Electrons in heavy group-IV monolayers, including silicene, germanene, and stanene, have the ability to exhibit rich physics due to the compatibility of the spin and valley degrees of freedom. We propose here that a valley-mediated giant Seebeck magnetoresistance (MR) effect, triggered and controlled by an interlayer electric field $E_z$, can be engineered near room temperature in a ferromagnetic/antiferromagnetic (FM/AFM) junction based on heavy group-IV monolayers, where the FM and AFM fields can be induced by the proximity effect, and $E_z$ is locally applied in the AFM region. Attributed to the specific tunneling mechanism of spin-valley matching, the high thermal MR state is dominated by a Seebeck state with nearly pure spin current (no charge current) from one valley under $E_z=0$, while the low thermal MR state is dominated by a spin filter state from the other valley under $E_z\ne 0$. We also demonstrate that such a giant Seebeck MR effect is robust against the small perturbation of the Fermi level, and is sensitive to $E_z$ by changing the electron-hole transport symmetry and tuning the bands. Further calculations indicate that the Seebeck MR effect is relatively too weak in other magnetic junctions, typically the FM/$E_z$ and AFM/$E_z$ junctions, even when the pure-spin-current state is present. These findings may pave the way for heavy group-IV monolayers in developing valley-assisted thermomagnetic storing and reading technologies in future spin caloritronic devices.

We present a study of the thermal transport in thin single crystals of iron-intercalated titanium disulphide, ${\mathrm{Fe}}_x{\mathrm{TiS}}_2$, for $0\le x\le 0.20$. We determine the distribution of intercalants using high-resolution crystallographic and magnetic measurements, confirming the insertion of Fe without long-range ordering. We find that iron intercalation perturbs the lattice very little, and suppresses the tendency of ${\mathrm{TiS}}_2$ to self-intercalate with excess Ti. We observe trends in the thermal conductivity that are compatible with our ab initio calculations of thermal transport in perfectly stoichiometric ${\mathrm{TiS}}_2$.

We present the theory of many-body corrections to cyclotron transition energies in graphene in strong magnetic field due to Coulomb interaction, considered in terms of the renormalized Fermi velocity. A particular emphasis is made on the recent experiments where detailed dependencies of this velocity on the Landau level filling factor for individual transitions were measured. Taking into account the many-body exchange, excitonic corrections and interaction screening in the static random-phase approximation, we successfully explained the main features of the experimental data, in particular that the Fermi velocities have plateaus when the 0th Landau level is partially filled and rapidly decrease at higher carrier densities due to enhancement of the screening. We also explained the features of the nonmonotonous filling-factor dependence of the Fermi velocity observed in the earlier cyclotron resonance experiment with disordered graphene by taking into account the disorder-induced Landau level broadening.

Key advances in the generation and shaping of spatially structured photonic fields in both the near and far fields render possible the control of the duration, the phase, and the polarization state of the field distributions. For instance, optical vortices having a structured phase are nowadays routinely generated and exploited for a range of applications. While the light-matter interaction with optical vortices have been well studied, the distinctive features of the interaction of quantum matter with vector beams, meaning fields with spatially inhomogeneous polarization states, have yet to be explored in full detail, which is done here. We analyze the response of atomic and low-dimensional quantum structures to irradiation with radially or azimuthally polarized cylindrical vector beams. Striking differences from vortex beams are found: Radially polarized vector beams drive radially breathing charge-density oscillations via electric-type quantum transitions. Azimuthally polarized vector beams do not affect the charge at all but trigger, via a magnetic vector potential, a dynamic Aharonov-Bohm effect, meaning a vector-potential-driven oscillating magnetic moment. In contrast to vortex beams, no unidirectional currents are generated. Atoms driven by a radially polarized vector beam exhibit angular-momentum-conserving quadrupole transitions tunable by a static magnetic field, while when excited with an azimuthally polarized beam, different final-state magnetic sublevels can be accessed.

We present a generic and systematic approach for constructing $D-\text{dimensional}$ lattice models with exactly solvable $d-\text{dimensional}$ boundary states localized to corners, edges, hinges, and surfaces. These solvable models represent a class of “sweet spots” in the space of possible tight-binding models—the exact solutions remain valid for any tight-binding parameters as long as they obey simple locality conditions that are manifest in the underlying lattice structure. Consequently, our models capture the physics of both (higher order) topological and nontopological phases as well as the transitions between them in a particularly illuminating and transparent manner.

