Mesoscale and Nanoscale Physics
Orbital-geometric spin response: inter-band coherence versus orbital-to-spin conversion
Dimitrie Culcer
Orbitronics seeks to convert orbital angular momentum (OAM) into
spin, but its microscopic dynamics is not understood. Here I show that, at moderate
spin-orbit coupling, an electrically induced
spin density contains a direct term and an orbital-
geometric spin response (OGSR) due to inter-band coherence of the spinless parent
Hamiltonian—related to OAM but not descended from it. In massive Dirac
fermions the OGSR survives as the net OAM vanishes, and generates counterflow, opposing the direct response. These results provide guidelines for optimising orbitronic systems.
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Mesoscale and Nanoscale Physics · Cross-listed from cond-mat.mtrl-sci
Unusual phase coexistence regime across the ferroelectric - paraelectric transition
M. Kopecký, J. Kub, E. de Prado et al. (+1 more)
A PbTiO3 epitaxial thin film grown on a DyScO3 substrate was studied by x-ray diffraction in the temperature range of 20-500 degrees Celsius. It is found that the ferroelectric to paraelectric phase transition proceeds through a peculiar transitional coexistence state extending between 402 degrees Celsius and 414 degrees Celsius. Within this temperature interval, at a temperature of 405 degrees Celsius, a domain structure transition occurs, at which the three-domain c/a1/a2 polytwin phase transforms into an ordinary c/a2 twinned phase. It is proposed that the coexistence of two independent twinned phases with the paraelectric phase is enabled by vertical strain gradients in the film.
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All authors M. Kopecký, J. Kub, E. de Prado, J. Hlinka
Mesoscale and Nanoscale Physics
Real-space Visualization of Emergent Electron Crystals in Rhombohedral Graphene
Yiming Sun, Jinghao Deng, Jiabin Xie et al. (+5 more)
Strongly interacting electrons can spontaneously break spatial
symmetries to form electron
crystals, exemplified by the Wigner
crystal. Recent studies of topological flat bands in rhombohedral
graphene have suggested more exotic forms of crystallization, including anomalous Hall
crystals that entangle charge order with nontrivial
topology and metallic electron
crystals in which localized and itinerant carriers coexist. Direct real-space observation of these states, however, has remained elusive. Here we use scanning tunneling microscopy and spectroscopy to visualize emergent electron
crystals in rhombohedral hexalayer
graphene. At low electric fields and over a finite range of hole doping, we observe electronic
lattice patterns that evolve from honeycomb to oblique order through a first-order
quantum phase transition with increasing hole density. The Fermi surface extracted from quasiparticle-interference measurements lacks the geometry needed to account for these patterns through conventional nesting. Together with metallic transport and a
crystal-site density much lower than the doped carrier density, this supports metallic electron
crystals in which a subset of carriers crystallizes. The honeycomb
crystal occupies the same phase space as the multiferroic orbital magnetism observed previously in transport and exhibits domain stabilization by a small
magnetic field, which may suggest a possible metallic anomalous Hall
crystal. With increasing
magnetic field, the oblique phase develops a $\sqrt{2}\times\sqrt{2}$ reconstruction with a
crystal-sublattice energy splitting that increases linearly with field, corresponding to a $g$-factor of 16. This may reflect an orbital-antiferromagnetic electron
crystal with alternating orbital magnetization across the
lattice. These results establish a new paradigm of electron crystallization in which charge order is intertwined with orbital magnetism.
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All authors Yiming Sun, Jinghao Deng, Jiabin Xie, Donghan Ge, Hongyuan Li, Takashi Taniguchi, Kenji Watanabe, Xiaomeng Liu
Mesoscale and Nanoscale Physics
Catastrophes, Optical Multistabilities, and Chiral Photocurrent Hysteresis in Driven Weyl Semimetals
Christopher Yang, Gil Refael and Frederik Nathan
The unique band
topology of Weyl semimetals provides them with a diverse array of strong photovoltaic response phenomena. Here we explore how
plasma screening affects these phenomena. We find that the screening field is highly nonlinear, can contain topological contributions, and can amplify the photoresponse, with the self-consistent response exhibiting the hallmarks of bistability, including hysteresis and catastrophes. The bistability emerges from a crossover from linear to saturated polarization as a function of the field amplitude, because carriers in a Weyl cone have bounded group speed $v_F$. When time-reversal
symmetry is broken, tilted Weyl nodes add a Berry-
curvature contribution to the in-plane current, making the nonlinear screening response helicity selective.
