The Brownian motion of domain walls in uniaxial and biaxial ferromagnetic nanowires is studied, comparing spin dynamics simulations with analytical calculations within the framework of a collective coordinate approach. Our results demonstrate that the interplay between spatial and angular dynamics gives rise to a complex time dependence of the MSD in biaxial nanowires and to a drastically reduced diffusion coefficient in uniaxial nanowires, analogous to magnetic skyrmions. This diffusion suppression is also responsible for the peculiar temperature dependence of the diffusion coefficient in biaxial systems: while it is found to scale linearly with temperature up to a certain threshold, the emergence of a Walker breakdown of Brownian motion is responsible for a reduction of the diffusion coefficient with increasing temperature above this threshold.

With intrinsic magnetism, two-dimensional (2D) $\mathrm{Cr}{\mathrm{I}}_3$ has attracted tremendous interest because of the potential application in magnetic devices of smaller size. We propose to use 2D $\mathrm{Mo}{\mathrm{Si}}_2{\mathrm{N}}_4$ material, which has remarkable stability, excellent electronic properties, and high mobility, as tunnel barrier in vertical magnetic tunnel junction (MTJ), and demonstrate that it is able to generate a giant tunnel magnetoresistance (TMR) over ${10}^5\%$ when integrated with $\mathrm{Cr}{\mathrm{I}}_3$ ferromagnetic layers and Ag electrodes. Combining with density-functional theory and nonequilibrium Green's function approach, we systematically investigate the electron transmission, band structures, and projected local density of states and elaborate the transmission mechanism. The TMR and spin injection efficiency maintain high values below 0.25 V. These results indicate that $\mathrm{Mo}{\mathrm{Si}}_2{\mathrm{N}}_4$ is a promising barrier material in future 2D vertical MTJs and provide important guidance for designing devices.

We present a method to calculate the optical conductivity of semi-Dirac and pseudospin models based on the evaluation of quasiparticle velocity correlators which also describe the phenomenon of Zitterbewegung. Applying this method to the semi-Dirac model with merging Dirac cones and gapped dice and Lieb lattice models we find exact analytical expressions for optical longitudinal and Hall conductivities. For the semi-Dirac model the obtained expressions allow us to analyze the role of spectrum anisotropy, Van Hove singularities, and Dirac cones in longitudinal conductivity. In addition, we predict signatures of topological phase transition with changing gap parameter in such a system that are manifested in dc transport at low temperatures. For the dice and Lieb lattices we emphasize the role of the spectral gap, which defines frequency thresholds related to transitions to and from a flat band.

A mirror asymmetric Janus structure induces Rashba spin splitting (RSS) and a piezoelectric response. Inspired by the recently synthesized layered material CrSSe [Yang, Shi, Wang, Yue, Zheng, Zhang, Gu, Yang, Shadike, Li, and Fu, J. Mater. Chem. A 8, 25739 (2020)], we use first-principles calculations to systematically study the Rashba effect and piezoelectricity of Janus chromium dichalcogenide monolayers $\mathrm{Cr}XY$ ($X\ne Y=\text{S,}\text{Se,}\text{Te}$), as well as their regulation with biaxial strain. Our results reveal that spin-orbit coupling (SOC) plays an important role in the electronic properties (such as the semiconductor type, RSS, and valley polarization) of a $\mathrm{Cr}XY$ monolayer. Due to the mirror symmetry break and strong SOC, the strain-free $\mathrm{Cr}XY$ exhibits large Rashba parameters. Specifically, the Rashba parameter of CrSeTe is as high as 1.23 eV Å. Due to the ${\mathit{k}}^3$ term in the valence-band edge, the CrSeTe exhibits a strong hexagonal warping effect along with a nonzero out-of-plane spin polarization ${\mathit{S}}_z$, which can also be found in the CrSSe and CrSTe monolayers in the lower energy valence bands. Moreover, the Janus $\mathrm{Cr}XY$ monolayer exhibits superior intrinsic piezoelectric responses (${\mathit{d}}_{31}$ = $0.4–0.83$ pm/V), which are orders of magnitude larger than those of the $\mathrm{Mo}XY$ monolayer. Furthermore, we reveal in detail the modulation of the band structure, RSS, and piezoelectric properties with biaxial strain. Tensile strain suppresses the band gap, whereas compressive strain increases the band gap. Thus, strain engineering can effectively tune the band structures resulting in semiconductor-metal and indirect-direct transitions. In addition, the strain has opposite effects on the RSS and the piezoelectricity; that is, unlike compressive strain-enhanced RSS, the tensile strain can significantly elevate the piezoelectric coefficients. Our results indicate that a Janus $\mathrm{Cr}XY$ monolayer has coexisting large intrinsic RSS and piezoelectricity, which can be efficiently regulated by strain engineering, opening opportunities for applications in spintronic and piezoelectric devices.

