The projection of time-dependent variational principle (TDVP) for matrix product states enables us to perform long-time simulations of one-dimensional quantum systems with the conservation of the total energy and the norm of wave functions. We compare long-time dynamics after a quantum quench simulated by TDVP with those by the exact diagonalization method in order to evaluate the performance of TDVP. We show that in a nonintegrable model the projection of TDVP clearly improves the long-time behaviors of global observables included in the Hamiltonian, such as the kinetic and interaction energies. In contrast, this projection can lead to larger error for other observables than that caused by the truncation of states.

Based on atomistic spin dynamics simulations, we report the ultrafast generation of single antiferromagnetic (AFM) skyrmions in a confined geometry. This process is achieved through an effective magnetic field induced by the athermal inverse Faraday effect from a short laser pulse. The resulting field can nucleate an isolated skyrmion as a topologically protected metastable state in a collinear antiferromagnet with small Dzyaloshinskii-Moriya interaction. The radius of a single skyrmion is shown to increase by applying a uniform dc magnetic field and at increasing temperature. To investigate possible AFM spin-caloritronics phenomena, we investigate the skyrmion dynamics under an applied temperature gradient both analytically and numerically. The antiferromagnetic skyrmions move longitudinally toward the hotter region, but in contrast, small skyrmions in the very low damping regime move toward the colder side, irrespective of the staggered topological charge number, with a speed that is much faster than that of their ferromagnetic counterparts.

We demonstrate laser-induced nonreciprocity of spin waves in the ferromagnetic-semiconductor structure. Surface spin waves in yttrium iron garnet film grown at the top of $n$-type gallium arsenide substrate were studied by means of Brillouin light-scattering spectroscopy. It is shown that spin-wave dispersion can be modified in a controlled manner by laser radiation. We observe the difference of up to 225 MHz when comparing the frequencies of counterpropagating spin waves. We attribute this frequency shift to the mutual influence of nonreciprocal spin-wave modal profiles and differences in magnetic anisotropies at two film surfaces as the result of laser-induced conductivity variation in GaAs substrate. We propose a simple model based on analytical dipole theory to describe the induced spin-wave nonreciprocity. Our results show the possibility of integration of magnonics and semiconductor electronics on the base of YIG/GaAs structures.

By means of experimental positron-annihilation-lifetime measurements and theoretical density functional theory (DFT) positron-lifetime calculations, vacancy-type defects in ${\mathrm{Ni}}_{50}{\mathrm{Mn}}_{50-x}{\mathrm{Sn}}_x$ ($x=25,20,15,13,10$) and ${\mathrm{Ni}}_{50}{\mathrm{Mn}}_{50-x}{\mathrm{In}}_x$ ($x=25,20,16,13$) systems are systematically studied. The study is extended to Ni-Mn-Ga systems as well. Experimental results are complemented with electron-positron DFT calculations carried out within the local density approximation and generalized gradient approximation, where five different parametrizations accounting for the $\gamma \left(\mathbf{r}\right)$ enhancement factor are analyzed. Theoretical results indicate that the Boronski-Nieminen parametrization of $\gamma \left(\mathbf{r}\right)$ is the one that best predicts the experimental results, which ultimately enables us to identify ${\mathrm{V}}_{\mathrm{Ni}}$ as the vacancy present in the studied samples. The characteristic positron lifetime related to ${\mathrm{V}}_{\mathrm{Ni}}$ ranges between 181 and 191 ps in Ni-Mn-Sn/In systems. Positron-annihilation-lifetime spectroscopy results in these two systems delimit the lower bound of the achievable vacancy concentration, which is much larger compared with the reported values in Ni-Mn-Ga systems. The present work, along with setting the basis for positron simulations in Ni-Mn based Heusler alloys, delimits the effect that the variation of vacancies has in the martensitic transformation in Ni-Mn-Sn systems.

The characterization of entanglement is a central problem for the study of quantum many-body dynamics. Here, we propose the quantum Fisher information (QFI) as a useful tool for the study of multipartite-entanglement dynamics in many-body systems. We illustrate this by considering the regular-to-ergodic transition in the Dicke model—a fully connected spin model showing quantum thermalization above a critical interaction strength. We show that the QFI has a rich dynamical behavior which drastically changes across the transition. In particular, the asymptotic value of the QFI as well as its characteristic timescales witness the transition both through their dependence on the interaction strength and through the scaling with the system size. Since the QFI also sets the ultimate bound for the precision of parameter estimation, it provides a metrological perspective on the characterization of entanglement dynamics in many-body systems. Here, we show that quantum ergodic dynamics allows for a much faster production of metrologically useful states.