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Mesoscale and Nanoscale Physics · Cross-listed from cond-mat.mtrl-sci
High-fidelity k$\cdot$p representations of first-principles electronic band structures through covariant renormalization
Kristian Berland
The k$\cdot$p method can be applied directly to first-principles energies and momentum
matrix elements, but the resulting models converge slowly with the number of bands and are inexact wherever the underlying
Hamiltonian is nonlocal. We show that both limitations can be largely removed by renormalizing the
eigenvalue spectra of the Hermitian momentum
matrices. These spectra are gauge invariant, and are identical along
symmetry-related directions; hence scaling the recurring magnitudes provides a modest set of parameters that preserves degeneracies and
crystal symmetry without needing to construct a
symmetry-adapted basis. We choose to renormalize the models against reference
eigenvalues and band velocities on rays out of a high-
symmetry point. For GaP, a 15-band model can reproduce the band structure in the near-gap reference regions within a few meV. For zincblende and wurtzite AlN, similar accuracy requires 30- or 85-band models, which in the case of zincblende is traced to the strong warping in the [110] direction. The scheme readily generalizes to rocksalt PbTe, which includes
spin-orbit coupling, and has $L$-centered valence- and conduction-band extrema. Further, we demonstrate, for GaP, how compact four-band
Hamiltonians can be obtained by downfolding within the same renormalization scheme. Moreover, by changing the fitting regime to encompass the entire Brillouin zone, the scheme can be used to generate full-zone models that reproduce the density of states. A practical use of the method is illustrated with a 59-band k$\cdot$p model for GaP, which can be evaluated on meshes far denser than the reference calculation, that in turn can be used both to resolve fine features in the density of states and to compute the hole conductivity at low temperature.
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Mesoscale and Nanoscale Physics · Cross-listed from cond-mat.mtrl-sci
Piezomagnetism in a model cubic noncollinear altermagnet
Sudarshan Sharma, Luca Buiarelli, Richard Spieker et al. (+4 more)
Altermagnets constitute a distinct class of
magnetic materials that combine compensated
magnetic order with
spin-polarized electronic bands, hence carrying characteristics of both antiferromagnets and ferromagnets. Piezomagnetism - the linear relationship between
lattice strain and a net
magnetic moment - has emerged as smoking-gun evidence of altermagnetism that distinguishes it from antiferromagnetism. Here, we uncover a large piezomagnetic response in MnTe2, a cubic altermagnet with a noncollinear
spin arrangement and weak
spin-orbit coupling. We combine dilatometry with nuclear
magnetic resonance, a bulk local probe, to observe signatures of both direct and inverse piezomagnetism consistent with
symmetry considerations and first-principles calculations. The dilatometry measurements reveal a shear deformation proportional to an applied
magnetic field, while
magnetic resonance detects a ferromagnetic moment induced by shear strain. The respective results for the piezomagnetic coupling strength are in good agreement, and they are captured by first-principles results. These findings for simple binary MnTe2 position this material as a model altermagnet, demonstrate the utility of nuclear
magnetic resonance in investigations of piezomagnetism, and open new avenues for
multimodal strain and
magnetic-field control in spintronic and
memory applications.