The orbital and spin magnetization of a cavity-embedded quantum dot array defined in a GaAs heterostructure are calculated within quantum-electrodynamical density-functional theory. To this end, a gradient-based exchange-correlation functional recently employed for atomic systems is adapted to the hosting two-dimensional electron gas submitted to an external perpendicular homogeneous magnetic field. Numerical results reveal the polarizing effects of the cavity photon field on the electron charge distribution and nontrivial changes of the orbital magnetization. We discuss its intertwined dependence on the electron number in each dot, and on the electron-photon coupling strength. In particular, the calculated dispersion of the photon-dressed electron states around the Fermi energy as a function of the electron-photon coupling strength indicates the formation of magnetoplasmon-polaritons in the dots.

We theoretically investigated the phonon statistics of an acoustic cavity coupled to a three-level ladder-type artificial atom. Based on the wave function method, we elucidated the optimal conditions for a strong phonon antibunching effect at zero temperature and observed the coexistence of both conventional and unconventional phonon blockades. Particularly, intersection points occurred between the antibunching structures of both blockades at a certain controlled field strength. Importantly, compared to the conventional/unconventional phonon blockade, simultaneously strong antibunching and high brightness (mean phonon number) were realized at the intersection point. Further, phonon blockade at a finite temperature was studied. We showed that thermal noise affected the phonon blockade only at temperatures exceeding a certain threshold value. More interestingly, the conventional phonon blockade was more robust against thermal noise than the unconventional phonon blockade. Thus, this work provides a scheme for preparing high-quality single-phonon sources at finite temperatures.

We analyze the optically driven dynamics of a qubit implemented on a singlet-triplet subspace of two-electron states in a self-assembled quantum dot molecule. We study two possible control schemes based on the coupling to an excited (four-particle) state either by two spectrally separated laser pulses or by a single spectrally broad pulse. We quantitatively characterize the imperfections of the qubit operation resulting from nonadiabatic evolution and from limited spectral selectivity in a real system, as compared to the ideal adiabatic Raman transfer of occupation in the $\mathrm{\Lambda }$ system. Next, we study the effects of decoherence induced by the coupling to the phonons of the surrounding crystal lattice and by radiative recombination. As a result, we are able to identify the optimization trade-offs between different sources of errors and indicate the most favorable conditions for quantum control of the singlet-triplet qubit in the two optical control schemes.

Electron-doped infinite-layer ${\mathrm{Sr}}_{1-x}{\mathrm{Eu}}_x\mathrm{Cu}{\mathrm{O}}_{2+y}$ films over a wide doping range have been prepared epitaxially on $\mathrm{Sr}\mathrm{Ti}{\mathrm{O}}_3$(001) using reactive molecular beam epitaxy. In-plane transport measurements of the single-crystalline samples reveal a dome-shaped nodeless superconducting phase centered at $x\sim$ 0.15, a Fermi-liquid behavior and pronounced upturn in low-temperature resistivity. We show that the resistivity upturn follows square-root temperature dependence, suggesting the emergence of superconductivity via a three-dimensional percolation process. The percolative superconductivity is corroborated spectroscopically by imaging the electronic phase separation between superconducting and metallic phases with low-temperature scanning tunneling microscopy. Furthermore, we visualize interstitial and apical oxygen anions that rapidly increase in number as $x>$ 0.12 and elucidate their impacts on the superconductivity and normal-state resistivity.

We propose a theoretical model of a ferroelectric metal where spontaneous electric polarization coexists with the conducting electrons. In our model we adopt a scenario when conducting electrons interact with two soft transverse optical phonons, generalize it to the case when there is a spontaneous ferroelectric polarization in the system, and show that a linear coupling to the phonons emerges as a result. We find that this coupling results in anisotropic electric transport which has a transverse to the current voltage drop. Importantly, the obtained transverse component of the resistivity has distinct linear dependence with temperature. Moreover, we show that the coupling enhances superconducting transition temperature of the ferroelectric metal. We argue that our results help to explain recent experiments on ferroelectric strontium titanate as well as provide new experimental signatures to look for.