While the Bloch spectrum of translationally invariant noninteracting lattice models is trivially obtained by a Fourier transformation, diagonalizing the same problem in the presence of open boundary conditions is typically only possible numerically or in idealized limits. Here we present exact analytic solutions for the boundary states in a number of lattice models of current interest, including nodal-line semimetals on a hyperhoneycomb lattice, spin-orbit coupled graphene, and three-dimensional topological insulators on a diamond lattice, for which no previous exact finite-size solutions are available in the literature. Furthermore, we identify spectral mirror symmetry as the key criterium for analytically obtaining the entire (bulk and boundary) spectrum as well as the concomitant eigenstates, and exemplify this for Chern and ${\mathcal{Z}}_2$ insulators with open boundaries of codimension one. In the case of the two-dimensional Lieb lattice, we extend this further and show how to analytically obtain the entire spectrum in the presence of open boundaries in both directions, where it has a clear interpretation in terms of bulk, edge, and corner states.

We present a detailed analysis of the temperature dependence of the interlayer shear mode that reveals a variation of the four-phonon scattering coupling with isotopic mass. Phonon anharmonic decay and isotopic disorder effects are very weak for the low-energy interlayer shear mode, and the overall temperature dependence is mainly governed by the thermal lattice expansion. This allows us to observe systematic differences in the temperature dependence of the low-energy mode with the isotopic mass that are related to the four-phonon scattering contribution in quartic anharmonicity. The change in the quartic anharmonicity coefficient between the isotopically pure samples is larger than would be purely expected from the mass variation.

Magnetic frustration and disorder are key ingredients to prevent the onset of magnetic order. In the disordered hexagonal double perovskite ${\mathrm{BaTi}}_{1/2}{\mathrm{Mn}}_{1/2}{\mathrm{O}}_3$, ${\mathrm{Mn}}^{4+}$ cations, with $S=3/2$ spins, can either form highly correlated states of magnetic trimers or dimers or remain as weakly interacting orphan spins. At low temperature ($T$), the dimer response is negligible, and magnetism is dominated by the trimers and orphans. To explore the role of magnetic frustration, disorder and possibly of quantum fluctuations, the low-$T$ magnetic properties of the remaining magnetic degrees of freedom of ${\mathrm{BaTi}}_{1/2}{\mathrm{Mn}}_{1/2}{\mathrm{O}}_3$ are investigated. Heat-capacity data and magnetic susceptibility display no evidence for a phase transition to a magnetically ordered phase but indicate the formation of a correlated spin state. The low-temperature spin dynamics of this state is then explored by $\mu \mathrm{SR}$ experiments. The zero-field ${\mu }^{+}$ relaxation rate data show no static magnetism down to $T=19$ mK and longitudinal field experiments support as well that dynamic magnetism persists at $\mathrm{low}T$. Our results are interpreted in terms of a spin-glass state which stems from a disordered lattice of orphans spins and trimers. A spin liquid state in ${\mathrm{BaTi}}_{1/2}{\mathrm{Mn}}_{1/2}{\mathrm{O}}_3$, however, is not excluded and is also discussed.

We report on the magnetic phase diagram of the distorted kagome-lattice antiferromagnet ${\mathrm{U}}_3{\mathrm{Ru}}_4{\mathrm{Al}}_{12}$ determined through measurements of magnetic and elastic properties. For field applied along the [100] and [120] axes of the hexagonal crystal structure, we find pronounced anomalies in the magnetization and elastic moduli that signal the existence of unknown magnetic phases. Our crystal-electric-field (CEF) analysis evidences interlevel quadrupolar interactions between the ground-state singlet and the first excited doublet. These interactions lead to a large softening of the shear elastic modulus $C_{44}$. The large number of phases and pronounced elastic softening suggest that geometric frustrations and CEF effects play an important role in the physical properties of ${\mathrm{U}}_3{\mathrm{Ru}}_4{\mathrm{Al}}_{12}$.

We propose to realize one-dimensional topological phases protected by $\mathrm{SU}\left(N\right)$ symmetry using alkali or alkaline-earth atoms loaded into a bichromatic optical lattice. We derive a realistic model for this system and investigate it theoretically. Depending on the parity of $N$, two different classes of symmetry-protected topological (SPT) phases are stabilized at half-filling for physical parameters of the model. For even $N$, the celebrated spin-1 Haldane phase and its generalization to $\mathrm{SU}\left(N\right)$ are obtained with no local symmetry breaking. In stark contrast, at least for $N=3$, a new class of SPT phases, dubbed chiral Haldane phases, that spontaneously break inversion symmetry, emerges with a twofold ground-state degeneracy. The latter ground states with open-boundary conditions are characterized by different left and right boundary spins, which are related by conjugation. Our results show that topological phases are within close reach of the latest experiments on cold fermions in optical lattices.

