape with strongly anisotropic warping, originating from the hybridization of two orthogonal quasi-one-dimensional bands. Through a quantitative comparison between experiment and theoretical calculations, we further reveal a large electron-hole scattering dichotomy: the quasiparticle lifetime on the electron FS is substantially shorter than that on the hole FS. This dichotomy is naturally explained by anisotropic, low-dimensional antiferromagnetic fluctuations, which selectively enhance scattering on the electron FS. This suggests a dominant role of the electron pockets for the emergence of superconductivity. Our results clarify a direct relation between FS geometry, magnetic fluctuations, and momentum-dependent quasiparticle lifetimes, and thus providing a crucial basis for the microscopic understanding of pairing mechanism, and impose stringent constraints on the gap symmetry of spin-triplet superconductivity in UTe2.
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All authors Motoi Kimata, Jun Ishizuka, Freya Husstedt, Yusei Shimizu, Ai Nakamura, Dexin Li, Yoshiya Homma, Atsushi Miyake, Yoshinori Haga, Hironori Sakai, Yoshifumi Tokiwa, Shinsaku Kambe, Yo Tokunaga, Dai Aoki, Toni Helm, Youichi Yanase

Materials Science · Cross-listed from physics.chem-ph

In-phase current and temperature oscillations reduce PEM fuel cell resistivity: A modeling study

Andrei Kulikovsky

We have developed a non-isothermal analytical model for the impedance of the cathode catalyst layer (CCL) in a PEM fuel cell. In-phase harmonic perturbations to the current density and temperature reduce the impedance and the static polarisation resistivity of the CCL due to lowering proton transport losses. A special selection of the current and temperature perturbation amplitudes allows for complete elimination of these losses.
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Materials Science

Chiral-Induced Spin Selectivity Effect in a 1 nm Thin 1,1'-Binaphthyl-2,2'-diyl Hydrogenphosphate Self-Assembled Monolayer on Nickel Oxide

Abin Nas Nalakath, Christian Pfeiffer, Anu Gupta et al. (+6 more)

The chiral-induced spin selectivity (CISS) effect describes an observed correlation between the orientation of an electron spin transported or transferred through a molecule and that molecule's chirality. Suitable molecules are usually arranged as self-assembled monolayers (SAMs), and the primary CISS systems are based on multiple nanometer-long biomolecules exhibiting helical chirality. Aside from these typically thiolate-anchored molecules, phosphonic and phosphoric acid SAMs may well become significant for those CISS applications that require a more robust molecular coupling to metal oxide surfaces. In this work, we report on our studies, employing the aromatic, low-molecular-mass, axially chiral organophosphoric acid derivative 1,1'-binaphthyl-2,2'-diyl hydrogenphosphate (BNP). Grown as a roughly 1 nm thin SAM on top of a NiOx/Ni substrate, a strong circular dichroism signal indicates that the thin films preserved chirality. The CISS response exhibits a high magnetoresistance with a spin polarization of 50-80% when measured using magnetic-conductive atomic force microscopy. For biases above 0.5 V, the magnetoresistance curves could be well fitted to the Fowler-Nordheim (FN) tunneling model. Using a minimal FN model, we determined that, depending on the magnetization direction and the handedness of the molecules, electrons of a certain spin direction face an effective tunneling barrier at high bias, which is either 80 % higher or 40 % lower compared to the barrier for electrons of the opposite spin direction. Due to the small size of the molecules, their compatibility with oxide materials, and their commercial availability, they are excellent candidates for the realization of novel (nanoscale) organic spintronic devices.
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All authors Abin Nas Nalakath, Christian Pfeiffer, Anu Gupta, Franziska Schölzel, Michael Zharnikov, Georgeta Salvan, Ron Naaman, Marc Tornow, Peer Kirsch

Soft Condensed Matter

Non-equilibrium phase coexistence in conserved chemically active mixtures

Florian Raßhofer and Erwin Frey

Chemical activity is known to affect phase coexistence and coarsening in liquid mixtures, most commonly through reaction-induced changes of intermolecular interactions. Here, we analyze a scenario in which chemical reactions regulate particle transport while leaving thermodynamic interactions unchanged. We study an incompressible mixture of thermodynamically identical solutes with unequal diffusivities that interconvert through driven chemical reactions. Using linear stability analysis and finite-element simulations, we show that the system can phase-separate into solute-rich and solute-poor domains via two qualitatively different pathways. When interactions are too weak to induce phase separation, patterns arise through a generalized mass-redistribution instability and coarsen uninterruptedly. When interactions favor phase separation, coarsening can be arrested if chemical activity locally enriches faster-diffusing solutes within dense domains. In the limit of fast chemical turnover, the system always coarsens, and phase coexistence is governed by an effective free energy that explicitly depends on kinetic parameters. Beyond this limit, we develop a sharp-interface theory that predicts the onset of arrested coarsening, stationary droplet sizes, and nucleation conditions under chemical driving. Taken together, our results establish kinetic regulation as a minimal and robust mechanism to control phase coexistence and coarsening in chemically active mixtures.
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