We study an isotropic Heisenberg spin-$\frac{1}{2}$ model on a trellis ladder which is composed of two $J_1-J_2$ zigzag ladders interacting through antiferromagnetic rung coupling $J_3$. $J_1$ and $J_2$ are ferromagnetic zigzag spin interaction between two legs and antiferromagnetic interaction along each leg of a zigzag ladder. A quantum phase diagram of this model is constructed using the density matrix renormalization group method and linearized spin-wave analysis. In the small $J_2$ limit, a short-range striped collinear phase is found in the presence of $J_3$, whereas, in the large $J_2/J_3$ limit, a noncollinear quasi-long-range phase is found. The system shows a short-range noncollinear state in the large $J_3$ limit. The short-range-order phase is the dominant feature of this phase diagram. We also show that the results obtained by DMRG and linearized spin-wave analysis show a similar phase boundary between the collinear striped and noncollinear short-range phases, and the collinear phase region shrinks with increasing $J_3$. We apply this model to understand the magnetic properties of ${\mathrm{CaV}}_2{\mathrm{O}}_5$ and also fit the experimental data of susceptibility and magnetization. We note that $J_3$ is a dominant interaction in this material, whereas $J_1$ and $J_2$ are approximately half of $J_3$. The variation of magnetic specific heat capacity as a function of temperature for various external magnetic fields is also predicted.

The elasto-optic effect, or photoelasticity, describes the linear change of dielectric tensor with applied strain and is a universal material property of insulators and semiconductors. Though the elasto-optic responses for solids can be computed directly from first principles (e.g., by using density functional perturbation theory) and measured experimentally, these methods do not provide sufficient insight into the governing microscopic physical principles of photoelasticity. In this work, we describe a microscopic first-principles analysis of photoelasticity in real space and apply it to investigate the elasto-optic responses of Si, diamond, NaCl, and MgO. By writing the random phase approximation (RPA) dielectric constant in the basis of maximally localized Wannier functions, we show that the strain-dependent change of dipole transitions between occupied and unoccupied Wannier functions are the main determinants of photoelasticity. By organizing the dipole transitions into spatially localized shells, we develop a “constrained sum” method that converges both the dielectric and photoelastic responses systematically and reveals a relatively long-ranged nature to these responses: one needs to sum up contributions of up to third neighbor shells to converge the elasto-optic coefficient with reasonable precision.

Terahertz radiation is demonstrated to generate phonon-induced resistance oscillations in two-dimensional electron gases. This effect dominates over microwave-induced resistance oscillations in the low terahertz range, but could not be detected in the gigahertz regime. Possible mechanisms for phonon generation are discussed.

We derived analytical results for the gapless edge states of two-dimensional topological insulators in the presence of electron–surface optical (SO) phonon interaction due to substrates. We followed an analytical algorithm, called the Lee-Low-Pines variational approximation in the conventional polaron theory, to examine the substrate-induced effects on both bulk and edge states of a two-dimensional topological insulator within the frame work of the Bernevig-Hughes-Zhang (BHZ) model. By implementing this algorithm, we propose a phonon-dressed BHZ Hamiltonian which allows one to investigate the effects of various substrates not only on bulk states but also on the associated gapless helical edge states (HESs). We found that both the bulk and HESs are significantly renormalized in the momentum space due to the substrate-related polaronic effects. The model we developed here clarifies which substrates favor the HESs of the quantum spin Hall system and which do not. Correspondingly, our work demonstrates that the substrate-related polaronic effects have a significant role in the emergence of HESs. In other words, we show that SO phonons due to substrates modify the electronic band topology of topological insulators together with the associated HESs, and therefore, they can be used to tune quantum phase transitions between topological insulators and nontopological ones.

