Spin-orbit interactions lead to a large spin splitting of the valence band maximum at K in ${\mathrm{MoSe}}_2$ monolayer. However, on stacking a second layer of ${\mathrm{MoSe}}_2$ in the same manner (2H) as found in the bulk, one finds that there is no net spin splitting. This has been attributed to the presence of inversion symmetry in the structures. An obvious route to making the bilayers useful for exploring the coupled spin and valley physics is through breaking inversion symmetry. We examined this by rotating the top layer by an angle $\theta$ with respect to the lower layer. We find that, for some angles of rotation, the spin splitting vanishes, even though there is no inversion symmetry in these structures. In this work we demonstrate and explain a mechanism for a vanishing of the spin splitting in these systems and provide a route to determining the presence/absence of spin splitting at an arbitrary angle of rotation.

Two-level systems (TLS) in amorphous materials limit coherence times of a number of solid-state quantum devices. Interactions between TLS become prominent below 100 mK, but the coupling mechanism and statistical properties are still unclear. Here we determine the homogeneous linewidth of ytterbium ions $\left({\mathrm{Yb}}^{3+}\right)$ in silica glass at 10–80 mK by using photon echo techniques as a probe of TLS. First, the homogeneous linewidth can be reduced by applying a magnetic field of 0.3 T. This effect is due to reduced magnetic interactions between adjacent ${\mathrm{Yb}}^{3+}$. Secondly, we observe saturation of the linewidth below 50 mK to a level of approximately 30 kHz, which is much larger than the lifetime-limited value of 0.2 kHz. This saturation behavior is in conflict with the coupling to independent TLS. We show that this effect can be explained by coherently coupled TLS.

By merging single-site typical medium theory with density-functional theory, we introduce a self-consistent framework for electronic-structure calculations of materials with substitutional disorder which takes into account Anderson localization. The scheme and details of the implementation are presented and applied to the hypothetical alloy ${\mathrm{Li}}_c{\mathrm{Be}}_{1-c}$, and the results are compared with those obtained with the coherent potential approximation. Furthermore, we demonstrate that Anderson localization suppresses ferromagnetic order for a very low concentration of (i) carbon impurities substituting oxygen in ${\mathrm{MgO}}_{1-c}{\mathrm{C}}_c$ and (ii) manganese impurities substituting magnesium in ${\mathrm{Mg}}_{1-c}{\mathrm{Mn}}_c\mathrm{O}$ for the low-spin magnetic configuration.

Ferromagnetic exchange stiffness is a key to describe the robustness of a ferromagnetic interaction. Because the experimental determination of the exchange stiffness constant $A$ continues to be elusive, especially at elevated temperatures, micromagnetic insights on the impact of thermal fluctuations on the ferromagnetic spin order have remained less tractable. In this study, the temperature dependence of $A$ is experimentally determined for a $\mathrm{N}{\mathrm{i}}_2\mathrm{M}{\mathrm{n}}_{0.8}\mathrm{I}{\mathrm{n}}_{1.2}$ single crystal by measuring the ${180}^{\circ }$ domain-wall width using off-axis electron holography and magnetocrystalline anisotropy. The theoretically expected power-law scaling with magnetization for $A$ is shown to be applicable only in the low-temperature region (approximately half of the Curie temperature), and $A$ starts to follow the power law with a significantly larger scaling exponent in the higher-temperature region. The decremental trends in the domain-wall width and the exchange length at elevated temperature seem to be attributed to this temperature dependence of $A$.

FeRh has a phase transition from an antiferromagnetic state (low temperature) to a ferromagnetic state (high temperature) at 360 K. Various explanations for this behavior have been proposed over the past 20 years. However, many of the mechanisms are inconsistent with all the data. Early models were Ising-like, but the large anisotropy fields necessary for this are not found in hysteresis curves. Using a four-spin Hamiltonian, we obtain a complete theoretical description of the field and temperature phase diagram and the magnetic properties for FeRh. The theoretical results are in good agreement with experiments. We use our approach to predict changes in behavior as a function of the thickness of an FeRh film. We find the four-spin Hamiltonian is particularly sensitive to the presence of a surface, with thinner films remaining ferromagnetic over a larger temperature range because the four-spin contribution to the energy (which favors the antiferromagnetic state) is smaller.