In this Letter we address the reentrance of magic-angle phenomena (band flatness and quantum-geometric transport) in twisted bilayer graphene (TBG) subjected to strong magnetic fluxes $\pm {\mathrm{\Phi }}_0,\pm 2{\mathrm{\Phi }}_0,\pm 3{\mathrm{\Phi }}_0...$ (${\mathrm{\Phi }}_0=h/e$ is the flux quantum per moiré cell). The moiré translation invariance is restored at the integer fluxes, for which we calculate the TBG band structure using accurate atomistic models with lattice relaxations. Similarly to the zero-flux physics outside the magic-angle condition, the reported effect breaks down rapidly with the twist. We conclude that the magic-angle physics reemerges in high magnetic fields, witnessed by the appearance of flat electronic bands distinct from Landau levels, and manifesting nontrivial quantum geometry. We further discuss the possible flat-band quantum geometric contribution to the superfluid weight in strong magnetic fields (28 T at $1.{08}^{\circ }$ twist), according to the Peotta-Törmä mechanism [Nat. Commun. 6, 8944 (2015)].

Two recent experiments from Cornell University and Columbia University have reported insulator-to-metal transitions in two-dimensional (2D) moiré transition metal dichalcogenides (mTMDs) induced by doping around half filling, where the system is a Mott insulator. In this Letter, we consider the temperature-dependent resistivity of this metallic phase in the doped situation away from half filling, arguing that it arises from the strongly temperature-dependent 2D Friedel oscillations (i.e., finite momentum screening) associated with random quenched charged impurities, leading to the observed strongly increasing linear-in-$T$ resistivity in the metallic phase. Our theory appears to account for the temperature-dependent metallic resistivity for doping around half filling of the effective moiré TMD band, showing that temperature-dependent screened Coulomb disorder is an essential ingredient of doped 2D mTMD physics.

A flexible control of wave scattering in complex media is of relevance in different areas of classical and quantum physics. Recently, great interest has been devoted to scattering engineering in non-Hermitian systems, with the prediction and demonstration of new classes of non-Hermitian potentials with unique scattering properties, such as transparent and invisible potentials or one-way reflectionless potentials. Such potentials have been found for both continuous and discrete (lattice) systems. However, wave scattering in lattice systems displays some distinct features arising from the discrete (rather than continuous) translational invariance of the system, characterized by a finite band of allowed energies and a finite speed of wave propagation on the lattice. Such distinct features can be exploited to realize invisibility on a lattice with methods that fail when applied to continuous systems. Here we show that a wide class of time-dependent non-Hermitian scattering potentials or defects with arbitrary spatial shape can be synthesized in an Hermitian single-band tight-binding lattice, which are fully invisible owing to the limited energy bandwidth of the lattice.

Phonon angular momentum (PAM) may exhibit exotic temperature dependence as it is sensitive to the phonon lifetime. Constant phonon-lifetime approximation fails to depict such behavior. Here, we study the PAM of AlN, GaN, and graphenelike boron nitride (g-BN) monolayer with full consideration of phonon lifetime using first-principles calculations. We show that wurtzite AlN and GaN acquire divergent PAM at low temperatures from their lowest-lying phonon branches. The g-BN monolayer, on the other hand, does not have finite PAM at equilibrium structure. Rather, it shows intriguing strain dependence on PAM; the compressive strain greater than the critical size generates divergent PAM at low temperatures due to the divergent lifetime of TA phonons. As PAM couples with rotational excitations in solids associated with charge, spin, or electromagnetic fields, our study demonstrates a possibility of mechanical and thermal engineering of such excitations.

Temporal metamaterials have been recently exploited as a novel platform for conceiving several electromagnetic and optical devices based on the anomalous scattering response arising at a single change or multiple sudden changes in the material properties. However, they are difficult to implement in realistic scenarios by switching the permittivity of a material in time, and new strategies to achieve time interfaces in a feasible manner must be identified. In this paper, we investigate the possibility to realize a temporal metamaterial without acting on the material properties, but rather on the effective refractive index and wave impedance perceived by the wave during the propagation in an empty guiding structure by varying the boundaries in time. We demonstrate analytically and through numerical experiments that suddenly changing the structural dispersion of a parallel-plate waveguide by varying boundary conditions will induce an effective temporal interface. The proposed concept can be extended to temporally controlled metasurfaces, opening an easier path to the design and realization of novel devices based on time-varying metamaterials at microwave and optical frequencies.