Simulation-size effect in evaluating the melting temperature of material is studied systematically by combining thermodynamic integration (TI) based on first-principles molecular-dynamics (FPMD) simulations and machine learning. Since the numerical integration to determine the free energies of two different phases as a function of temperature is very time consuming, the FPMD-based TI method has only been applied to small systems, i.e., less than 100 atoms. To accelerate the numerical integration, we here construct an interatomic potential based on the artificial neural-network (ANN) method, which retains the first-principles accuracy at a significantly lower computational cost. The free energies of the solid and liquid phases of rubidium are accurately obtained by the ANN potential, where its weight parameters are optimized to reproduce FPMD results. The ANN results reveal a significant size dependence up to 500 atoms.

We have used the Boltzmann kinetic equation for the phonon distribution function to analyze the relaxation kinetics of the spin system of a ferromagnetic insulator (F) lying on a massive dielectric substrate with high thermal conductivity. Under periodic heating of the spin system, the relaxation depends on the thickness of the F layer and on the frequency of the thermal source $\omega$. When the thickness of the F layer is much greater than the phonon-magnon scattering length, the magnon temperature dependence on the frequency has two features related to specific characteristic times of the system. One of them determines the dependence in the low-frequency regime and is related to the average phonon escape time from the F layer to the substrate ${\tau }_{\mathrm{es}}$. In turn, the high-frequency behavior is determined by the magnon-phonon collisions time ${\tau }_{mp}$. From the latter, the time of phonon-magnon collisions ${\tau }_{pm}$ can be found. In contrast, the response of effectively thin F layers is characterized by just one feature, which is determined by the time ${\tau }_{mp}$. Thus, based on the obtained theoretical results, the times ${\tau }_{\mathrm{es}},{\tau }_{mp}$, and ${\tau }_{pm}$ can be deduced from experiments on the parametric excitation of spin waves by electromagnetic radiation modulated at frequency $\omega$.

By means of the plane-wave method, we study nonuniform, i.e., mode- and $\mathit{k}$-dependent, effects in the spin-wave spectrum of a two-dimensional bicomponent magnonic crystal. We use the crystal based on a hexagonal lattice squeezed in the direction of the external magnetic field wherein the squeezing applies to the lattice and the shape of inclusions. The squeezing changes both the demagnetizing field and the spatial confinement of the excitation, which may lead to the occurrence of an omnidirectional magnonic band gap. In particular, we study the role played by propagational effects, which allows us to explain the $\mathit{k}$-dependent softening of modes. The effects we found enabled us not only to design the width and position of magnonic band gaps, but also to plan their response to a change in the external magnetic field magnitude. This allows the reversible tuning of magnonic band gaps, and it shows that the studied structures are promising candidates for designing magnonic devices that are tunable during operation.

We report scanning tunneling microscopy results on 25- and 50-nm-thick films of superconducting TiN that show Andreev tunneling behavior at 0.5 K. At most locations on the topographically rough surfaces, we observe tip-sample current-voltage characteristics with a clear superconducting gap, as expected for superconductor-normal (S-N) tunneling through a low-transparency barrier, while in some places, we find a zero-voltage conductance peak, as expected for S-N Andreev tunneling through a highly transparent barrier. Fitting the Blonder-Tinkham-Klapwijk model to the conductance data allows an accurate extraction of the TiN superconducting gap $\mathrm{\Delta }$, by accounting for local variations in the tip-sample barrier height $Z$ and junction temperature $T$. From spatial maps of the model parameters, we find that both films show a strong inhomogeneity, with $\mathrm{\Delta }$ varying by as much as a factor of 2 from grain to grain. In the thicker film, however, correlations between $T,Z$, and $\mathrm{\Delta }$ suggest the grains are thermally isolated, perhaps due to internal stress. We discuss possible mechanisms that could produce these large correlated variations, including local heating and surface contamination, and consider some of the implications for devices made from such films.

We study the Berezinskii-Kosterlitz-Thouless mechanism for vortex-antivortex pair formation in two-dimensional superfluids for nonequilibrium condensates. Our numerical study is based on a classical field model for driven-dissipative quantum fluids that is applicable to polariton condensates. We investigate the critical noise needed to create vortex-antivortex pairs in the systems, starting from a state with uniform phase. The dependence of the critical noise on the nonequilibrium and energy relaxation parameters is analyzed in detail.

