Metallic heterostructure as promising terahertz (THz) spintronic emitters has promoted the development of both spintronics and THz science. However, the underlying physics at the metallic interface, such as the nonlinear optical effect, remains unclear. Herein, we present interface magnetization induced THz generation from metallic heterostructure consisting of Heusler alloy CoFeMnSi (CFMS) and Pd thin films. THz generation is ascribed to 35% contribution from the magnetization-induced optical rectification (MOR) and 65% contribution from inverse spin Hall effect (ISHE) based on the pump polarization and sample azimuthal angle dependent measurement. Furthermore, the contribution ratio of the MOR decreases to 12% via lowering the CFMS grown temperature, which is due to the reduced crystalline quality and possible metal to semiconductor transformation in CFMS. Our results not only clarify MOR and ISHE in metallic heterostructure for the scientific field, but they also benefit THz source optimization for the technology field.

We report an observation of the linear Stark effect in a ${\mathrm{Tm}}^{3+}:{\mathrm{Y}}_3{\mathrm{Al}}_5{\mathrm{O}}_{12}$ crystal with the distribution of the Stark coefficient over the ion ensemble. We associate this effect with local lattice distortions near the positions of ${\mathrm{Tm}}^{3+}$ ions. Using this effect, the addressable storage of a series of weak light pulses in a cavity-assisted scheme of the revival of silenced echo quantum memory protocol is implemented. In this memory scheme, we also demonstrate storage of a light pulse on the few photon level. The application of an optical resonator makes it possible to increase the memory efficiency in this crystal and to reduce the minimal number of photons in the input signal pulse to 5.6 for the signal-to-noise ratio of 1 in the retrieved echo pulse. The results are in good agreement with the theoretical analysis. The possible ways of the further improvement of the implemented memory scheme are also discussed.

A mobility edge (ME) in energy separating extended from localized states is a central concept in understanding various fundamental phenomena, such as the metal-insulator transition in disordered systems. In one-dimensional quasiperiodic systems, there exist a few models with exact MEs, and these models are beneficial to provide exact understanding of ME physics. Here we investigate two widely studied models including exact MEs, one with an exponential hopping and one with a special form of incommensurate on-site potential. We analytically prove that the two models are mutually dual and further give the numerical verification by calculating the inverse participation ratio and Husimi function. Our result may provide insight into realizing and observing exact MEs in both theory and experiment.

In the present paper we investigate macroscopic two-fluid effects in systems composed of two elastic media. Using two strain tensors as macroscopic variables, we present a nonlinear analysis in the regime of long wavelengths and low frequencies. We also discuss the description in terms of relative strains and total strains. Generalizing earlier work on relative rotations for systems such as nematic elastomers or uniaxial magnetic gels, we investigate how this concept can be applied to the case of two elastic media in the linear domain. For the two strain fields and relative rotations between the two elastic media, we find a number of reversible and dissipative cross-coupling terms that couple velocity differences, mean velocities, strain fields, and relative rotations to each other as well as to temperature and concentration gradients. The question of relative translations is also analyzed. A linearized description using relative translations is physically meaningful as well as technically consistent with using strain fields and relative rotations. Finally, we apply this description to the swinging of two coupled homogeneous elastic media relative to each other, and to the oscillating actuation or active stress introduced through one of the elastic compounds.

Studies of the Grüneisen ratio, i.e., the ratio between thermal expansion and specific heat, have become a powerful tool in the context of quantum criticality since it was shown theoretically that the Grüneisen ratio displays characteristic power-law divergencies upon approaching the transition point of a continuous quantum phase transition. Here we show that the Grüneisen ratio also diverges at a symmetry-enhanced first-order quantum phase transition, albeit with mean-field exponents, as the enhanced symmetry implies the vanishing of a mode gap which is finite away from the transition. We provide explicit results for simple pseudospin models, both with and without Goldstone modes in the stable phases, and discuss implications.

