Single-spin state evolution induced by the Landau-Zener-Stückelberg-Majorana (LZSM) interference in a Zeeman-spit four-level system in a periodically driven double quantum dot is studied theoretically by the Floquet stroboscopic method. An interplay between spin-conserving and spin-flip tunneling processes with the electric dipole spin resonance (EDSR) that is induced in an individual dot and enhanced by the LZSM multiple level crossings with the neighboring quantum dot is investigated as a function of the microwave (MW) frequency, driving amplitude, interdot detuning, and magnetic field. A number of special points in the parameter space are identified, out of which all three features are merged. Under this triple-crossing resonance condition, the interdot tunneling is combined with a fast spin evolution in each dot at the EDSR frequency. Harmonics of the EDSR are revealed in the spin-dependent tunneling maps versus variable magnetic field and MW frequency. The results are applicable for both electron and hole systems with strong spin-orbit interaction and may be useful for developing new time-efficient schemes of the spin control and readout in qubit devices.

We studied fcc praseodymium (Pr) with the ab initio correlation matrix renormalization theory (CMRT) explicitly calculating the Coulomb interactions among basis orbitals and without using any adjustable parameters to work for strongly correlated electron systems. We calculated its total energy in a paramagnetic ground state and studied the role of the correlated $4f$ electrons in the system. Good agreement was obtained between CMRT and experiments in the pressure volume dependence of the fcc phase. We also compared the CMRT results against other theoretical methods including local density $\text{approximation}+\text{dynamical}$ mean-field theory and showed consistent results among them. Moreover, we found the normalized local charge fluctuation of the $4f$ electrons as the function of volume exhibits a clear slope change at the volume collapse region, indicating a switch in their correlation nature there.

We report detailed angle-resolved photoemission measurements on the electronic structure of an unconventional multifold Dirac fermionic semimetal ${\mathrm{PdSb}}_2$ with a pyrite structure. By exploiting the photon energy and polarization dependence of the matrix element in photoemission intensity and by comparing photoemission data with ab initio band calculations, we experimentally identify the exact electronic structure, including the orbital characters of the electron pockets at the $R$ point in the Brillouin zone of ${\mathrm{PdSb}}_2$. Each electron pocket and hole-pocket-like structure consists of three doubly degenerate parabolic bands, respectively, which cross one another at the $R$ point, forming a sixfold Dirac fermion. The overall electronic structure is consistent with the band calculations, but the gap size between two sextuple points is very sensitive to the Wyckoff position of Sb atoms, which is a result of the competition between the band energies and crystal symmetries.

In this paper we study thermoelectric transport in interacting two-dimensional Dirac-type systems using a phenomenological Boltzmann approach. We consider a setup that can accommodate electrons, holes, and collective modes. In the first part of the paper we consider the electron-hole hydrodynamics, a model that is popular in the context of graphene, and its transport properties. In a second part, we propose a unique type of hydrodynamics. In that setup, the “fluid” consists of electrons, holes, and plasmons. We study its transport properties, especially the thermoelectric behavior. The results of this part can also be adapted to the study of a fluid consisting of electrons and phonons. This paper is accompanied by a technical paper [K. Pongsangangan et al., Phys. Rev. B 106, 205127 (2022)], in which we give a detailed derivation of the Boltzmann equations and the encoded conservation laws.

We study the hydrodynamic properties of ultraclean interacting two-dimensional Dirac electrons with Keldysh quantum field theory. We study it from a weak-coupling and a strong-coupling perspective. We demonstrate that long-range Coulomb interactions play two independent roles: (i) they provide the inelastic and momentum-conserving scattering mechanism that leads to fast local equilibration; (ii) they facilitate the emergence of collective excitations, for instance plasmons, that contribute to transport properties on equal footing with electrons. Our approach is based on an effective field theory of the collective field coupled to electrons. Within a conserving approximation for the coupled system we derive a set of coupled quantum-kinetic equations. This builds the foundation of the derivation of the Boltzmann equations for the interacting system of electrons and plasmons. From this, we explicitly derive all the conservation laws and identify the extra contributions of energy density and pressure from the plasmons. We demonstrate that plasmons show up in thermoelectric transport properties as well as in quantities that enter the energy-momentum tensor, such as the viscosity. In a parallel paper [K. Pongsangangan et al., Phys. Rev. B 106, 205126 (2022)] we discuss some of the phenomenology of the corresponding hydrodynamic equations with an eye on thermoelectric transport properties.

