The peculiar characteristics of relaxors, viz., a frequency-dependent dielectric permittivity peak and good functional properties (dielectric, electromechanical, electrocaloric, etc.), are attributed to nanoscale regions with correlated dipoles, or polar nanoregions (PNRs). However, the exact nature of PNRs and their contribution to relaxor behavior remains debatable. In recent years, solid solutions of $\mathrm{BaTi}{\mathrm{O}}_3\text{−}\mathrm{Bi}\mathrm{Me}{\mathrm{O}}_3$ (where Me is a metal), have emerged as an interesting system with characteristics in between that of relaxors and dipole glasses. Here, we have examined the atomistic origins of weakly coupled relaxor behavior, specifically with regard to formation of PNRs, in Sn-doped $\left(1-x\right)\left(\mathrm{Ba},\mathrm{Ca}\right)\mathrm{Ti}{\mathrm{O}}_3\text{−}x\mathrm{BiSc}{\mathrm{O}}_3$ using macroscopic polarization and neutron dynamic pair distribution function measurements. We show that the short-range atomic correlations observed within the PNRs dynamically fluctuate with frequencies of the order of THz. Furthermore the composition-dependent dielectric and polarization behaviors are critically influenced by the relative stability of the atomic correlations near ∼1 THz, while the instantaneous atomic correlations are largely independent of $x$. The current results are discussed based on a model of intrinsic local modes distributed in a dielectrically soft matrix.

Breaking the reciprocity of electromagnetic interactions is of paramount importance in photonic and microwave technologies, as it enables unidirectional power flows and other unique electromagnetic phenomena. Here we explore a method to break the reciprocity of electromagnetic guided waves utilizing an electron beam with a constant velocity. By introducing an effective dynamic conductivity for the beam, we theoretically demonstrate how nonreciprocal guided waves and a one-way propagating regime can be achieved through the interaction of swift electrons with electromagnetic waves in two-dimensional (2D) parallel-plate and three-dimensional (3D) circular-cylindrical waveguides. Unlike the conventional electron beam structures such as traveling wave tubes and electron accelerators, here the goal is neither to generate and/or amplify the wave nor to accelerate electrons. Instead, we study the salient features of nonreciprocity and unidirectionality of guided waves in such structures. The relevant electromagnetic properties such as the modal dispersion, the field distributions, the operating frequency range, and the nonreciprocity strength and its dependence on the electron velocity and number density are presented and discussed. Moreover, we compare the dispersion characteristics of waves in such structures with some electric-current-based scenarios in materials reported earlier. This broadband tunable magnet-free method offers a unique opportunity to have a switchable strong nonreciprocal response in optoelectronics, nanophotonics, and THz systems.

Microscopic model of the interaction of spins with a microwave field in a random-anisotropy magnet has been developed. Numerical results show that microwave absorption occurs in a broad range of frequencies due to the distribution of ferromagnetically correlated regions on sizes and effective anisotropy. That distribution is also responsible for the weak dependence of the absorption on the damping. At a fixed frequency of the ac-field, spin oscillations are localized inside isolated correlated regions. Scaling of the peak absorption frequency agrees with the theory based upon the Imry-Ma argument. The effect of the dimensionality of the system related to microwave absorption by thin amorphous magnetic wires and foils has been studied.

Single crystalline samples of the van der Waals antiferromagnet ${\mathrm{CrPS}}_4$ were studied by measurements of specific heat and comprehensive anisotropic temperature- and magnetic-field-dependent magnetization. In addition, measurements of the heat capacity and magnetization were performed under pressures of up to $\sim 21$ and $\sim 14$ kbar, respectively. At ambient pressure, two magnetic transitions are observed: second order from a paramagnetic to an antiferromagnetic state at $T_N\sim 37$ K, and a first-order spin reorientation transition at $T^{*}\sim 34$ K. Anisotropic $H\text{−}T$ phase diagrams were constructed using the $M(T,H)$ data. As pressure is increased, $T_N$ is weakly suppressed with $d{T}_N/dP\approx -0.1$ K/kbar. $T^{*}$, on the other hand, is suppressed quite rapidly, with $d{T}^{*}/dP\approx -2$ K/kbar, extrapolating to a possible quantum phase transition at $P_c\sim 15$ kbar.