We investigate the resonance energy transfer (RET) rate between two quantum emitters near a suspended graphene sheet in vacuum under the influence of an external magnetic field. We perform the analysis for low and room temperatures and show that, due to the extraordinary magneto-optical response of graphene, it allows for an active control and tunability of the RET even in the case of room temperature. We also demonstrate that the RET rate is extremely sensitive to small variations of the applied magnetic field and can be tuned up to a striking six orders of magnitude for quite realistic values of magnetic field. Moreover, we highlight the fundamental role played by the magnetoplasmon polaritons supported by the graphene monolayer as the dominant channel for the RET within a certain distance range. Our results suggest that magneto-optical media may take the manipulation of energy transfer between quantum emitters to a whole new level and broaden even more its great spectrum of applications.

The Dzyaloshinskii-Moriya (DM) interaction, as well as symmetric anisotropic exchange, are important ingredients for stabilizing topologically nontrivial magnetic textures, such as, e.g., skyrmions, merons, and hopfions. These types of textures are currently in focus from a fundamental science perspective and they are also discussed in the context of future spintronics information technology. While the theoretical understanding of the Heisenberg exchange interactions is well developed, it is still a challenge to access, from first principles theory, the DM interaction as well as the symmetric anisotropic exchange, which both require a fully-relativistic treatment of the electronic structure, in magnetic systems where substantial electron-electron correlations are present. Here, we present results of a theoretical framework which allows to compute these interactions in any given system and demonstrate its performance for several selected cases, for both bulk and low-dimensional systems. We address several representative cases, including the bulk systems CoPt and FePt, the B20 compounds MnSi and FeGe as well as the low-dimensional transition metal bilayers Co/Pt(111) and Mn/W(001). The effect of electron-electron correlations is analyzed using dynamical mean-field theory on the level of the spin-polarized $T$-matrix $+$ fluctuating exchange (SPTF) approximation, as regards the strength and character of the isotropic (Heisenberg) and anisotropic (DM) interactions in relation to the underlying electronic structure. Our method can be combined with more advanced techniques for treating correlations, e.g., quantum Monte Carlo and exact diagonalization methods for the impurity solver of dynamical mean-field theory. We find that correlation-induced changes of the DM interaction can be rather significant, with up to fivefold modifications in the most distinctive case.

The recent discovery of two-dimensional (2D) superconductivity at the interface of $\mathrm{Ba}\mathrm{Pb}{\mathrm{O}}_3$ (BPO) and $\mathrm{Ba}\mathrm{Bi}{\mathrm{O}}_3$ (BBO) has motivated us to study in depth the electronic and structural properties and the relation between them in this particular heterostructure, by means of first-principles calculations. Our results indicate that the breathing distortions, the charge ordering, and the semiconducting behavior that characterize the parent compound BBO in its bulk form are preserved at the innermost layers of the BBO side of the BPO/BBO bilayer. On the other hand, at the interface, there is a partial breaking of the breathing distortions with a concomitant charge transfer between the interfacial Bi ions and the on top BPO layer. We show that two types of carriers coexist at the interface, the delocalized three-dimensional-like $sp$ states coming from Pb ions and the quasi-2D $s$ states from the Bi ones. We obtain a substantial electron-phonon coupling between the 2D Bi states with the interfacial stretching phonon mode and a large density of states that can explain the critical temperature experimentally observed below 3.5 K. We hope these findings will motivate future research to explore different interfaces with charge-ordered semiconductors such as BBO in order to trigger this fascinating 2D behavior.