We demonstrated the electric field (EF) effect on exchange bias (EB) in a perpendicularly magnetized $\mathrm{Co}/\mathrm{Co}{\mathrm{O}}_x$ layered structure. The antiferromagnetic $\mathrm{Co}{\mathrm{O}}_x$ layer was formed by naturally oxidizing the Co surface. The modulation of the EB field and the coercivity by gate voltage application through a dielectric layer was clearly observed below the blocking temperature $T_{\mathrm{B}}$. The modulation ratio of the EB field exhibited strong temperature dependence, and it increased as the temperature approached $T_{\mathrm{B}}$. One possible cause of the EB modulation detailed herein is the modulation of the electronic state at the $\mathrm{Co}/\mathrm{Co}{\mathrm{O}}_x$ interface, which is fundamentally different from the case in which multiferroic antiferromagnets are used.

The diameter dependence of the thermal conductivity of nanowires is usually modeled using Matthiessen's rule, by putting the mean free path of phonons equal to the diameter $d$ of the nanowire. This results in a thermal conductivity $\kappa$ that decreases with decreasing $d$, due to the increase in boundary scattering. Recent molecular dynamics studies of heat transport in thin silicon nanowires have shown a nonmonotonic diameter dependence of $\kappa$, where a decrease with decreasing $d$ is followed by an increase to a value of $\kappa$ exceeding the bulk thermal conductivity. This increase of $\kappa$ was explained by an increase of the importance of hydrodynamic transport effects in the thinner wires, where the normal scattering by phonon-phonon interaction increases, but the Umklapp scattering decreases [Y. Zhou, X. Zhang, and M. Hu, Nano Lett. 17, 1269 (2017)]. Here, we study heat transport in thin nanowires of the compound semiconductor gallium-phosphide in the wurtzite crystal structure, using molecular dynamics simulations. A similar nonmonotonic $d$ dependence of $\kappa$ is found as in silicon nanowires, but with a minimum in $\kappa$ occurring at a much larger diameter of $d\approx 8\mathrm{nm}$ instead of 2–3 nm.

We perform a theoretical investigation of the magnetization switching in polycrystalline Ni nanoparticles induced by ultrashort pulses of surface acoustic waves via magnetoelastic interactions. In our numerical simulations, a Ni nanoparticle is modeled as an ellipsoidal disk deposited on a dielectric substrate. The in-plane external magnetic field breaks the symmetry and allows us to adjust the height of the energy barrier between two metastable magnetization states of the free-energy density and dramatically lower the amplitude of elastic strain pulses required for magnetization switching. The switching threshold is shown to depend on the duration of an acoustic pulse, the magnetic shape anisotropy of an elliptical nanoparticle, the amplitude of the external magnetic field, and the magnetostriction coefficient. We introduce the magnetoelastic switching diagram, allowing for the simultaneous visualization of the switching threshold and its characteristic timescale as a function of various physical parameters.

We compare the accuracy of two prime time evolution algorithms involving matrix product states—tDMRG (time-dependent density matrix renormalization group) and TDVP (time-dependent variational principle). The latter is supposed to be superior within a limited and fixed auxiliary space dimension. Surprisingly, we find that the performance of algorithms depends on the model considered. In particular, many-body localized systems as well as the crossover regions between localized and delocalized phases are better described by tDMRG, contrary to the delocalized regime where TDVP indeed outperforms tDMRG in terms of accuracy and reliability. As an example, we study many-body localization transition in a large size Heisenberg chain. We discuss drawbacks of previous estimates [Phys. Rev. B 98, 174202 (2018)] of the critical disorder strength for large systems.