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All authors Sudarshan Sharma, Luca Buiarelli, Richard Spieker, Ivan Jakovac, Damjan Pelc, Turan Birol, Martin Greven
Mesoscale and Nanoscale Physics
Interacting Tomonaga-Lüttinger liquid with impurity for interaction constant $K = 1/2$: Thermopower investigation, entropy variation and heat capacity densities associated with thermoelectric particle transport
Abdellah Touati, Redoune Zamoum and Farhi Houssam Eddine
We investigate thermoelectric and thermodynamic properties of a Tomonaga-Luttinger
liquid with interaction parameter $K=1/2$ in the presence of a localized impurity. Using bosonization and refermionization, we derive an exact expression for the thermopower in the nonlinear regime and obtain the corresponding Seebeck coefficient in linear response. At low temperature, the Seebeck coefficient follows a Mott-like relation governed by the energy dependence of the transmission coefficient and is found to be closely related to the
entropy per particle. This connection allows us to derive using Kelvin formula the thermopower contribution to the
entropy density variation and the charge-carriers contribution to the heat capacity density. Both quantities exhibit Mott-like behavior in the low-temperature regime. We apply our results to a fractional
quantum Hall
quantum point contact and to a one-channel
quantum conductor coupled to an Ohmic environment. Our results demonstrate the close connection between thermoelectric transport and thermodynamic properties in interacting one-dimensional
quantum systems.
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Mesoscale and Nanoscale Physics
Hilbert-space selected switch of helical edges in an artificial quantum Hall insulator
Naijie Ren, Zhiren Xiong, Kaining Yang et al. (+9 more)
Quantum Hall effects (QHE) host one-dimensional topologically-protected edge channels, which can serve as an essential ingredient in exotic
quantum electronic systems. Yet the manual reconstruction of Landau-level
topology, by electrostatic confinement or
symmetry breaking, remains experimentally challenging. Here, we show that interfacial charge transfer in between CrOCl and large-angle twisted bilayer
graphene offsets the two otherwise decoupled Dirac Landau-level ladders in each
graphene layer, creating a new sequence of composite filling configurations. At charge neutrality, the composited $(+2,-2)$ state involves only the zeroth Landau levels and becomes fully insulating, with longitudinal resistance reaching the G$\Omega$ regime. By contrast, higher composite zero-filling
quantum Hall states, including $(+6,-6)$ and $(+10,-10)$, retain counter-propagating helical edge channels and exhibit pronounced non-local transport, reaching up to $50\%$ of the local response. We attribute such switching-behavior to the Landau-spinor
Hilbert space – as the filling is reduced from $(+6,-6)$ to $(+2,-2)$, the orthogonal $N=\pm1$ orbital components are removed, eliminating the edge-compatible channel and gapping both bulk and boundary transport. The interaction nature of the observed gapped sates was further examined both experimentally and theoretically. Our results suggest that charge transfer provides a direct route to engineer artificial
quantum Hall
insulators, opening possibilities for wavefunction-selective control of helical edge modes.
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All authors Naijie Ren, Zhiren Xiong, Kaining Yang, Yanran Shi, Hanwen Wang, Kenji Watanabe, Takashi Taniguchi, Neng Wan, Xiaojun Jia, Jianpeng Liu, Zheng Vitto Han, Yaning Wang
Mesoscale and Nanoscale Physics
Interplay of Kekulé order, flat bands and electron-phonon coupling in graphene
Dominik Szczȩśniak
The Kekulé order in
graphene is known to coexist with both an extended flat band near the van Hove singularity (VHS) and enhanced electron-
phonon coupling. However, the microscopic origin of this interplay is still not fully understood. Here, this behavior is traced to the Kekulé-induced bond distortion and the resulting redistribution of electronic delocalization. In this picture, the conduction band below the VHS is observed to flatten with increasing Kekulé order, extending its intrinsically weakly dispersive region around the saddle point over a broader momentum range. In parallel, the VHS and dimensionless electron-
phonon coupling exhibit approximately quadratic upward responses. The resulting enhancement of the electron-
phonon coupling is found to be predominantly governed by the increasing density of states, with the effective deformation potential remaining weakly affected by the distortion. As such, the obtained results are consistent with previous theoretical and experimental studies, while also pointing toward the narrowing of the conduction band and a possible departure of the electron-
phonon coupling from the conventional adiabatic regime. Altogether, they highlight the potential of controlled Kekulé ordering for engineering emergent correlated
quantum phases in
graphene.