We report the evolution of the physical properties of FeSe single crystals with Cu substitution. We show that introducing Cu suppresses bulk superconductivity quickly, much faster than the decrease in structural transition temperature, and further doping Cu induces a metal-insulator transition. In contrast, zero-field $ab$-plane resistivity ${\rho }_{xx}$ exhibits an unusual temperature dependence ${\rho }_{xx}\left(T\right)\sim A{T}^n$, with $n\sim$ 1, that is almost unchanged with the variation of Cu content. This result implies that magnetic fluctuations in FeSe are insensitive to the Cu substitution. The field dependence of the Hall resistivity of ${\mathrm{Fe}}_{1-x}{\mathrm{Cu}}_x\mathrm{Se}$ with $x>$ 0 shows a positive slope in the low-temperature region which can be ascribed to the relatively higher hole mobility than the electron one. Therefore, the low-concentration Cu dopant as a strong scattering source can lead to the significant decrease in electron mobility which is detrimental to superconductivity, but it has minor effects on the carrier densities of electrons and holes as well as the shapes of the Fermi surfaces. Correspondingly, the structural transition and magnetic fluctuations in ${\mathrm{Fe}}_{1-x}{\mathrm{Cu}}_x\mathrm{Se}$ change slowly with $x$.

A detailed zero-field and transverse-field muon spin relaxation/rotation ($\mu \mathrm{SR}$) experiments have been carried out on the recently discovered noncentrosymmetric superconductor ${\mathrm{W}}_3{\mathrm{Al}}_2\mathrm{C}$ to speculate about its superconducting ground state. Bulk nature of superconductivity below 7.6 K is confirmed through magnetization and specific heat measurements. No change in the zero field $\mu \mathrm{SR}$ spectra collected above and below $T_c$ is visible, ruling out the possibility of spontaneous magnetic field below $T_c$. This confirms that time-reversal symmetry is preserved for ${\mathrm{W}}_3{\mathrm{Al}}_2\mathrm{C}$ upon entering in the superconducting ground state. Temperature-dependent superfluid density [${\rho }_s\left(T\right)$], which directly reflects the superconducting gap symmetry is obtained by the analysis of spectra obtained from the transverse-field $\mu \mathrm{SR}$ experiments. Despite a noncentrosymmetric structure, ${\mathrm{W}}_3{\mathrm{Al}}_2\mathrm{C}$ adopts a fully gapped spin-singlet superconducting ground state as confirmed through the analysis of ${\rho }_s\left(T\right)$ and $C_e\left(T\right)$. The zero-temperature value of gap is ${\mathrm{\Delta }}_0$ = 1.139(5) meV with gap-to-$T_c$ ratio $2{\mathrm{\Delta }}_0/k_B{T}_c\approx 3.48$, classifying this material as a weakly coupled superconductors. We have also revealed that the ratio $T_c/{\lambda }_{\mathrm{eff}}^{-2}$ is comparable to the class of unconventional superconductors, which points towards an unconventional pairing mechanism in ${\mathrm{W}}_3{\mathrm{Al}}_2\mathrm{C}$.

Our direct atomic simulations reveal that a thermally activated phonon mode involves a large population of elastic wave packets. These excitations are characterized by a wide distribution of lifetimes and coherence times expressing particlelike and wavelike natures. In agreement with direct simulations, our theoretical derivation yields a generalized law for the decay of the phonon number taking into account coherent effects. Before the conventional exponential decay due to phonon-phonon scattering, this law introduces a delay proportional to the square of the coherence time. This additional regime leads to a moderate increase of the relaxation times and to a different dependence of phonon relaxation to those. This work opens new horizons in the understanding of the origin and the treatment of thermal phonons.