We show that a one-dimensional lattice of spinless fermions can undergo a topological phase transition only due to the effect of interaction. By allowing a dimerized interaction among the particles we show that the system exhibits a symmetry protected topological phase which does not exist in the absence of interaction and the system is a gapless state. The nontrivial topological character appears due to the onset of two degenerate bond-order phases as a function of dimerized interaction which are found to be topologically distinct from each other. As a result a topological phase transition occurs between these bond-order phases through a gap closing point. However, in the limit of strong interaction, the topologically distinct bond-order phases are separated from each other through a gapped charge density wave phase possessing a local antiferromagnetic order. We characterize this emergent topological nature by the edge states, Berry phase, and a nonlocal string order parameter and provide possible experimental signatures in terms of Thouless charge pumping and density-density correlation.

We present a general relation between the electromechanical couplings of infinitesimal strain and electric field to arbitrary order, measured at fixed voltage and at fixed electric field. We show that the improper response at fixed field can be written as the strain derivative of the $n\mathrm{th}$-order susceptibility tensor, and the proper response at fixed voltage drop can be written as the response at fixed field plus corrections for dilations and ${90}^{\circ }$ rotations induced by strain. Our theory correctly reproduces the proper piezoelectric response and we go beyond with the electrostrictive response. We present first-principles calculations of the improper electrostrictive response at fixed field, and illustrate how the correction is used to obtain the proper response at fixed voltage. This distinction is of high importance given the recent interest in giant electrostrictors exhibiting electromechanical responses as large as the piezoelectric ones.

Ferroelectricity is usually suppressed by pressure. Here, we establish a general phenomenological theory to describe pressure-induced ferroelectrics and then relate it to real materials. Based on first-principles calculations, we identify a pressure-induced ferroelectric transition in which the nonpolar LiBC in the ZrBeSi structure type transforms into the polar state in the LiGaGe structure type at about 108 GPa. The pressure-induced ferroelectric LiBC has a larger polarization of $1.95\mathrm{C}/{\mathrm{m}}^2$ and band gap of 3.27 eV at 110 GPa. A ferroelectric multiple-well energy as a function of polar distortion is found. The pressure-induced ferroelectric transition is driven by the softness of one of the $A_{2u}$ modes in the nonpolar LiBC above 60 GPa. We also find an enhancement of a dynamic transfer of charge along the B–C bond under pressure, which indicates that the ionic electrostatic interactions become stronger. Our work demonstrates the existence of pressure-induced ferroelectrics and thus provides opportunities to design devices with ferroelectrics working on high-pressure conditions.

The Fe/MgO interface is an essential ingredient in spintronics as it shows giant tunneling magnetoresistance and strong perpendicular magnetic anisotropy (PMA). A recent study demonstrated that the insertion of an ultrathin LiF layer between the Fe and MgO layers enhances PMA significantly. In this paper, we perform x-ray magnetic circular dichroism measurements on Fe/LiF/MgO multilayers to reveal the origin of the PMA enhancement. We find that the LiF insertion increases the orbital-magnetic-moment anisotropy and thus the magnetic anisotropy energy. We attribute the origin of this orbital-magnetic-moment-anisotropy enhancement to the stronger electron localization and electron-electron correlation or the better interface quality with fewer defects.

In the quest for materials hosting Kitaev spin liquids, much of the efforts have been focused on the fourth- and fifth-row transition metal compounds, which are spin-orbit coupling–assisted Mott insulators. Here we study the structural and magnetic properties of $3d$ transition metal oxides, ${\mathrm{Na}}_2{\mathrm{Co}}_2{\mathrm{TeO}}_6$ and ${\mathrm{Na}}_3{\mathrm{Co}}_2{\mathrm{SbO}}_6$. The partial occupancy of sodium in former compound is addressed using a cluster expansion, and a honeycomb lattice of sodiums is found to be energetically favored. Starting from the ab initio band structures, a many-body second-order perturbation theory leads to a pseudospin-$\frac{1}{2}$ Hamiltonian with estimated magnetic interactions. We show that the experimentally observed zigzag magnetic state is stabilized only when the first-neighbor Kitaev coupling dominates over the Heisenberg term, both of which are highly suppressed due to presence of $e_g$ orbitals. A third-neighbor Heisenberg interaction is found dominant in both these compounds. We also present a phase diagram for ${\mathrm{Na}}_2{\mathrm{Co}}_2{\mathrm{TeO}}_6$ by varying the electron-electron and spin-orbit interactions. The computed spin excitation spectra are found to capture essential features of recent experimental magnon spectrum, lending support to our results.