We report magnetic properties in the random spinel magnet ${\mathrm{CoGa}}_2{\mathrm{O}}_4$. Rietveld analysis of the x-ray diffraction profile for ${\mathrm{CoGa}}_2{\mathrm{O}}_4$ reveals that the Co and Ga ions are distributed randomly in the tetrahedral $A$ sites and octahedral $B$ sites in the cubic spinel structure. ${\mathrm{CoGa}}_2{\mathrm{O}}_4$ exhibits a spin-glass transition at $T_{\mathrm{SG}}=8.2\mathrm{K}$ that is confirmed by measurements of the dc and ac susceptibilities and thermoremanent magnetization (TRM) that develops below $T_{\mathrm{SG}}$. From the frequency dependence of the freezing temperature $T_{\mathrm{f}}$ for ${\mathrm{CoGa}}_2{\mathrm{O}}_4$, it is indicated that the relaxation time $\tau \left(T\right)$ follows a Vogel-Fulcher law $\tau ={\tau }_0\exp [-E_a/k_{\mathrm{B}}(T-T_0\left)\right]$. An analysis of specific heat suggested that a doublet ground state of the octahedrally coordinated ${\mathrm{Co}}^{2+}$ was stabilized by spin-orbit and crystal field couplings. The relaxation rate of TRM is considerably enhanced at $T_{\mathrm{SG}}$ and decays rapidly above and below $T_{\mathrm{SG}}$. The time course of TRM is reproduced by nonexponential relaxation forms, such as a stretched exponential (Kohlrausch) as well as Ogielski and Weron relaxation forms. This behavior is displayed universally in glass systems, and the characteristic parameters associated with these functions were reasonable.

We report the comparative effects of disorder and hydrostatic pressure on charge density wave (CDW) and superconductivity (SC) in $2H\text{−}{\mathrm{TaS}}_2$ by measuring electrical resistivity and ac magnetic susceptibility. For the crystals in the clean limit (low disorder level), CDW ground state is suppressed completely at a critical pressure $P_{\mathrm{c}}\sim 6.24\left(5\right)\mathrm{GPa}$ where a dome-shaped pressure dependence of superconducting transition temperature $T_{\mathrm{c}}$(P) appears with a maximum value of ${T_{\mathrm{c}}}^{\max }\sim 9.15\mathrm{K}$, indicating strong competitions between CDW and SC. The temperature exponent $n$ of low-temperature resistivity data decreases from ∼3.36 at ambient pressure (AP) to ∼1.29(2) at $P_{\mathrm{c}}$ and then retains a saturated value ∼2.10(4) when the pressure is higher than 7.5 GPa; accordingly, the quadratic temperature coefficient of normal-state resistivity A peaks out just at $P_{\mathrm{c}}$ with an enhancement by nearly one order in magnitude. These features strongly manifest that the enhanced critical CDW fluctuations near $P_{\mathrm{c}}$ are possible important glues for superconducting pairings. High-pressure magnetic susceptibility indicates that superconducting shielding volume increases with pressure and retains a nearly constant value above $P_{\mathrm{c}}$, which evidences that the enhancement of $T_{\mathrm{c}}$(P) is accompanied by the expense of CDW. For those crystals in dirty limit (high disorder level), there is no clear CDW phase transition in resistivity; the pressure dependence of $T_{\mathrm{c}}$(P) and $n$ broadens up and becomes less apparent in comparison with the clean crystals. Our results suggest that disorder scattering and the melting of CDW are two factors affecting SC, and the melting of CDW dominates the change of $T_{\mathrm{c}}$ below $P_{\mathrm{c}}$; the enhancement of $T_{\mathrm{c}}$(P) is associated with the suppression of CDW by pressure and the increase in the density of states at Fermi level; however, after the CDW collapse, superconducting pairing strength is strongly weakened by impurity scattering above $P_{\mathrm{c}}$ according to Anderson's theorem.