The quest for novel reliable and fast-performing logic and memory elements in classical and quantum computing requires discoveries of new effects in novel quantum materials. Skyrmions are such magnetic textures where electron scattering can essentially change the shape and location of the skyrmion, which can be used as a memory element being more efficient than domain walls as a memory device in classical computing. Because the skyrmion motion is sensitive even to very small currents, we study electron scattering by skyrmions in an ideal skyrmion gas in a ferromagnetic environment. In such systems, the direct and Hall currents become spin-dependent. For applications, it is important to consider a Hall effect in the whole range of temperatures under the assumption of the skyrmion existence. In this study, we find the nonmonotonic temperature dependence of the direct spin-up conductivity, i.e., the conductivity for the current directed along the applied electric field due to the electrons with the spin parallel to the ferromagnetic moment. Such a behavior contradicts the traditional understanding where the temperature only increases the value of the conductivity. The spin-down Hall conductivity is found to be even more dramatic exhibiting a conductivity sign change (i.e., a change in the current direction) with temperature for small skyrmion sizes. The found effects strongly depend on Fermi energy. The most pronounced dependencies take place if the Fermi energy is slightly below and above the bottom of the upper (spin-down) energy band of an ideal two-dimensional electron gas. In addition, we also find that the direct and Hall resistivities, ${\rho }_{xx}$ and ${\rho }_{xy}$, are independent of the exchange integral $J$ for $2J>{\varepsilon }_F$.

Alkali-metal rare-earth chalcogenide $AREC{h}_2$ (A = alkali or monovalent metal, RE = rare earth, Ch = O, S, Se, Te) is a large family of quantum spin liquid candidates we discovered recently. Unlike ${\mathrm{YbMgGaO}}_4$, most members in the family, except for the oxide ones, have a relatively small crystalline electric field (CEF) energy gap from the ground state to the first excited state. This makes the conventional Curie-Weiss analysis at finite temperatures inapplicable and the CEF excitations may play an essential role in understanding the low-energy spin physics. Here we consider an effective magnetic Hamiltonian incorporating the CEF excitations and spin-spin interactions, to accurately describe thermodynamics in such a system. By taking ${\mathrm{NaYbSe}}_2$ as an example, we were able to analyze magnetic susceptibility, magnetization under pulsed high fields, and heat capacity systematically and comprehensively. The analysis allows us to produce accurate anisotropic exchange coupling energies and unambiguously determine a crossover temperature ($\sim 25$ K in the case of ${\mathrm{NaYbSe}}_2$), below which the CEF effects fade away and pure spin-spin interactions stand out. We further validated the effective picture by successfully explaining the anomalous temperature dependence of electron spin resonance spectral width. The effective scenario in principle can be generalized to other rare-earth spin systems with small CEF excitations.

Despite the robustness of the chiral edge modes of quantum Hall systems against the superconducting proximity effect, Cooper pairs can penetrate into the chiral edge channels and carry the Josephson current in an appropriate setup. In our work, the Josephson junction of a spin-polarized quantum anomalous Hall insulator (QAHI) with a Chern number $\nu =1$ connecting conventional superconductors is studied from the perspective of pairing symmetry consistent with the chiral edge mode. Induced pairing states are equal-spin triplet, a combination of the even- and odd-frequency components, nonlocally extended, and have a finite momentum $2k_F$. The signature of the equal-spin triplet pairings is confirmed via the dependence on the interface-magnetization direction, and that of the finite-momentum pairing states via the width dependence of the critical current and the spatial profile of the anomalous Green's function. In the presence of disorder, the robustness of the chiral edge mode leads to high sensitivity of the critical current and the equilibrium phase difference to disorder configurations, which is resulting from the interference of current-carrying channels. The numerical calculations on a lattice model are also examined by a simplified analytical model.

Tensor network states form a variational ansatz class widely used, both analytically and numerically, in the study of quantum many-body systems. It is known that if the underlying graph contains a cycle, e.g., as in projected entangled pair states, then the set of tensor network states of given bond dimension is not closed. Its closure is the tensor network variety. Recent work has shown that states on the boundary of this variety can yield more efficient representations for states of physical interest, but it remained unclear how to systematically find and optimize over such representations. We address this issue by defining an ansatz class of states that includes states at the boundary of the tensor network variety of given bond dimension. We show how to optimize over this class in order to find ground states of local Hamiltonians by only slightly modifying standard algorithms and code for tensor networks. We apply this method to different models and observe favorable energies and runtimes when compared with standard tensor network methods.