Recent tunneling conductance measurements of Majorana nanowires show a strong variation in the magnetic-field dependence of the superconducting gap among different devices. Here, we theoretically study the magnetic field dependence of the gap closing feature and establish that the degree of convexity (or concavity) of the gap closing as a function of Zeeman field can provide critical constraints on the underlying microscopic parameters of the semiconductor-superconductor hybrid system model. Specifically, we show that the gap closing feature is entirely concave only for strong spin-orbit coupling strength relative to the chemical potential. Additionally, the nonlinearity (i.e., concavity or convexity) of the gap closing as a function of magnetic field complicates the simple assignment of a constant effective $g$-factor to the states in the Majorana nanowire. We develop a procedure to estimate the effective $g$-factor from recent experimental data that accounts for the nonlinear gap closing, resulting from the interplay between chemical potential and spin-orbit coupling. Thus, measurements of the magnetic field dependence of the gap closure on the trivial side of the topological quantum phase transition can provide useful information on parameters that are critical to the theoretical modeling of Majorana nanowires.

We theoretically consider a quantum magnetic impurity coupled to a superconductor and obtain the local density of states at the position of the impurity taking into account the effect of spin fluctuations and single-ion magnetic anisotropy. We particularly focus on the spectrum of subgap Yu-Shiba-Rusinov (YSR or Shiba) states induced by a quantum impurity with easy- or hard-axis uniaxial anisotropy. Although this is a relevant experimental situation in, e.g., magnetic adatoms on the surface of clean metals, it is customary that theoretical descriptions assume a classical-spin approximation which is not able to account for single-ion anisotropy and other quantum effects. Here, quantum fluctuations of the spin are taken into account in the equations of motion of the electronic Green's function in the weak-coupling limit and considerably modify the energy of the Shiba states compared to the classical-spin approximation. Our results point towards the importance of incorporating quantum fluctuations and anisotropy effects for the correct interpretation of scanning tunneling microscopy (STM) experiments.

${\mathrm{U}}_2\mathrm{N}{\mathrm{i}}_2\mathrm{Sn}$ is a member of a large family of intermetallic compounds with the tetragonal $\mathrm{M}{\mathrm{o}}_2\mathrm{Fe}{\mathrm{B}}_2$ crystal structure. It orders antiferromagnetically at 25 K with propagation vector $q=(0,0,\frac{1}{2})$. Magnetization, magnetoacoustic, and neutron-diffraction experiments on a single crystal provide evidence that the uranium moments align parallel to the $c$ axis with the anisotropy energy of ≈170 K, indicating that ${\mathrm{U}}_2\mathrm{N}{\mathrm{i}}_2\mathrm{Sn}$ can be classified as an Ising system. The results are at variance with previous studies on polycrystals, which indicated different magnetic structure, and which were incompatible with the $5f\text{−}5f$ two-ion anisotropy model dominant in most U band systems. High-field magnetization studies exhibit a weak linear response for fields along the basal plane up to the highest field applied (60 T), while the $c$-axis magnetization curve exhibits three metamagnetic transitions at approximately 30, 39, and 50 T. The U magnetic moments of $0.87{\mu }_{\mathrm{B}}$, the low magnetic entropy, and the enhanced Sommerfeld coefficient $\gamma =187\mathrm{mJ}/\mathrm{mol}\mathrm{f}.\mathrm{u}.{\mathrm{K}}^2$ suggest that ${\mathrm{U}}_2\mathrm{N}{\mathrm{i}}_2\mathrm{Sn}$ can be classified as an itinerant antiferromagnet with strong electron-electron correlations.

Magnetic excitations of the recently discovered frustrated spin-1/2 two-leg ladder system ${\mathrm{Li}}_2{\mathrm{Cu}}_2\mathrm{O}{\left({\mathrm{SO}}_4\right)}_2$ are investigated using inelastic neutron scattering, magnetic susceptibility, and infrared absorption measurements. Despite the presence of a magnetic dimerization concomitant with the tetragonal-to-triclinic structural distortion occurring below 125 K, neutron scattering experiments reveal the presence of dispersive triplet excitations above a spin gap of $\mathrm{\Delta }=10.6\text{meV}$ at $1.5\text{K}$, a value consistent with the estimates extracted from magnetic susceptibility. The likely detection of these spin excitations in infrared spectroscopy is explained by invoking a dynamic Dzyaloshinskii-Moriya mechanism in which light is coupled to the dimer singlet-to-triplet transition through an optical phonon. These results are qualitatively explained by exact diagonalization and higher-order perturbation calculations carried out on the basis of the dimerized spin Hamiltonian derived from first principles.

We theoretically investigate qualitative features of the field-temperature ($H\text{−}T$) phase diagram of superconductors with strong attractive interaction lying in the BCS-BEC crossover regime. Starting with a simple attractive Hubbard model, we estimate three kinds of characteristic fields, i.e., the pair-formation field $H^{*}$, the vortex-liquid-formation field $H_{\mathrm{c}2}$, and the vortex-lattice-formation field $H_{\mathrm{melt}}$. The region between $H_{\mathrm{c}2}$ and $H_{\mathrm{melt}}$, as well as that between $H_{\mathrm{c}2}$ and $H^{*}$, is found to be enlarged as the interaction is stronger. In other words, a strong attractive interaction can stabilize both the vortex-liquid and preformed-pair regions. We also point out the expected particle-density dependence of the $H\text{−}T$ phase diagram.