Even though the Hubbard model is one of the most fundamental models of highly correlated electrons, analytical and numerical data describing its thermodynamics at nonzero magnetization are relatively scarce. We present a detailed investigation of the thermodynamic properties for the one-dimensional repulsive Hubbard model in the presence of an arbitrary magnetic field for all values of the filling fraction and temperatures as low as $T\sim 0.005t.$ Our analysis is based on the system of integral equations derived in the quantum transfer matrix framework. We determine the critical exponents of the quantum phase transitions and also provide analytical derivations for some of the universal functions characterizing the thermodynamics in the vicinities of the quantum critical points. Extensive numerical data for the specific heat, susceptibility, compressibility, and entropy are reported. The experimentally relevant double occupancy presents an interesting doubly nonmonotonic temperature dependence at intermediate values of the interaction strength and also at large repulsion and magnetic fields close to the critical value. The susceptibility in zero magnetic field has a logarithmic singularity at low temperatures for all filling factors similar to the behavior of the same quantity in the spin-1/2 isotropic Heisenberg model. We determine the density profiles for a harmonically trapped system and show that while the total density profile seems to depend mainly on the value of chemical potential at the center of the trap the distribution of phases in the inhomogeneous system changes dramatically as we increase the magnetic field.

We present single-crystal x-ray diffraction measurements, optical investigations, and electronic structure calculations for the organic charge-transfer salt ${\beta }^{\prime }$-(BEDT-TTF)${}_2{\mathrm{CF}}_3{\mathrm{CF}}_2{\mathrm{SO}}_3$ synthesized by electrocrystallization. Electronic structure calculations confirm the quasi-one-dimensional behavior of the compound and optical conductivity measurements reveal the dimer-Mott insulating nature of the system. The splitting of the charge-sensitive ${\nu }_2$ mode in Raman spectra demonstrates the onset of an interlayer charge-ordered phase below 25 K, also suggested by the crystal structure considerations. This transition is accompanied by clear signatures of a lattice distortion in the BEDT-TTF donor layer, as shown by a splitting of the vibrational ${\nu }_3$ mode in infrared spectra. At the same time, the sharp redshift of the ${\nu }_1$ mode involving the BEDT-TTF ethylene groups strongly suggests a significant modification of the hydrogen-type bonding present between the BEDT-TTF donor layer and the ${\mathrm{CF}}_3{\mathrm{CF}}_2{\mathrm{SO}}_3^{-}$ anion layer. These observations point to a subtle interplay of charge and lattice degrees of freedom at the phase transition.

Understanding exotic solids is a difficult task as interactions are often hidden by the symmetry of the system. Here, we study the electronic properties of a noncentrosymmetric solid, BiPd, which is a rare material exhibiting both superconductivity and the topological phase of matter. Employing high-resolution photoemission spectroscopy with photon energies ranging from the hard x-ray to extreme-ultraviolet regime, we show that hard x-ray spectroscopy alone is not enough to reveal surface-bulk differences in the electronic structure. We derive the escape depths close to the extreme surface sensitivity and find that the photon energies used for high-resolution photoemission measurements fall in the surface sensitive regime. In addition, we discover a deviation of the branching ratio of Bi core level features derived from conventional quantum theories of the core hole final states. Such a breakdown of the atomic description of the core level spectroscopy can be attributed to the absence of a center of symmetry and spin-orbit interactions.

For the Haldane phase, the magnetic field usually tends to drive the system into a topologically trivial phase. Here we report a novel reentrance of the Haldane phase at zero temperature in the spin-1 antiferromagnetic Heisenberg model on a sawtooth chain. A partial Haldane phase is induced by the magnetic field, which is the combination of the Haldane state in one sublattice and a ferromagnetically ordered state in the other sublattice. Such a partial Haldane phase should be protected by the bond centered inversion symmetry, and is a result of the zero-temperature entropy due to quantum fluctuations and geometrical frustration.