We show that gapless spin liquids, which are potential candidates to describe the ground state of frustrated Heisenberg models in two dimensions, become trivial insulators on cylindrical geometries with an even number of legs. In particular, we report calculations for Gutzwiller-projected fermionic states on strips of square and kagome lattices. By choosing different boundary conditions for the fermionic degrees of freedom, both gapless and gapped states may be realized, the latter ones having a lower variational energy. The direct evaluation of static and dynamical correlation functions, as well as overlaps between different states, allows us to demonstrate the sharp difference between the ground-state properties obtained within cylinders or directly in the two-dimensional lattice. Our results shed light on the difficulty to detect bona fide gapless spin liquids in such cylindrical geometries.

We numerically study quenches from a fully ordered state to the ferromagnetic regime of the chiral ${\mathbb{Z}}_3$ clock model, where the physics can be understood in terms of sparse domain walls of six flavors. As in the previously studied models, the ballistic spread of entangled domain wall pairs generated by the quench leads to a linear growth of entropy with time, up to a time $\ell /2v_g$ in size-$\ell$ subsystems in the bulk, where $v_g$ is the maximal group velocity of domain walls. In small subsystems located in the bulk, the entropy continues to grow, approaching $ln3$, as domain walls traverse the subsystem and increment the population of the two oppositely ordered states, restoring the ${\mathbb{Z}}_3$ symmetry. The latter growth in entropy is seen also in small subsystems near an open boundary in a nonchiral clock model. In contrast to this, in the case of the chiral model, the entropy of small subsystems near an open boundary saturates. We rationalize the difference in behavior in terms of qualitatively different scattering properties of domain walls at the open boundary in the chiral model. We also present empirical results for entropy growth, correlation spread, and energies of longitudinal-field-induced bound states of domain wall pairs in the chiral model.

A perspective of the Green's function in a boundary value problem as the only eigenstate in an auxiliary formulation is introduced. In this treatment, the Green's function can be perceived as a defect state in the presence of a $\delta$-function potential, the height of which depends on the Green's function itself. This approach is illustrated in one-dimensional and two-dimensional Helmholtz equation problems, with an emphasis on systems that are open and have a non-Hermitian potential. We then draw an analogy between the Green's function obtained this way and a chiral edge state circumventing a defect in a topological lattice, which shines light on the local minimum of the Green's function at the source position.

Optical second harmonic generation (SHG) on excitons is studied in polar ZnO/(Zn,Mg)O quantum wells grown on ZnO and sapphire substrates. We observe SHG signal on exciton resonances in a symmetry-forbidden geometry for light propagation along the $c$ axis of the structure. A symmetry analysis allows us to suggest that the signals are induced by a symmetry reduction with respect to ZnO bulk crystals by the well-barrier interfaces in combination with built-in electric fields due to spontaneous and piezoelectric polarization as well as structural defects.

Through a global structure search combined with density functional theory calculations, we predict a two-dimensional (2D) electride ferromagnet, yttrium iodide (YI). The ground-state YI structure has YI layers bound by relatively weak van der Waals interactions; the interlayer binding energy is only $–0.016\mathrm{eV}/{\AA }^2$. Each YI layer has strongly localized anionic electrons in interstitial spaces, which is a crucial characteristic of electrides. The hybridization of these interstitial $s$-band lone-pair electrons and the $d$-band electrons of Y gives rise to a Stoner-type instability, resulting in 2D ferromagnetic orders with magnetic moments of $1.28{\mu }_{\mathrm{B}}$ per unit cell. Our Monte Carlo simulations further demonstrate that the Curie temperatures of bulk and monolayer YI are as high as 380 and 383 K, respectively. Phonon dispersion calculations and ab initio molecular dynamics simulations also show that the YI layers are dynamically and thermodynamically stable at room temperature. These findings suggest that monolayer YI is a promising candidate for a room-temperature 2D ferromagnet.