We use our recently developed functional renormalization group (FRG) approach for quantum spin systems to investigate the phase diagram of the frustrated $J_1{J}_2{J}_3$ quantum Heisenberg model on a cubic lattice. From a simple truncation of the hierarchy of FRG flow equations for the irreducible spin vertices, which retains only static spin fluctuations and neglects the flow of the four-spin interaction, we can estimate the critical temperature with a similar accuracy as the numerically more expensive pseudofermion FRG. In the regime where the ground state exhibits either ferromagnetic or antiferromagnetic order, a more sophisticated truncation including the renormalization of the four-spin interaction as well as dynamic spin fluctuations reveals the underlying renormalization group fixed point and yields critical temperatures which deviate from the accepted values by at most $4\%$.

It is conventional wisdom that a left-hand microwave cannot efficiently excite the spin wave (SW) in the ferromagnet due to the constraint of angular momentum conservation. In this work, we show that the left-hand microwave can drive nonreciprocal SWs in the presence of a strong ellipticity mismatch between the microwave and precessing magnetization. A compensation frequency is predicted, at which the left-hand microwave cannot excite SWs. Away from it the SW amplitude sensitively depends on the microwave ellipticity, in sharp contrast to the case driven by right-hand microwaves. By tuning the driving frequency, we observe a switchable SW nonreciprocity in the ferromagnetic layer. A mode-dependent mutual demagnetizing factor is proposed to explain this finding. Our work advances the understanding of photon-magnon conversion and paves the way to designing diodelike nanoscale magnonic devices.

We point out that in superconductors there may exist localized quantum excitations of the electric and condensate fields surrounding a static charge, which cannot be interpreted as simply the ground state of the screened charge plus some number of massive photons. This is illustrated via a lattice Monte Carlo calculation of the energy spectrum of a pair of separated static charges in an effective Ginzburg-Landau model of superconductivity.

In the presence of an AC drive, multiterminal Josephson junctions exhibit the inverse AC Josephson effect, where the oscillations of the superconducting phase of each junction can lock onto one another or onto the external drive. The competition between these different phase-locked states results in a complex array of quantized voltage plateaus whose stability strongly depend on the circuit parameters of the shunted junctions. This phase diagram cannot be explored with low-temperature transport experiments alone, given the breadth of the parameter space, so we present an easily tunable analog circuit whose dynamical properties emulate those of a three-terminal junction. We focus on the observation of the multiterminal inverse AC Josephson effect, and we discuss how to identify Shapiro steps associated with each of the three junctions as well as their quartet states. We only observe integer phase-locked states in strongly overdamped networks, but fractional Shapiro steps appear as well when the quality factor of the junctions increases. Finally, we discuss the role of transverse coupling in the synchronization of the junctions.

Charge order is commonly believed to compete with superconducting order. An intertwined form of a superconducting wave function, known as pair density wave (PDW) order, has been proposed; however, direct evidence, theoretical or experimental, that it forms the ground state of any cuprate superconductor is lacking. As a test case, we consider ${\mathrm{La}}_{2-x}{\mathrm{Ba}}_x{\mathrm{CuO}}_4$ with $x=1/8$, where charge and spin stripe orders within the ${\mathrm{CuO}}_2$ planes compete with three-dimensional superconducting order. We report measurements of the superconducting critical current perpendicular to the planes in the presence of an in-plane magnetic field. The variation of the critical current with orientation of the field is inconsistent with a theoretical prediction specific to the PDW model. It appears, instead, that the orientation dependence of the critical-current density might be determined by a minority phase of $d$-wave superconductivity that is present as a consequence of doped-charge inhomogeneity.

The low-energy spectrum of a zigzag graphene ribbon contains two gapless bands with highly nonlinear dispersion, $\epsilon \left(k\right)={\pm |\pi -k|}^W$, where $W$ is the width of the ribbon. The corresponding states are located at the two opposite zigzag edges. Their presence reflects the fact that the clean ribbon is a quasi-one-dimensional system naturally fine-tuned to the topological multicritical point. This quantum critical point separates a topologically trivial phase from the topological one with the index $W$. Here we investigate the influence of the (chiral) symmetry-preserving disorder on such a multicritical point. We show that the system harbors delocalized states with the localization length diverging at zero energy in a manner consistent with the $W=1$ critical point. The same is true regarding the density of states (DOS), which exhibits the universal Dyson singularity, despite the clean DOS being substantially dependent on $W$. On the other hand, the zero-energy localization length critical exponent, associated with the lattice staggering, is not universal and depends on the topological index $W$.