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Mesoscale and Nanoscale Physics
Bridging steady-state and time-domain descriptions of molecular electron transport
Thibaut Lacroix, Namgee Cho, Clemens Vittmann et al. (+3 more)
Electron transmission from an input electrode, through a molecular system, to an output electrode has been widely studied using the steady-state non-
equilibrium Green's function (NEGF) method. Recently, the wave packet method, which provides access to the transient dynamics of electrons as well as internal molecular degrees of freedom, has been employed to investigate enantiospecific electron transport through chiral molecules. In this work, we derive the quantitative relation between the transmission of a finite-size wave packet and the energy-resolved NEGF transmission, showing that the former corresponds to a spectral average of the latter weighted by the wave packet energy distribution. Exploiting this correspondence, we construct non-Gaussian auxiliary wave packets whose spectral weight encodes the Landauer energy-window, allowing current-voltage characteristics to be obtained directly from time propagation. We further show that the correspondence extends to
spin-resolved transport in a
spin-
phonon model of chirality-induced
spin selectivity.
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All authors Thibaut Lacroix, Namgee Cho, Clemens Vittmann, James Lim, Susana F. Huelga, Martin B. Plenio
Mesoscale and Nanoscale Physics · Cross-listed from quant-ph
Nonlinear dynamics and mechanical frequency combs with a Meissner-levitated micromagnet
Yiqi Wang, Trisha Madhavan, Vanessa Wachter et al. (+6 more)
Nonlinearities in multimode mechanical systems can give rise to rich dynamical phenomena with great potential for sensing applications and for future
quantum experiments. We demonstrate that the coupled center-of-mass and rotational motion of a Meissner-levitated micromagnet offer a promising platform for nonlinear dynamics, combining low dissipation,
magnetic tunability, strong intrinsic Duffing nonlinearities, and nonlinear intermodal couplings. We use this tunability to demonstrate the generation of a mechanical frequency comb in the micromagnet dynamics, realized by parametric excitation of two translational modes followed by cascaded nonlinear frequency mixing, which produces a phononic comb with tunable spacing. At large amplitudes, nonlinear coupling to a low-frequency librational mode of the magnet leads to parametric excitation and phase locking of that mode, generating a dense spectral fine structure at subharmonics of the drive. These results establish levitated micromagnets as a platform for nonlinear multimode mechanics, with potential applications in precision sensing and
quantum-limited metrology.
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All authors Yiqi Wang, Trisha Madhavan, Vanessa Wachter, J. DaLi Schaefer, Addison NewRingeisen, Zhiyuan Wei, Frankie Fung, Benjamin A. Stickler, Mikhail D. Lukin
Mesoscale and Nanoscale Physics · Cross-listed from cond-mat.mtrl-sci
Competition between vacancy creation and filling in defect-engineering of hBN
Shrirang Chokappa, Manuel Längle, Barbara Maria Mayer et al. (+13 more)
Hexagonal boron nitride (hBN) has recently become the focus of intense research as a material that can host
quantum emitters. It is known that such emission is related to point defects, but in order to conclusively correlate specific defects to their spectra, having control over the defect creation mechanism is required. Here, we prepare freestanding, monolayer hBN samples and irradiate them with ultra-low-energy (150 eV) Ar+ ions. The samples are characterized before and after irradiation via scanning transmission electron microscopy to assess the defect density and distribution. Contrary to what analytical potential molecular dynamics simulations have predicted, we predominantly observe boron single vacancies after ion irradiation, followed by double vacancies at half the count. Moreover, we also observe that vacancy filling with Si and C impurity atoms plays a more significant role in the created defects than previously assumed, potentially posing a problem for selective creation of
quantum emitters in hBN.