We develop an analytical model to describe the phonon dispersion relations of host-guest lattices with heavy guest atoms (rattlers). Crucially, the model also accounts for phonon damping arising from anharmonicity. The spectrum of low-energy states contains acousticlike and (soft) optical-like modes, which display the typical avoided crossing, and which can be derived analytically by considering the dynamical coupling between host lattice and guest rattlers. Inclusion of viscous anharmonic damping in the model allows us to compute the vibrational density of states (VDOS) and the specific heat, unveiling the presence of a boson peak (BP) linked to an anharmonicity-smeared van Hove singularity. Upon increasing the coupling strength between the host and the guest dynamics, and by decreasing the energy of the soft optical modes, the BP anomaly becomes stronger and it moves towards lower frequencies. Moreover, we find a robust linear correlation between the BP frequency and the energy of the soft optical-like modes. This framework provides a useful model for tuning the thermal properties of host-guest lattices by controlling the VDOS, which is crucial for optimizing thermal conductivity and hence the energy conversion efficiency in these materials.

We present a scheme to control the spin-exchange interactions by manipulating the orbital degrees of freedom using a periodic drive. We discuss two different protocols for orbital Floquet engineering. In one case, a periodic drive modifies the properties of the ligand orbitals which mediate magnetic interactions between transition-metal ions. In the other case, we consider drive-induced mixing of $d$ orbitals on each magnetic ion. We first find that an AC Stark shift of orbitals induces a change comparable to that induced from photoinduced hopping schemes, but expands the applicable frequency ranges. Next, we find that radiatively induced coherent vibrations provide a realistic path for Floquet orbital engineering with short pulses of electric fields weaker than 0.5 V/Å producing $5\%–10\%$ changes in the magnetic coupling of Mott insulators such as the rare-earth titanates.

We investigate the ultrafast magnetization dynamics of FePt in the $\mathrm{L}{1}_0$ phase after an optical heating pulse, as used in heat-assisted magnetic recording. We compare continuous and nano-granular thin films and emphasize the impact of the finite size on the remagnetization dynamics. The remagnetization speeds up significantly with increasing external magnetic field only for the continuous film, where domain-wall motion governs the dynamics. The ultrafast remagnetization dynamics in the continuous film are only dominated by heat transport in the regime of high magnetic fields, whereas the timescale required for cooling is prevalent in the granular film for all magnetic field strengths. These findings highlight the necessary conditions for studying the intrinsic heat transport properties in magnetic materials.

We propose to use the recently introduced orbital angular momentum spectrometer in a transmission electron microscope to perform electron magnetic chiral dichroism experiments, dispersing the inelastically scattered electrons from a magnetic material in both energy and angular momentum. The technique offers several advantages over previous formulations of electron magnetic chiral dichroism as it requires much simpler experimental conditions in terms of specimen orientation and thickness. A simulation algorithm, based on the multislice description of the beam propagation, is used to anticipate the advantages of the approach over current electron magnetic chiral dichroism implementations. Numerical calculations confirm an increased magnetic signal to noise ratio with in plane atomic resolution.

We consider the effects of an externally applied electrostatic field on superconductivity, self-consistently within a BCS mean-field model, for a clean three-dimensional (3D) metal thin film. The electrostatic change in superconducting condensation energy scales as $\mathrm{\Delta }/\mu$ close to subband edges as a function of the Fermi energy $\mu$ and follows 3D scaling ${(\mathrm{\Delta }/\mu )}^2$ away from them. We discuss nonlinearities beyond the gate effect and contrast results with recent experiments on gating effects on Josephson junctions.