Whether self-correcting quantum memories can exist at nonzero temperature in a physically reasonable setting remains a great open problem. It has recently been argued [S. Roberts and S. D. Bartlett, Phys. Rev. X 10, 031041 (2020)] that symmetry-protected topological (SPT) systems in three space dimensions subject to a strong constraint—that the quantum dynamics respect a 1-form symmetry—realize such a quantum memory. We illustrate how this works in Walker-Wang codes, which provide a specific realization of these desiderata. In this setting we show that it is sufficient for the 1-form symmetry to be enforced on a subvolume of the system. This strongly suggests that the SPT character of the state is not essential. We confirm this by constructing an explicit example with a trivial (paramagnetic) bulk that realizes a self-correcting quantum memory. We therefore show that the enforcement of a 1-form symmetry on a measure-zero subvolume of a three-dimensional system can be sufficient to stabilize a self-correcting quantum memory at nonzero temperature.

Recently, we developed a general laser control scheme, i.e., the Stark control of electrons at interfaces (SCELI), based on Stark shifts, that is able to manipulate the electron dynamics at material interfaces [A. J. Garzón-Ramírez and I. Franco, Phys. Rev. B 98, 121305 (2018)]. Here, we investigate how SCELI is influenced by the band bending effects introduced by interfacial dipoles and by the laser screening due to the polarization response of the material. For this, we follow the quantum dynamics of a model one-dimensional tight-binding semiconductor-semiconductor heterojunction driven by nonresonant few-cycle laser pulses of intermediate intensity. Band bending effects are introduced through an interfacial electrostatic potential term dictated by the depletion approximation of a neutral $p\text{−}n$ junction. In turn, screening effects are captured through the general boundary conditions of the field vectors at interfaces dictated by Maxwell's equations. For field amplitudes where SCELI dominates ($E_0\le 0.55$ V/Å in the model), laser screening leads to a 46% reduction of the effect that can be partially compensated by increasing the intensity of the incident field. Surprisingly, band bending mildly affects SCELI. When both band bending and screening are considered simultaneously, the charge transfer is reduced, on average, 40% for $E_0\le 0.46$ V/Å. Overall, we observe that screening and band bending change the magnitude of SCELI but leave the underlying mechanism for electron transfer intact.

Nonzero bulk viscosity signals breaking of the scale invariance. We demonstrate that disorder in a two-dimensional noninteracting electron gas in a perpendicular magnetic field results in nonzero disorder-averaged bulk viscosity. We derive an analytic expression for the bulk viscosity within the self-consistent Born approximation. This residual bulk viscosity provides the lower bound for the bulk viscosity of 2D interacting electrons at low enough temperatures.

The ability to manipulate the frequency of light is a cornerstone of a wide range of applications in photonics expanding spectral ranges of the light sources. To boost the second harmonic generation efficiency, intense efforts toward exploiting artificial microstructures for frequency doubling were carried out. The sought-after approach of utilizing photonic crystals is inspired by their unusual light dispersion. In such structures, the dynamics of the field of femtosecond laser pulses is of paramount importance for frequency conversion. In this work, we investigate the phase-matched second harmonic generation under the effect of diffraction-induced femtosecond laser pulse splitting in one-dimensional photonic crystals at the Bragg diffraction in the Laue geometry. We demonstrate that the interaction of the two pulses at the fundamental frequency that appears in photonic crystals through the dynamical light diffraction leads to a nonmonotonic dependence of the frequency up-conversion efficiency with respect to the laser pulse duration, thus allowing to increase the second harmonic power by tuning the pulse duration.