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All authors Shrirang Chokappa, Manuel Längle, Barbara Maria Mayer, Vladimir Zobač, Jacob Madsen, Diana Propst, David Lamprecht, Philipp Irschik, Arixin Bo, Clara Kofler, Vinzent Hana, Fabian Kraft, Clemens Mangler, Lado Filipovic, Toma Susi, Jani Kotakoski
Mesoscale and Nanoscale Physics
Phase-Controlled Majorana Zero Modes in Altermagnetic Topological-Insulator Josephson Junctions
Hao Dong, Xun-Jiang Luo, Xiao-Hong Pan et al. (+1 more)
We exploit facet-dependent Andreev phase shifts to control topological superconductivity with a phase bias in a three-dimensional altermagnetic topological-
insulator Josephson junction. In the weak link between two conventional $s$-wave
superconductors, the $d$-wave altermagnetic order produces anisotropic momentum shifts of the surface Dirac cones. The resulting net momentum of the states involved in Andreev reflection generates additional propagation phases that differ between facets. Consequently, the facet-resolved Andreev spectra exhibit gap closings at distinct phase biases, giving rise to topological superconducting regimes that host Majorana zero modes (MZMs). We further show that the spatial locations of the MZMs can be controlled by the phase bias. Moreover, these topological superconducting transitions are only weakly affected by moderate variations in the chemical potential, obviating the need for fine-tuning to the Dirac point. Our results establish a platform for realizing and spatially controlling MZMs by tuning the superconducting phase bias in altermagnetic topological-
insulator Josephson junctions.
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All authors Hao Dong, Xun-Jiang Luo, Xiao-Hong Pan, Xin Liu
Mesoscale and Nanoscale Physics
Néel-Vector-Dependent Altermagnetic Spin Splitting in the One-Dimensional Limit
Xin Chen and Duo Wang
Altermagnetism is normally identified by nonrelativistic
spin-split bands in a compensated collinear magnet. Under one-dimensional confinement, however, this diagnostic can disappear: a boundary-compatible sublattice-exchange operation may leave the only Bloch momentum unchanged, forcing the nonrelativistic
spin-up and
spin-down spectra to coincide. We show that nonrelativistic
spin degeneracy can coexist with relativistic
spin splitting in a compensated one-dimensional magnet. In a Lieb-based construction, confinement along the diagonal cancels the projected $d$-wave
spin splitting while preserving the real-space sublattice-exchange motif.
Spin-orbit coupling (SOC) then locks
spin to the
lattice, so the Néel-vector orientation selects a
magnetic line group that can reveal
spin-split bands. Using fully compensated $[110]$ Ta$_2$TeSeO nanoribbons as a prototype, we find
spin-degenerate nonrelativistic bands and sizable SOC splitting in the inherited easy-axis domain, $\mathbf N\parallel[100]$ or $[010]$. For sufficiently wide ribbons that retain the parent easy-axis order, this splitting is an
equilibrium property. The easy-axis ribbon also supports a right-moving-mode
spin polarization of about $35\%$ at finite ideal conductance. Our results establish one-dimensional fully compensated altermagnetic functionality hidden inside apparently conventional antiferromagnetic bands.
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Mesoscale and Nanoscale Physics · Cross-listed from cond-mat.mtrl-sci
Bottom-up Synthesis of Metastable 2D Hexagonal Copper(I) Iodide on Monolayer and within Bilayer Graphene
David Kaiser, Guobin Jia, Janis Köster et al. (+9 more)
Copper(I) iodide (CuI) is a wide-bandgap
semiconductor crystallizing in the 3D $\gamma$-phase under ambient conditions; its layered van der Waals bulk phase $\beta$-CuI is stable only between 643 and 673 K. The two-dimensional (2D) h-CuI form has been obtained via
liquid-phase exfoliation of mechanochemically prepared precursors and via encapsulation between
graphene sheets, whereas bottom-up growth of 2D h-CuI on open surfaces has not yet been demonstrated. Here, we report a vapor-phase synthesis of h-CuI directly on low-defect, large-area monolayer and within bilayer reduced oxo-
graphene (r-oxo-G) at low temperatures. Using a copper TEM grid as the solid-state precursor for copper, HI-vapor exposure at 40 $\deg$C initiates nucleation, while annealing at 180 $\deg$C promotes the growth of extended h-CuI domains. Aberration-corrected HRTEM resolves the atomic structure, local twist angles, and
lattice anisotropy of the CuI/r-oxo-G nanohybrid, while STEM-EDX yields a Cu:I ratio consistent with 1:1. First-principles calculations show that van der Waals adhesion to
graphene stabilizes the supported hexagonal layer. Under the presented low-temperature precursor conditions, pathways for nucleation of the $\gamma$-phase are not available, allowing the hexagonal phase to form selectively at the
graphene interface. Ab initio molecular dynamics simulations show that the
heterostructure retains its hexagonal
lattice order at 600 K, including on an open monolayer
graphene support. The lateral extent of the growth is limited mainly by remaining interfacial adsorbates. These results establish a route to metastable 2D h-CuI on a chemically inert
graphene template, which may be useful for wide-bandgap electronic and optoelectronic devices.