We propose an interferometry within the framework of the quantum Kibble-Zurek mechanism by exemplifying two prototypical quench protocols, namely, the round-trip and quarter-turn ones, on the transverse Ising and quantum $XY$ chains. Each protocol contains two linear ramps that drive the system across quantum critical point twice. The two linear ramps arouse two respective nonadiabatic critical dynamics that are well described by the quantum Kibble-Zurek mechanism. However, in combination, the two critical dynamics can interfere with each other deeply. As an effect of the interference, the dynamical phase is exposed in the final excitation probability, which leads to a quantum coherent many-body oscillation in the density of defects with predictable characteristic period. Thus, such an interference is available for direct experimental observations. In the quantum $XY$ model, we show that an interference can also arise from the interplay between two different critical dynamics derived from a critical point and a tricritical point. Furthermore, we demonstrate a phenomenon of multiple length scales in the defect-defect correlator, which is due to the interplay between the interference and the quantum dephasing of excited quasiparticle modes. It turns out that the dephased result relies on how the diagonal and off-diagonal lengths are modulated by the controllable parameters in a quench protocol.

Artificial spin ice (ASI) is a special class of engineered lattice of highly shape anisotropic single domain magnetic nanostructures, which is used as one of the model systems to study the spin ice behavior observed in pyrochlore oxides. The nanomagnets interact via dipolar interaction, resulting in correlated magnetization dynamics exhibiting macroscopic spin configuration states. Here, we exploit the tunability of magnetic microstates in the presence of an external field to study the excitation of spin waves in a finite-size square artificial spin ice system due to a locally applied pulse field. Also, we have studied the excitation in other nanomagnets which are not subjected to the pulse field, which we term as “propagation” of the waves from the perturbed nanomagnets to unperturbed ones purely due to the dipolar coupling. We report that careful selection of vertices with local magnetic charges can effectively direct the anisotropic spin wave excitation in neighboring nanomagnets in the presence of an external field. Further, the investigation has been carried out for spin wave excitation due to the magnetic monopole-polaronic environment forming in closed-edge ASI vertices. Our studies based on micromagnetic simulations suggest that magnetically charged vertices and monopole-polaron formation in square ASI vertices carry spectral signatures such as splitting in higher-frequency bulk mode.

Monolayer transition metal dichalcogenides are promising materials for spintronics due to their robust spin-valley locked valence states, enabling efficient charge-to-spin conversion via the valley Hall effect with nonequilibrium spins possessing long spin diffusion lengths of hundreds of nanometers. In this paper, we show that the injection of a pure valley current, induced by the valley Hall effect in a ${\mathrm{WSe}}_2$ monolayer, imparts a spin torque on the magnetization of an overlaid Fe or CoFe in a tunneling structure. The torque efficiency is found to be comparable to that in conventional perpendicular magnetic tunnel junctions and can be further optimized with the valley Hall angle in ${\mathrm{WSe}}_2$. The valley nature of the spin torque gives rise to out-of-plane dampinglike torques in a current-in-plane configuration, vanishing charge transport perpendicular to the plane, as well as torque efficiency tunable through gating.

A nonequilibrium state shares macroscopic properties, such as conductivity and superconductivity, with a static state when the two states exhibit an identical average value of an observable over a period. We studied the quench dynamics of a Kitaev chain on the basis of two types of order parameters associated with two channels of pairing: local pairing in real space and Bardeen-Cooper-Schrieffer (BCS)-like pairing in momentum space. Based on the exact solution, we found that the two order parameters were identical for the ground state, which indicated a balance between the two pairing channels and would help determine the quantum phase diagram. However, for a nonequilibrium state obtained through time evolution from an initially prepared vacuum state, the two parameters varied, but both parameters could still help determine the phase diagram. In the region of nontrivial topological phases, nonequilibrium states favored BCS-like pairing. In this paper, we present an alternative approach to dynamically generate a superconducting state from a trivial empty state and illuminate the pairing mechanism.