Quantum many-body systems have a rich structure in the presence of boundaries. We study the ground states of conformal field theories (CFTs) and Lifshitz field theories in the presence of a boundary through the lens of the entanglement entropy. For a family of theories in general dimensions, we relate the universal terms in the entanglement entropy of the bulk theory with the corresponding terms for the theory with a boundary. This relation imposes a condition on certain boundary central charges. For example, in $2+1$ dimensions, we show that the corner-induced logarithmic terms of free CFTs and certain Lifshitz theories are simply related to those that arise when the corner touches the boundary. We test our findings on the lattice, including a numerical implementation of Neumann boundary conditions. We also propose an ansatz, the boundary extensive mutual information model, for a CFT with a boundary whose entanglement entropy is purely geometrical. This model shows the same bulk-boundary connection as Dirac fermions and certain supersymmetric CFTs that have a holographic dual. Finally, we discuss how our results can be generalized to all dimensions as well as to massive quantum field theories.

The experimentally observed bad-metal behavior in parent iron pnictides and chalcogenides suggests that these systems contain strong electronic correlations and are on the verge of a metal-to-insulator transition. The magnetic excitations in this bad-metal regime mainly derive from the incoherent part of the electronic spectrum away from the Fermi energy. We present a microscopic study of the exchange interactions in such a regime within a slave rotor approach. We find that the exchange interaction is maximized near the Mott transition. Generalizations to the multiorbital case are discussed, as are the implications for the strength of superconducting pairing amplitude in iron-based superconductors.

We propose an insulating two-dimensional phase of ${\mathrm{IrO}}_2$, predicted by ab initio evolutionary algorithms. The predicted phase is a van der Waals crystal, in which Ir forms a triangular lattice, and is energetically competitive with the metastable spinel phase, observed experimentally. Electronic structure calculations show that the magnetic properties of this phase are highly nontrivial, with an almost perfect degeneracy of ${120}^{\circ }$ Néel and $Y$-stripe orders, and unusually soft magnetic moments. The resulting behavior, which we term easy plane anisotropy, is entirely different from what is realized in previously explored Kitaev honeycomb lattices. Our results thus suggest that ${\mathrm{IrO}}_2$ may be an ideal candidate to realize highly unusual magnetic properties.

Motivated by recent experiments, here we study the indirect interactions between magnetic impurities deposited on top of a clean Pb(110) surface, induced by the underlying conduction electrons. Our approach makes use of ab initio calculations to characterize the clean Pb(110) surface and avoids self-consistency, a feature that greatly reduces the computational cost. In combination with second-order perturbation theory in the microscopic s-d exchange parameter $J_{sd}$ between a magnetic adatom and the conduction electrons, we are able to systematically derive the Ruderman-Kittel-Kasuya-Yosida, the Dzyaloshinskii-Moriya, and the anisotropic tensor interactions emerging at the Pb(110) surface between magnetic impurities. The only adjustable parameter is $J_{sd}$, which is fitted to reproduce the experiments. Our results show important anisotropy effects arising both from the rectangular geometry of the (110) unit cell and from the strong Rashba spin-orbit interaction due to the broken inversion symmetry at the Pb(110) surface. In addition to Pb(110), the characterization of the indirect spin interactions described here could be extended to other realistic metallic surfaces for weakly coupled impurities and would enable us to fabricate atomic-size nanostructures with engineered interactions and on-demand magnetic properties, anticipating useful applications in nanotechnology.

We consider a model describing a spin field-effect transistor based on a quantum nanowire with a tunable spin-orbit interaction embedded between two ferromagnetic leads with anticollinear magnetization. We investigate a regime of a strong interplay between resonance Kondo scattering and interference associated with the Aharonov-Casher effect. Using the Keldysh technique at a weak-coupling regime we calculate perturbatively the charge current. It is predicted that the effects of the spin-orbit interaction result in a nonvanishing current for any spin polarization of the leads including the case of fully polarized anticollinear contacts. We analyze the influence of the Aharonov-Casher phase and degree of spin polarization in the leads onto a Kondo temperature.