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All authors David Kaiser, Guobin Jia, Janis Köster, Sadegh Ghaderzadeh, Christian E. Halbig, Siegfried Eigler, Andrey Turchanin, Benjamin Dietzek-Ivanšić, Arkady V. Krasheninnikov, Jonathan Plentz, Elena Besley, Ute Kaiser
Mesoscale and Nanoscale Physics · Cross-listed from cond-mat.mtrl-sci
Vacancy Order and Physical Properties of a Ternary Compound Fe0.68Pd0.80Te with an {\alpha}-Fe1+xTe-Type Structure
Bingxian Shi, Manyu Wang, Chenglin Shang et al. (+14 more)
We report the identification and characterization of a new compound Fe0.68Pd0.80Te with an {\alpha}-Fe1+xTe prototype structure. Different from the Fe-square net and minor occupancy of interstitial Fe-sites in Fe1+xTe, Fe0.68Pd0.80Te is featured by a Pdsquare net and near 68% occupancy of the corresponding interstitial Fe-sites. Furthermore, noncontact atomic force microscopy and X-ray diffraction provide evidence for the existence of a 3*3*3 Pd-vacancy order in this layered material. A
spin-glass ground state below Tg 40 K is identified via
magnetic characterization. Electrical transport measurements show that Fe0.68Pd0.80Te is a
semiconductor with a very small band gap below 10 meV. It has weak negative magnetoresistance and holelike charge carriers below room temperature. Our results demonstrate its potentials for further exploring various
quantum phenomena.
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All authors Bingxian Shi, Manyu Wang, Chenglin Shang, Yanyan Geng, Yangyang Fan, Xiaoxiao Pei, Li Huang, Daye Xu, Juanjuan Liu, Jinchen Wang, Hongxia Zhang, Hongliang Wang, Lijie Hao, Rui Xu, Zhongyi Lu, Zhihai Cheng, Peng Cheng
Mesoscale and Nanoscale Physics · Cross-listed from physics.app-ph
Influence of magnetic fields on the performance of spin-orbit torque magnetic random-access memory
Jiaxin Liu, Yuqing Zhou, Guoyi Shi et al. (+1 more)
Spin-orbit torque
magnetic random-access
memory (SOT-MRAM) offers high speed, ultrahigh endurance, and compatibility with advanced
semiconductor processes, making it a promising candidate for next-generation nonvolatile
memory. However, intrinsic bias fields in
magnetic tunnel junctions (MTJs), originating from reference-layer stray fields and interlayer coupling, cause asymmetric critical switching currents and increased energy consumption. Existing compensation approaches usually introduce additional
magnetic layers into the MTJ stack, which increases fabrication complexity and limits wafer-scale integration. Here, we propose a bias-compensation strategy without modifying the MTJ stack by engineering local stray
magnetic fields through
magnetic filling materials in vertical interconnect access (VIA) channels during the back-end-of-line process. Micromagnetic simulations show that the proposed
magnetic filling layer can provide the required auxiliary field for deterministic switching and significantly suppress write-current asymmetry. By optimizing the MTJ position relative to the
magnetic filling structure, the write-current bias ratio is reduced from 21.6% in the conventional design to 1.3%. The approach is also applicable to in-plane
magnetic anisotropy SOT-MTJs, reducing the bias ratio from 19.8% to -0.2%. Scaling analysis further demonstrates that the compensation effect remains effective when the device size is reduced to 20% of the original dimension (MTJ diameter approximately 10 nm), indicating its potential for high-density SOT-MRAM integration.