Transmission through a (100)-oriented single crystal of bismuth ferrite $\left({\mathrm{BiFeO}}_3\right)$ was studied at frequencies of 0.485–0.765 THz with a continuous-wave source and in the temperature range of 30–350 K as a function of polarization angle. The fine splitting of spin-wave modes that could be detected with our vector network analyzer-based spectrometer is shown to depend on polarization angle. The crystal shows strong, temperature-dependent, linear dichroism. The interference pattern produced in the sample slab revealed a huge birefringence. The fine splitting dependence on polarization angle suggests correlation with the observed dichroism and the birefringence.

Critical points and phase transitions are characterized by diverging susceptibilities, reflecting the tendency of the system toward spontaneous symmetry breaking. Equilibrium statistical mechanics bounds these instabilities to occur at zero frequency, giving rise to static order parameters. In this work we argue that a prototype model of correlated driven-dissipative lattice bosons, of direct relevance for upcoming generation of circuit QED arrays experiments, exhibits a susceptibility sharply diverging at a finite nonzero frequency, which is an emerging scale set by interactions and nonequilibrium effects. In the broken-symmetry phase the corresponding macroscopic order parameter becomes nonstationary and oscillates in time without damping, thus breaking continuous time-translational symmetry. Our work, connecting breaking of time translational invariance to divergent finite frequency susceptibilities, which are of direct physical relevance, could potentially be extended to study other time-domain instabilities in nonequilibrium quantum systems, including Floquet time crystals and quantum synchronization.

Reducing the size of a superconductor below its characteristic length scales can either enhance or suppress its critical temperature ($T_c$). Depending on the bulk value of the electron-phonon coupling strength, electronic and phonon confinement effects will play different roles in the modification of $T_c$. Experimentally disentangling each contribution has remained a challenge. We have measured both the phonon density of states and $T_c$ of Sn nanowires with diameters of 18, 35, and 100 nm in order to quantify the effects of phonon confinement on superconductivity. We observe a shift of the phonon frequency towards the low-energy region and an increase in the electron-phonon coupling constant that can account for the measured increase in $T_c$.

We report on the symmetry-dependent Raman scattering studies of a FeSe single crystal over a broad frequency range of $60-7000{\mathrm{cm}}^{-1}$. Our experimental data feature abundant excitation spectra of phonon, spin, charge, and electronic degrees of freedom. The $207\text{−}{\mathrm{cm}}^{-1}{B}_{1g}$ mode, involving the out-of-plane vibrations of the Fe ions, undergoes a large frequency shift by $14{\mathrm{cm}}^{-1}$ due to orbital-phonon coupling. The Raman spectra are well separated into strongly symmetry-dependent excitations at energies below $3000{\mathrm{cm}}^{-1}$ and weakly symmetry-dependent electron-hole excitations above $3000{\mathrm{cm}}^{-1}$. This is taken as evidence for the coexistence of itinerant and localized spins. A prominent low-energy Raman response peaked at $531{\mathrm{cm}}^{-1}$ observed only in $B_{2g}$ symmetry is interpreted in terms of coupled spin and charge quadrupole fluctuations. Our Raman results suggest that the dichotomic character of electrons together with the small energy scale involved in orbital ordering of the degenerated $d_{xz}$ and $d_{yz}$ orbitals promotes the intriguing physics of FeSe.

We report on quantum emission from Pb-related color centers in diamond following ion implantation and high-temperature vacuum annealing. First-principles calculations predict a negatively charged Pb-vacancy (PbV) center in a split-vacancy configuration, with a zero-phonon transition around 2.4 eV. Cryogenic photoluminescence measurements performed on emitters in nanofabricated pillars reveal several transitions, including a prominent doublet near 520 nm. The splitting of this doublet, 5.7 THz, exceeds that reported for other group-IV centers. These observations are consistent with the PbV center, which is expected to have a combination of narrow optical transitions and stable spin states, making it a promising system for quantum network nodes.

Characterizing charge noise is of prime importance to the semiconductor spin qubit community. We analyze the echo amplitude data from a recent experiment [Yoneda et al., Nat. Nanotechnol. 13, 102 (2018)] and note that the data show small but consistent deviations from a $1/f^{\alpha }$ noise power spectrum at the higher frequencies in the measured range. We report the results of using a physical noise model based on two-level fluctuators to fit the data and find that it can mostly explain the deviations. While our results are suggestive rather than conclusive, they provide what may be an early indication of a high-frequency cutoff in the charge noise. The location of this cutoff, where the power spectral density of the noise gradually rolls off from $1/f$ to $1/f^2$, crucial knowledge for designing precise qubit control pulses, is given by our fit of the data to be around 200 kHz.

