Papers

Altermagnetic $\alpha$-MnTe exhibits huge anomalous Hall conductivity (AHC) up to room-temperature together with weak ferromagnetism arising from spin and orbital polarizations. We clarify the origin of the large value of the AHC by identifying two sets of distinct symmetry-enforced nodal lines in the valence bands with Mn character, located at $k_z=0$ and $k_z=\frac{\pi}{c}$, protected by mirror symmetry $M_z$ and glide symmetry $G_z = \{M_z\,|\,0,0,\tfrac{c}{2}\}$, respectively. Both nodal lines are energy-dependent with an approximate C$_6$ symmetry, which is reduced to an exact C$_2$ symmetry due to the presence of the Néel vector. The highest valence band exhibits a Mexican-hat dispersion, whereas the second-highest valence band exhibits an inverted Mexican-hat dispersion, with nodal lines at the crossing between the two bands. Within first-principles accuracy, we demonstrate that these nodal lines give rise to the large AHC observed experimentally and exhibit a strong interplay with the weak ferromagnetism. We further show that even a small spin canting strongly modifies the nodal lines and the AHC, making them both magnetically tunable. By disentangling the altermagnetic and ferromagnetic contributions to the AHC, the altermagnetic contribution dominates at small canting angles, while the ferromagnetic contribution becomes sizeable for larger values. Using linear dichroism in angle-resolved photoemission spectroscopy, we show a signature of the nodal line at the border of the Brillouin zone.

Many-body localization (MBL) is a dynamical phenomenon that describes the non-ergodicity of isolated quantum many-body systems. In contrast to thermalization, this phenomenon leads to a long-lived memory of initial states of local systems and slow growth of entanglement. In this work, we study Stark MBL in a 12-qubit correlated fermionic system described by the one-dimensional Fermi-Hubbard model using Hamiltonian simulation on an IBM superconducting qubit quantum computer. To enable such a computation on current-day noisy hardware, we combine a series of compilation steps, including the use of the spin-resolved Jordan-Wigner transformation, employing SWAP networks, and integrating a tensor-network-based quantum circuit optimization routine on top of a standard circuit optimization pipeline. As a result, there is approximately an 88$\%$ and 87$\%$ reduction in two-qubit gate count and circuit depth, respectively. Through such simulations of the real-time dynamics using Trotterized quantum circuits, we exhibit a crossover from thermalizing dynamics of the system at a weak tilt of the field to a strongly localized behavior at large tilt with short evolution times. We also benchmark our obtained results with respect to those from exact simulations.

We study the migration of a single excitation excited at a structural element of a finite molecular segment, which is a part of a long biomolecular chain. The excitation cannot leave the segment and is locally coupled to thermal vibrations of the lattice, forming a self-trapped state corresponding to a nonadiabatic polaron. The time-dependent probability distribution of finding the excitation at the nodes of the segment is calculated, with particular emphasis on the role of the initial excitation position. A formal analogy is observed between the present model and continuous-time quantum walk models on finite chains with reflecting boundaries. The results reveal an asymmetry in the probability distribution for nodes symmetrically positioned with respect to the initially excited site, which arises solely from the asymmetric placement of the initial excitation within the finite segment. The only exception occurs when the initially excited node is located at the center of the segment, where the probability distribution becomes symmetric. The complex interference pattern and the absence of well-defined revivals stem from the non-equidistant spectrum of mode frequencies, leading to progressive dephasing of the constituent modes. As a result, the initially well-localized probability maximum fragments into one dominant maximum accompanied by several secondary maxima of lower intensity. These findings highlight the importance of boundary conditions and initial-state geometry in controlling quantum transport in finite molecular systems.