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All authors Jiaxin Liu, Yuqing Zhou, Guoyi Shi, Kaiming Cai
Mesoscale and Nanoscale Physics
Enhancing charge stability of Ge quantum well heterostructures via SiGe layer composition engineering
Ding-Ming Huang, Jun-Hang Liu, Han Gao et al. (+10 more)
Composition modulation is a powerful technique for designing materials with tailored properties, fueling the development of advanced
semiconductor devices. In this work, we have implemented this technique into Ge
quantum well
heterostructures, offering a promising avenue to address the critical challenge of charge stability in
spin qubit devices. Harnessing the atomic-scale precision of molecular beam epitaxy, we have engineered the band structure of the SiGe top barrier via graded composition modulation, thereby reducing charge accumulation states at the SiGe-dielectric interface and strengthening the effective confinement to the hole gases in the Ge
quantum wells. The enhanced charge stability of composition-modulated SiGe/Ge
quantum well
heterostructures is confirmed in Hall devices, featuring an enlarged stable gate voltage range. We have further fabricated
quantum dot devices from the composition-modulated SiGe/Ge
quantum well
heterostructures and observed remarkably low charge noise with an averaged amplitude of $0.46\,\mathrm{\mu eV}/\mathrm{\sqrt{Hz}}$ at $1\,\mathrm{Hz}$—the lowest reported value for Ge
quantum wells grown on silicon. This exceptional charge stability of the
quantum dots persists in the few-hole regime, with no observable voltage drift over $\sim$hours. With reduced charge noise and enhanced energy stability, composition-modulated SiGe/Ge
heterostructures exhibit significant potential for applications in building high-performance
quantum devices, including
spin qubits with a long coherence time.
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All authors Ding-Ming Huang, Jun-Hang Liu, Han Gao, Jie-Yin Zhang, Jian-Huan Wang, Fang-Ze Liu, Xin-Yu Zhou, Yi Luo, Bin-Xiao Fu, Xiao-Fei Liu, Ji-Yin Wang, Jian-Jun Zhang, H. Q. Xu
Mesoscale and Nanoscale Physics
Fingerprints of Excitonic Collective Modes in the Two-Dimensional Electron Gas
Jakob Wolff, Silvana Botti, Lucia Reining et al. (+1 more)
The two-dimensional homogeneous electron gas (2D HEG) is a prototype system of fundamental interest that can be realized experimentally over a wide range of densities. Here, we investigate its collective charge excitations at low densities, using time-dependent density functional theory. We demonstrate that, beyond traditional plasmons, new collective excitonic modes emerge for a Wigner-Seitz radius larger than $r_{\mathrm s} \approx 1$. These excitonic modes leave characteristic fingerprints in experimentally accessible quantities, namely, asymmetric peak structures in the loss function and very strong Friedel-like oscillations in the static linear density response that increase when the system approaches a regime of instability. Indeed, at low enough densities the collective modes cross the zero energy axis, indicating an instability of the paramagnetic 2D HEG towards the formation of a charge-density-wave phase with excitonic origin. These findings provide valuable insights for the experimental detection of excitonic collective modes in tunable 2D electron systems and contribute to the fundamental understanding of many-body effects in low-density electron gases.
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All authors Jakob Wolff, Silvana Botti, Lucia Reining, Matteo Gatti
Mesoscale and Nanoscale Physics
In-plane magnetic field control of anomalous Hall response enabled by magnetic anisotropy engineering
J. C. Rodriguez E., E. De Biasi, J. I. Facio et al. (+7 more)
Engineering
magnetic anisotropy provides a powerful route to control magnetization orientation and unlock emerging functionalities in opto-spintronic and current-driven devices. Beyond its role in magnetization reversal, the effective anisotropy can strongly influence the magnetotransport response, offering an additional degree of freedom to tune new device functionalities. In this work, we report a magnetotransport study of a ferrimagnetic [Tb/Co]$_{\times 5}$ multilayer grown with a Tb thickness gradient, whose wedge-shaped tilts the uniaxial anisotropy axis slightly away from the film normal. Anomalous Hall resistivity measurements from 80 K to 300 K reveal a
spin reorientation transition, while the angular dependence of the magnetotransport responses exposes the crucial role of the tilted anisotropy. A simplified macrospin model reproduces the full angular response across the transition and shows that the observed anomalous Hall effect when the in-plane
magnetic field is applied originates from the tilt of the uniaxial anisotropy axis, which supplies a built-in
symmetry-breaking mechanism, enabling in-plane field control over the out-of-plane anomalous Hall response, sign included. These findings establish tilted
magnetic anisotropy as a promising route toward Hall effect-based sensor applications and highlight Tb/Co multilayers as a versatile platform for anisotropy-engineered spintronic devices.