The solution of complex many-body lattice models can often be found by defining an energy functional of the relevant density of the problem. For instance, in the case of the Hubbard model the spin-resolved site occupation is enough to describe the system's total energy. Similarly to standard density functional theory, however, the exact functional is unknown, and suitable approximations need to be formulated. By using a deep-learning neural network trained on exact-diagonalization results, we demonstrate that one can construct an exact functional for the Hubbard model. In particular, we show that the neural network returns a ground-state energy numerically indistinguishable from that obtained by exact diagonalization and, most importantly, that the functional satisfies the two Hohenberg-Kohn theorems: for a given ground-state density it yields the external potential, and it is fully variational in the site occupation.

Using scanning tunneling microscopy, we report the observation of moiré patterns (MPs) on van der Waals heterostructures comprised of various 2D allotropes of bismuth and antimony grown on highly ordered pyrolytic graphite and ${\mathrm{MoS}}_2$. The spatial periods of the MPs range from $\lambda \sim 1$ to $\sim 10$ nm. For all the reported cases ($\alpha$-bismuthene, $\alpha$-antimonene, $\beta$-antimonene, and monolayer bismuthene), we model the observations using a simple superposition model (SSM). Where possible, the results obtained from the SSM are compared to analytical prediction. MPs emerging from mixed symmetry stacking (hexagonal on rectangular) are explained without requiring commensuration of the layers.

We study the transport properties of a monolayer transition-metal dichalcogenide (TMDC) ribbon subject to a time-periodic circularly polarized laser field. First, we calculate the quasienergy spectrum within the framework of the Floquet theory and analyze the nontrivial topology of the Floquet bulk gaps. The latter is revealed by the presence of chiral edge states inside the bulk gaps in finite samples, in agreement with the calculation of the appropriate winding numbers as a function of both the energy and the amplitude of the laser field. The effect of the time-dependent perturbation on the equilibrium edge states is also analyzed. Finally, we calculate the two-terminal conductance and discuss how the above mentioned effects manifest on it. In particular, besides the expected suppression of the bulk conductance and the emergence of edge transport at the Floquet gaps, we find that there is an asymmetry between left and right transmission coefficients (in the zigzag case), leading to pumping effects. In addition, we found that the laser field can lead to a complete switch off of the linear conductance when the latter is dominated by the equilibrium edge states.

Fractional Chern insulators (FCIs) realized in fractional quantum Hall systems subject to a periodic potential are topological phases of matter for which space group symmetries play an important role. In particular, lattice dislocations in an FCI can host non-Abelian topological defects, known as genons. Genons can increase the ground-state degeneracy of the system and are thus potentially useful for topological quantum computing. In this work, we study FCI edges and how they can be used to detect genons. We find that translation symmetry can impose a quantized momentum difference between the edge electrons of a partially filled Chern band. We propose layer-resolved lattice contacts, which utilize this momentum difference to selectively contact a particular FCI edge electron. The relative current between FCI edge electrons can then be used to detect the presence of genons in the bulk FCI. Recent experiments have demonstrated graphene is a viable platform to study FCI physics. We describe how the lattice contacts proposed here could be implemented in graphene subject to an artificial lattice, thereby outlining a path forward for experimental dectection of non-Abelian topological defects.

Spin-orbit entangled magnetic dipoles, often referred to as pseudospins, provide a new avenue to explore novel magnetism inconceivable in the weak spin-orbit coupling limit, but the nature of their low-energy interactions remains to be understood. We present a comprehensive study of the static magnetism and low-energy pseudospin dynamics in the archetypal spin-orbit Mott insulator ${\mathrm{Sr}}_2{\mathrm{IrO}}_4$. We find that in order to understand even basic magnetization measurements, a formerly overlooked in-plane anisotropy is fundamental. In addition to magnetometry, we use neutron diffraction, inelastic neutron scattering, and resonant elastic and inelastic x-ray scattering to identify and quantify the interactions that determine the global symmetry of the system and govern the linear responses of pseudospins to external magnetic fields and their low-energy dynamics. We find that a pseudospin-only Hamiltonian is insufficient for an accurate description of the magnetism in ${\mathrm{Sr}}_2{\mathrm{IrO}}_4$ and that pseudospin-lattice coupling is essential. This finding should be generally applicable to other pseudospin systems with sizable orbital moments sensitive to anisotropic crystalline environments.