$\mathrm{Ca}{\mathrm{Cu}}_3{\mathrm{Ru}}_4{\mathrm{O}}_{12}$ (CCRO) is considered a $d$-electron-based heavy-fermion metallic system with intriguing electronic properties. The magnetotransport measurements on metallic CCRO thin films reveal weak antilocalization (WAL) effects; however, it is not straightforward to infer whether the same originates from $e\text{−}e$ Coulomb interaction (EEI), spin-orbit interaction (SOI), or both present in CCRO. By evaluating quantum correction to sheet conductance for CCRO metallic thin films in the two-dimensional limit, it is observed that SOI gives rise to a dominant contribution to negative magnetoconductance (MC) in the weak-field regime. Based on weak localization (WL) and WAL analysis, SOI and inelastic scattering lengths ($l_{so}$ and $l_{\varphi }$) are obtained to be 30 and 74 nm, respectively, which indicates that WAL $(l_{so}<l_{\varphi })$ remains at play. The anisotropic effect of SOI is reflected in the field-direction-dependent in-plane and out-of-plane MC measurements. The presence of the EEI contribution is evidenced from (i) the linear increment (positive slope) of sheet conductance with $ln\left(T\right)$ in the quantum interference regime [only the WAL effect should otherwise show a negative slope with $ln\left(T\right)]$ and (ii) $ln\left(T\right)$ dependence of the Hall coefficient with a negative slope. Further, in the high-magnetic-field regime where WL or WAL is not valid, MC follows $ln\left(B\right)$-type behavior, indicating the presence of EEI. These findings have implications for the basic understanding of quantum magnetotransport properties in the presence of SOI and EEI in CCRO.

Recent work has demonstrated a new route to discrete time crystal physics in quantum spin chains by periodically driving nearest-neighbor exchange interactions in gate-defined quantum dot arrays [H. Qiao et al., Nat. Commun. 12, 2142 (2021)]. Here, we present a detailed analysis of exchange-driven Floquet physics in small arrays of GaAs quantum dots, including phase diagrams and additional diagnostics. We also show that emergent time-crystalline behavior can benefit the protection and manipulation of multispin states. For typical levels of nuclear spin noise in GaAs, the combination of driving and interactions increases the coherence times of entangled states by orders of magnitude. Similar results can be obtained for other quantum dot systems such as in Si. We further show how to construct a time-crystal-inspired cz gate between singlet-triplet qubits with high fidelity. These results show that periodically driving exchange couplings can enhance the performance of quantum dot spin systems for quantum information applications.

Starting from the numerical solution of the 6-band $\mathbf{k}·\mathbf{p}$ description of a lattice-mismatched ellipsoidal quantum dot situated inside a nanowire, including a spin Zeeman effect with values appropriate to a dilute magnetic semiconductor, we propose and test phenomenological models of the effect of the built-in strain on the heavy hole, light hole, and exciton states. We test the validity and the limits of a description restricted to a $\left({\mathrm{\Gamma }}_8\right)$ quadruplet of ground states and we demonstrate the role of the interactions of the light hole state with light hole excited states. We show that the built-in axial strain not only defines the character, heavy hole or light hole, of the ground state, but also mixes significantly the light hole state with the split-off band's states: Even for a spin-orbit energy as large as 1 eV, that mixing induces first-order modifications of properties such as the spin value and anisotropy, the oscillator strength, and the electron-hole exchange, for which we extend the description to the light hole exciton. CdTe/ZnTe quantum dots are mainly used as a test case but the concepts we discuss apply to many heterostructures, from mismatched II-VI and III-V quantum dots and nanowires, to III-V nanostructures submitted to an applied stress, and to silicon nanodevices with even smaller residual strains.

We theoretically study magnetic field, temperature, and energy band-gap dependences of magnetizations in the Dirac fermions. We use the zeta function regularization to obtain analytical expressions of thermodynamic potential, from which magnetization of graphene for strong field/low temperature and weak field/high temperature limits are calculated. Further, we generalize the result by considering the effects of impurity on orbital susceptibility of graphene. In particular, we show that in the presence of impurity, the susceptibility follows a scaling law which can be approximated by the Faddeeva function. In the case of massive Dirac fermions, we show that a large band gap gives a robust magnetization with respect to temperature and impurity. In the doped Dirac fermion, we discuss the dependences of period and amplitude of the de Haas-van Alphen oscillation on band gap.

It is known that phonons have angular momentum, and when the time-reversal symmetry (TRS) is broken, the total phonon angular momentum in the whole system becomes nonzero. In this paper, we propose that as an angular momentum of phonons for a crystal without TRS, we need to consider the canonical angular momentum, as opposed to the kinetic angular momentum in previous works. Next, we show that the angular momentum of phonons without TRS exhibits universal behaviors near the $\mathrm{\Gamma }$ point. We focus on in-plane oscillations in two-dimensional crystals as an example. By breaking the TRS, one of the acoustic phonon branches at the $\mathrm{\Gamma }$ point acquires a gap. We show that the angular momentum of its acoustic phonon with a gap has a peak with the height $\pm \hslash$ regardless of the details of the system. From this, we find that this peak height changes discontinuously by changing the sign of the TRS-breaking parameter.