Synchrotron-radiation angle-resolved and core-level photoemission spectroscopy are used together to investigate the origin of the two-dimensional (2D) electron gas on the surface of single-crystal CdO (100) films. A reduction in the two-dimensional electron density of the surface state is observed under the synchrotron beam during angle-resolved photoemission spectroscopy, which is shown to be accompanied by a concomitant reduction in the surface-adsorbed species (monitored through the O $1s$ core-level signal). This shows that surface adsorbates donate electrons into the surface accumulation layer. When the surface is cleaned, the surface conduction band state empties. A surface doped with atomic H is also studied. Here, interstitial H increases the two-dimensional electron density at the surface. This demonstrates that reversible donor doping is possible. The surface band-bending profiles, 2D electron densities, and effective masses are calculated from subband dispersion simulations.

We demonstrate the ability of an epitaxial semiconductor-superconductor nanowire to serve as a field-effect switch to tune a superconducting cavity. Two superconducting gatemon qubits are coupled to the cavity, which acts as a quantum bus. Using a gate voltage to control the superconducting switch yields up to a factor of 8 change in qubit-qubit coupling between the on and off states without detrimental effect on qubit coherence. High-bandwidth operation of the coupling switch on nanosecond time scales degrades qubit coherence.

We apply support vector machine (SVM) to study the phase transition between many-body localized and thermal phases in a disordered quantum Ising chain in a transverse external field. The many-body eigenstate energy $E$ is bounded by a bandwidth $W=E_{\text{max}}-E_{\text{min}}$. The transition takes place on a phase diagram spanned by the energy density $\epsilon =2(E-E_{\text{min}})/W$ and the disorder strength $\delta J$ of the spin interaction uniformly distributed within $[-\delta J,\delta J]$, formally parallel to the mobility edge in Anderson localization. In our paper, we use the labeled probability density of eigenstate wave functions belonging to the deeply localized and thermal regimes at two different energy densities ($\epsilon$'s) as the training set, i.e., providing labeled data at four corners of the phase diagram. Then we employ the trained SVM to predict the whole phase diagram. The obtained phase boundary qualitatively agrees with previous work using entanglement entropy to characterize these two phases. We further analyze the decision function of the SVM to interpret its physical meaning and find that it is analogous to the inverse participation ratio in configuration space. Our findings demonstrate the ability of the SVM to capture potential quantities that may characterize the many-body localization phase transition.

The vibrational properties of three sodosilicate glasses have been investigated in the framework of density functional theory. The pure vibrational density of states has been calculated for all systems and the different vibrational modes have been assigned to specific atoms or structural units. It is shown that the Na content affects several vibrational features as the position and intensity of the $R$ band or the mixing of the rocking and bending atomic motions of the Si-O-Si bridges. The calculated Raman spectra have been found to agree with experimental observations and their decomposition indicated the dominant character of the nonbridging oxygen contribution on the spectra, in particular for the high-frequency band, above $800{\mathrm{cm}}^{-1}$. The decomposition of the high-frequency Raman feature into vibrations of the depolymerized tetrahedra (i.e., $Q_n$ units) has revealed spectral shapes of the partial contributions that cannot be accounted for by simple Gaussians as frequently assumed in the treatment of experimentally obtained Raman spectra.

We show that the two-dimensional spatial-inversion-symmetry (SIS) broken square-lattice antiferromagnet with easy-plane spin anisotropy exhibits a thermal Hall effect and the edge modes characterized by the ${\mathbb{Z}}_2$ topological invariant. These topological properties require a nonzero Berry curvature, and its origin is ascribed to the Dzyaloshinskii-Moriya (DM) interactions or the noncoplanar magnetic ordering generating a U(1) gauge field that couples to the kinetic motion of magnons. Although this picture is established in ferromagnets on the kagome and pyrochlore lattices, it does not apply to our square-lattice model since such gauge field cancels out in an edge-shared geometry. Instead, our case has an analogy with the anomalous Hall effect of Rashba electronic system where the spin-orbit coupling generates an SU(2) gauge field. The two species of magnons defined on antiferromagnetic sublattices can be regarded as the pseudospin degrees of freedom of magnons. The DM interactions that emerge due to the SIS breaking serve as a pseudo-spin-orbit coupling of magnons and generate a Berry curvature, when the direction of the magnetic moments is properly controlled by the magnetic field. The thermal Hall conductivity of our antiferromagnet shows rapid growth in temperature $T$ beyond the power law, reflecting the almost gapless low-energy branch of the antiferromagnet, which distinctively differs from the $T^{7/2}$ dependence in the pyrochlore ferromagnets. The present system serves as a standard model for the noncentrosymmetric crystals ${\mathrm{Ba}}_2{\mathrm{MnGe}}_2{\mathrm{O}}_7$ and ${\mathrm{Ba}}_2{\mathrm{CoGe}}_2{\mathrm{O}}_7$.