We have performed ${}^{53}\mathrm{Cr}$ NMR measurements on a high-purity polycrystalline sample to investigate the static and dynamic properties of a half-metallic ferromagnet ${\mathrm{CrO}}_2$. Two ${}^{53}\mathrm{Cr}$ NMR lines, corresponding to magnetically nonequivalent Cr nuclei, were observed in the ferromagnetic phase of ${\mathrm{CrO}}_2$ despite all of the Cr ions being situated on the crystallographic equivalent sites. We measured the temperature dependences of the ${}^{53}\mathrm{Cr}$ spin-lattice relaxation rate ${\left(T_1\right)}^{-1}$ in the ferromagnetic phase for the temperature range $T=4.2–360\mathrm{K}$. It was found that in the range of low temperatures ($T\le 60\mathrm{K}$) the relaxation of nuclear magnetic moments is determined mainly by the orbital contribution proportional to the temperature, conditioned by the fluctuation of the orbital currents of $d$-band electrons. At temperatures $T>60\mathrm{K}$, the main mechanism leading to the nuclear spin-lattice relaxation is a three-magnon process of scattering at which the relaxation of the nuclear spin is accompanied by the absorption of a magnon and the creation of two magnons. Based on the analysis of temperature dependences of ${\left(T_1\right)}^{-1}$ for two nonequivalent Cr ions, we found that their valence state is the same and corresponds to valence ${\mathrm{Cr}}^{4+}$, whereas the difference of resonance frequencies for these ion nuclei is conditioned by the different magnetic local fields in their location.

We examine the dynamics of a $(1+1)$-dimensional measurement-only circuit defined by the stabilizers of the [[5,1,3]] quantum error correcting code interrupted by single-qubit Pauli measurements. The code corrects arbitrary single-qubit errors and it stabilizes an area law entangled state with a $D_2={\mathbb{Z}}_2\times {\mathbb{Z}}_2$ symmetry protected topological (SPT) order, as well as a symmetry breaking (SB) order from a twofold bulk degeneracy. The Pauli measurements break the topological order and induce a phase transition into a trivial area law phase. Allowing more than one type of Pauli measurement increases the measurement-induced frustration and the SPT and SB order can be broken either simultaneously or separately at nonzero measurement rate. This yields a rich phase diagram and unanticipated critical behavior at the phase transitions. Although the correlation length exponent $\nu =\frac{4}{3}$ and the dynamical critical exponent $z=1$ are consistent with bond percolation, the prefactor of the logarithmic entanglement growth may take noninteger multiples of the percolation value. Remarkably, we identify a robust transient scaling regime for the purification dynamics of $L$ qubits. It reveals a modified dynamical critical exponent $z^{*}\ne z$, which is observable up to times $t\sim L^{z^{*}}$ and is reminiscent of the relaxation of critical systems into a prethermal state.

We present a comprehensive study of static and dynamic magnetic properties in the Ising-like dipolar antiferromagnet (AFM) ${\mathrm{DyScO}}_3$ by means of DC and AC magnetization measurements supported by classical Monte Carlo calculations. Our AC-susceptibility data show that the magnetic dynamics exhibit a clear crossover from an Arrhenius-like regime to quantum tunneling of magnetization (QTM) at $T^{*}=10\mathrm{K}$. Below $T_{\mathrm{N}}=3.2\mathrm{K}, {\mathrm{DyScO}}_3$ orders in an antiferromagnetic $GxAy$-type magnetic structure and the magnetization dynamics slow down to the minute timescale. The low-temperature magnetization curves exhibit complex hysteretic behavior, which depends strongly on the magnetic field sweep rate. We demonstrate that the low-field anomalies on the magnetization curve are related to the metamagnetic transition, while the hysteresis at higher fields is induced by a strong magnetocaloric effect. Our theoretical calculations, which take into account dipolar interaction between ${\mathrm{Dy}}^{3+}$ moments, reproduce essential features of the magnetic behavior of ${\mathrm{DyScO}}_3$. We demonstrate that ${\mathrm{DyScO}}_3$ represents a rare example of an inorganic compound, which exhibits QTM at a single-ion level and magnetic order due to classical dipolar interaction.

Topological materials hosting kagome lattices have drawn considerable attention due to the interplay between topology, magnetism, and electronic correlations. The $\left({\mathrm{Fe}}_{1-x}{\mathrm{Co}}_x\right)\mathrm{Sn}$ system not only hosts a kagome lattice but has a tunable symmetry-breaking magnetic moment with temperature and doping. In this study, angle-resolved photoemission spectroscopy and first-principles calculations are used to investigate the interplay between the topological electronic structure and varying magnetic moment from the planar to axial antiferromagnetic phases. Evidence for a theoretically predicted gap at the Dirac point is revealed in the low-temperature axial phase, but no gap opening is observed across a temperature-dependent magnetic phase transition. However, topological surface bands are observed to shift in energy as the surface magnetic moment is reduced or becomes disordered over time during experimental measurements. The shifting surface bands may preclude the determination of a temperature-dependent bulk gap, but this highlights the intricate connections between magnetism and topology with a surface/bulk dichotomy that can affect material properties and their interrogation.