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All authors J. C. Rodriguez E., E. De Biasi, J. I. Facio, D. Salomoni, S. Auffret, R. C. Sousa, I. L. Prejbeanu, A. Bruchhausen, J. Curiale, L. Avilés-Félix
Mesoscale and Nanoscale Physics
A substrate booster for P-type 2D ferromagnetic semiconductor
Hai Wang, Woye Pei, Ridong Cong et al. (+14 more)
Spin transistors with its both charge and
spin properties tuned via electrostatic gating are believed capable for widespread use, which however have proven challenging due to the extreme rareness of their physical base –
magnetic semiconductors. The latter are limited within very few systems including diluted
magnetic semiconductors (DMS) and two-dimensional ferromagnetic
semiconductors (2D-FMS), and known to suffer from inadequate gate-tunability of their electric and/or
magnetic properties. Here, we show a substrate engineering paradigm by interfacing few-layered Cr$_{2}$Ge$_{2}$Te$_{6}$ (FL-CGT) with an antiferromagnetic
insulator CrOCl. Owing to the subtle interfacial charge transfer couplings, CGT can be drastically turned from an ambipolar
semiconductor into a high performance P-type
semiconductor. When cooled below the Curie temperature, the ON-OFF ratio in such substrate-boosted FMS field-effect transistor (FET) reaches 10$^{5}$ with its coercive field $H_{c}$ of
magnetic hysteresis loop tunable by a factor of more than 200$\%$, enabling {gate-assisted
magnetic switching in the prototype semiconducting
spin transistor architecture}. A crossover from critical power-law scaling to a dual power-law behaviour under heavy hole doping was further observed. Our findings {signify} an efficient interfacial charge transfer and electrically modulated
magnetic anisotropy energy supported by calculations. This high performance P-type FMS-FET system suggests that active substrate-boosting paradigm might be a powerful path for the investigation of future gate-tunable spintronic devices.
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All authors Hai Wang, Woye Pei, Ridong Cong, Xiaoyan Liu, Yuping Tian, Kenji Watanabe, Teng Yang, Takashi Taniguchi, Xiangru Kong, Weijiang Gong, Guowei Zhou, Xiaoxi Li, Hanwen Wang, Jixuan Wu, Jiezhi Chen, Xiaohong Xu, Tongyao Zhang
Mesoscale and Nanoscale Physics
Emergence of spin-orbit coupling among spin, atomic orbital, and Bloch dynamics in Janus double-transition-metal MXenes
Tetsuro Habe
We found a
spin-orbit coupling to cause a simultaneous correlation among three degrees of freedom, the electronic
spin, orbital, and Bloch dynamics in an investigation into the electronic structure of Janus double-transition-metal MXenes, Mo$_2$HfC$_2$OS and W$_2$HfC$_2$OS. In this paper, it is also revealed that the
spin-orbit coupling causes a staggered
spin configuration with a trigonal pattern around the $\Gamma$ point near the insulating gap. We developed a reduced
Hamiltonian describing the electronic states and show that the
spin-orbit coupling cannot be equated with conventional forms for a single electron in solids, LS, Rashba, and Dresselhaus couplings, even in the approximation under the low-energy and small wave number condition. Because of the intrinsic shape of the conduction band, a trigonally alternating
spin-momentum locking emerges with the
spin axis perpendicular to the layer plane. The theoretical analysis shows that these Janus materials can provide a platform for exploring the
spin-related phenomena due to the trigonal
spin-momentum locking other than Rashba and Dresselhaus types.
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