Understanding the joint dynamics of electron and nuclear spins is central to core concepts in solid-state magnetic resonance such as spin-lattice relaxation and dynamic nuclear polarization, but a generalization that capitalizes on competing polarization loss and gain channels is still lacking. Here, we study theoretically the non-Hermitian dynamics of hybrid electron/nuclear spin systems in the simultaneous presence of electron spin pumping and spin-lattice relaxation. Focusing on periodic, one-dimensional chains, we find that, by adjusting the electron spin pumping to a critical level, it is possible to steer the flow of nuclear polarization to create site-dependent distributions where either end of the array polarizes in opposite ways, irrespective of the initial state. By contrast, we show that ring-like patterns—where the limit nuclear polarization is uniform—exhibit a non-decaying, externally driven nuclear spin current. Interestingly, cyclic magnetic field modulation can render these processes largely robust to defects in the chain, a response featuring some interesting similarities—and differences—with recent findings in other non-Hermitian physical platforms.

Electric, piezoelectric, and elastic characteristics of the quantum spin-1/2 chain system are calculated. Using the exact analytical solution we show that electric permittivity, piezoelectric and elastic modules, and magnetic characteristics can manifest strong dependencies on the values of the external magnetic, electric field, external strain, and temperature.

Recent realizations of intrinsic magnetic order in truly two-dimensional materials have opened new avenues in the fundamental knowledge and spintronic applications. Here we develop an anisotropic Heisenberg model with relativistic exchange interactions that are obtained from the first-principles calculations. We demonstrate the crucial importance of magnetic interactions beyond the first neighbor to qualitatively and quantitatively reproduce the experimental results. Once we ascertain the predictive capacity of the model for chromium trihalides and $\mathrm{Cr}\mathrm{Ge}{\mathrm{Te}}_3$, we investigate the feasibility of tuning the magnetic ordering by electrical means in these materials. A remarkable fivefold increase in the ferromagnetic Curie temperature is predicted in monolayer ${\mathrm{CrI}}_3$ within experimentally obtainable hole density. The elusive microscopic mechanism behind the doping-dependent Curie temperature is illustrated. Furthermore, in the present context, the effects of induced biaxial strain and chemical doping are also addressed. The results should trigger further experimental attention to test the present conclusions.

Inelastic neutron scattering spectra in the antiferromagnetic ferroborates ${\mathrm{Nd}}_{1-x}{\mathrm{Tb}}_x{\mathrm{Fe}}_3{\left({\mathrm{BO}}_3\right)}_4$ ($x$ = 0, 0.1, 0.2, and 1) reveal various magnetic excitations of the interacting iron and rare-earth subsystems. We observe an evolution of the magnetic system from “easy-plane”, in the Tb-free ($x$ = 0) case, to “easy-axis” anisotropy for samples substituted with Tb. The spectra show hybridized Fe and Nd branches, which are determined by the Fe-Nd exchange splitting of the ground-state ${\mathrm{Nd}}^{3+}$ doublet. In the easy-plane configuration, near the Brillouin zone center, there are two different pairs of anticrossing quasiacoustic Fe and Nd modes in contrast to the easy-axis state, where the two corresponding pairs of the branches are degenerated. The high-energy (exchange) branches are similar in both spin configurations. The Ising-type anisotropy of the Tb ion prevents the magnetic moment from precession. The increasing of the Tb content changes the effective magnetic anisotropy and stabilizes the easy-axis state. The spin-wave dispersion in the substituted and pure $\mathrm{Tb}{\mathrm{Fe}}_3{\left({\mathrm{BO}}_3\right)}_4$ compounds, which have the same, easy-axis magnetic structure, but different crystal symmetry, strongly differ. The observed spectra were analysed in the frame of linear spin-wave theory and the exchange parameters were determined.