Manipulation of quantum systems is the basis for many promising quantum technologies. However, how quantum mechanical principles can be used to manipulate the dynamics of quantum dissipative systems remains unanswered because of strong decoherence effects arising from interaction with the surrounding environment. In this work, we demonstrate that electron transfer dynamics in molecular loop structures can be manipulated with the use of Floquet engineering by applying a laser field. Despite strong dephasing, the system's dynamics spontaneously breaks the chiral symmetry of the loop in a controllable fashion, followed by the generation of a robust steady-state electronic current without an external voltage. An exponential scaling law that relates the magnitude of the current to the system-environment coupling strength is revealed numerically. The breaking of chiral symmetry and the consequent controllable unidirectional flow of electrons could be employed to construct functional molecular electronic circuits.

Due to their possibility to encode information and realize low-energy-consumption quantum devices, control and manipulation of the valley degree of freedom have been widely studied in electronic systems. In contrast, the phononic counterpart—valley phononics—has been largely unexplored, despite the importance in both fundamental science and practical applications. In this work, we demonstrate that the control of “valleys” is also applicable for phonons in graphene by using a grain boundary. In particular, perfect valley filtering effect is observed at certain energy windows for flexural modes and found to be closely related to the anisotropy of phonon valley pockets. Moreover, valley filtering may be further improved using Fano-like resonance. Our findings reveal the possibility of valley phononics, paving the road towards purposeful phonon engineering and future valley phononics.

${\mathrm{NaCrGe}}_2{\mathrm{O}}_6$ and ${\mathrm{NaCrSi}}_2{\mathrm{O}}_6$ are isostructural compounds exhibiting different magnetic ground states. ${\mathrm{NaCrGe}}_2{\mathrm{O}}_6$ adopts a ferromagnetic ground state with $Tc=6\mathrm{K}$, whereas ${\mathrm{NaCrSi}}_2{\mathrm{O}}_6$ orders antiferromagnetically below $T_N=3.4\mathrm{K}$. Although it has been proposed that the intriguing magnetic behavior in Cr-based pyroxenes involves competition between antiferromagnetic direct exchange and ferromagnetic superexchange interactions, a delicate balance that is sensitive to Cr-Cr distance and local distortion, no spectroscopy study has been done to determine the microscopic interactions in these compounds. To delve deeper into the evolution from ferromagnetism to antiferromagnetism, we performed a doping-dependent study to investigate how the substitution of Ge by Si affects the magnetic properties of ${\mathrm{NaCrSi}}_x{\mathrm{Ge}}_{2-x}{\mathrm{O}}_6$ ($x=0$, 0.5, 1, 1.5, 2). Neutron diffraction and magnetization measurements show that replacing larger Ge with smaller Si simultaneously suppresses the ferromagnetic order. The lattice constants and the unit-cell volume contract, i.e., chemical pressure effect, and the Cr-Cr distance within the chain gradually decreases with increasing Si doping. High-resolution inelastic neutron-scattering studies of the spin waves of ${\mathrm{NaCrGe}}_2{\mathrm{O}}_6$ and ${\mathrm{NaCrSi}}_2{\mathrm{O}}_6$ indicate that replacing Ge with Si has profound effect on the intrachain coupling, whereas it has negligible effect on the interchain couplings. We compare our results, which indicate ${\mathrm{NaCrGe}}_2{\mathrm{O}}_6$ is magnetic quasi-one-dimensional (1D) and ${\mathrm{NaCrSi}}_2{\mathrm{O}}_6$ is three-dimensional (3D), with $\mathrm{LiCr}{(\mathrm{Si},\mathrm{Ge})}_2{\mathrm{O}}_6$, where ${\mathrm{LiCrSi}}_2{\mathrm{O}}_6$ is proposed to be magnetic quasi-1D and ${\mathrm{LiCrGe}}_2{\mathrm{O}}_6$ is 3D, and discuss the different behaviors in magnetic dimensionality crossover in the context of how substituting Ge with Si fine-tunes the relative ratio between the intrachain and interchain couplings that defines the magnetic dimensionality in these materials.