We have measured the resistivity of magnetically doped Ag:Mn mesoscopic wires as a function of temperature and magnetic field. The doping has been made using ion implantation, allowing a distribution of the dopants in the middle of the sample. Comparison with an undoped sample, used as a reference sample, shows that the resistivity of the doped sample exhibits nonmonotonic behavior as a function of both magnetic field and temperature, revealing the competition between the Kondo effect and the RKKY interactions between spins. This proves that transport measurements are still a reliable probe of the spin-glass transition in nanoscopic metallic wire doped using implantation.

Topological nodal-line semimetals support protected band crossings, which form nodal lines or nodal loops between the valence and conduction bands and exhibit novel transport phenomena. Here we address the topological state of the nodal-line semimetal candidate material CaCdSn, and report magnetotransport properties of its single crystals grown by the self-flux method. Our first-principles calculations show that the electronic structure of CaCdSn harbors a single nodal loop around the $\mathrm{\Gamma }$ point in the absence of spin-orbit coupling effects. The nodal crossings in CaCdSn are found to lie above the Fermi level and yield a Fermi surface that consists of both electron and hole pockets. CaCdSn exhibits high mobility ($\mu \approx 3.44\times {10}^4{\mathrm{cm}}^2{\mathrm{V}}^{-1}{\mathrm{s}}^{-1}$) and displays a field-induced metal-semiconductor-like crossover with a plateau in resistivity at low temperature. We observe an extremely large and quasilinear nonsaturating transverse as well as longitudinal magnetoresistance (MR) at low temperatures ($\approx 7.44\times {10}^3\%$ and $\approx 1.71\times {10}^3\%$, respectively, at 4 K). We also briefly discuss possible reasons behind such a large quasilinear magnetoresistance and its connection with the nontrivial band structure of CaCdSn.

An investigation of the unoccupied electronic structure of the transition-metal dichalcogenide tin disulphide has been conducted using core-hole clock spectroscopy. Polarization-dependent x-ray absorption in the tender x-ray regime at the S K edge and maps of the resonant Auger spectra in the S KLL Auger kinetic energy range have been recorded. Supported with ab initio calculations of the unoccupied electronic structure, these allow us to relate resonances in the absorption cross section to excitations along various directions in the Brillouin zone. We observe anisotropy in the x-ray absorption cross section in polarization directions in plane and out of plane of the crystal. There is also anisotropy in the charge transfer dynamics as inferred from the coherent and noncoherent parts of the resonant Auger spectra. This approach can be generally used to interpret dynamics in unoccupied states, e.g., in layered structure or heterogenous interfaces.

In addition to the known pseudomagnetic field, nonuniform strains independently induce a position-dependent Fermi velocity (PDFV) in graphene. Here we demonstrate that, due to the presence of a PDFV, the Dirac fermions on a nonuniform (strained) honeycomb lattice may experiment a sort of magnetic effect, which is linearly proportional to the momentum of the quasiparticle. As a consequence, the quasiparticles have a sublinear dispersion relation. Moreover, we analyze the general consequence of a PDFV on the Klein tunneling of electrons through pseudomagnetic barriers. In particular, we report an anomalous (Klein) tunneling for an electron passing across velocity barriers with magnetic features. Our findings about the effects induced by a PDFV on Dirac fermions in a [two-dimensional (2D)] strained honeycomb lattice could be extended to (3D) Dirac and Weyl semimetals and/or its analogous artificial systems.

The subgap transport in electrostatically gapped bilayer graphene (BLG) is explored by adopting ultrashort constrictions of BLG. We confirmed the emergence of the edge conducting channels, with a residual conductance of $\sim 2e^2/h$ per edge of realistic disorder. Transport along the edge state exhibits either Coulomb blockade oscillations or Fabry-Perot interference, depending on the interfacial transparency between the reservoirs and the edge channel. Both residual conductance and band-gap dependence of the penetration range of the edge state from a constriction edge coincide well with the behavior of the theoretically predicted robust edge conducting channel, which forms irrespective of the degree of edge disorder.

We report on the far-infrared magnetospectroscopy of HgTe quantum wells with inverted band ordering at different electron concentrations. We particularly focus on optical transitions from zero-mode Landau levels, which split from the edges of electronlike and holelike bands. We observe a pronounced dependence of the transition energies on the electron concentration varied by persistent photoconductivity effect. This is striking evidence that in addition to the already well-documented crystalline and interface asymmetries, electron-electron interactions also have a significant impact on the usual behavior of the optical transitions from zero-mode Landau levels.

We develop an extension of the Gutzwiller approximation (GA) formalism that includes the effects of Coulomb interactions of arbitrary range (including density density, exchange, pair hopping, and Coulomb-assisted hopping terms). This formalism reduces to the ordinary GA formalism for the multiband Hubbard models in the presence of only local interactions. This is accomplished by combining the $1/z$ expansion—where $z$ is the coordination number, and only the leading-order terms contribute in the limit of infinite dimensions—with a $P_R^{†}{P}_R-I$ expansion, where $P_R$ is the Gutzwiller projector on the site $R$. The method is conveniently formulated in terms of a Gutzwiller Lagrange function. We apply our theory to the extended single-band Hubbard model. Similarly to the usual Brinkman-Rice mechanism, we find a Mott transition. A valence skipping transition is observed, where the occupation of the empty and doubly occupied states for the Gutzwiller wave function is enhanced with respect to the uncorrelated Slater determinant wave function.

We present a theoretical study exposing the dominant microscopic electronic transport mechanisms underlying a recent molecular spin-transistor experiment [C. Godfrin et al., ACS Nano 11, 3984 (2017)], where purely electrical readout of the spin of a Tb(III)-based single-molecule magnet was achieved. To identify the relevant spin-to-charge conversion mechanisms enabling opposite spin polarizations of the Tb(III) ion $4f$ electrons to generate different magnetoconductance responses, we investigate both incoherent sequential tunneling charge transport, and coherent cotunneling corrections. Contrary to previous interpretations invoking the highly coherent Kondo transport regime, we find that all reported experimental observations, including the temperature and magnetic field dependence of the differential conductance, can be reproduced reasonably well within a sequential tunneling transport regime explicitly accounting for broadening of the device energy levels due to molecule-lead coupling.

The utilization of periodic structures such as photonic crystals and metasurfaces is common for light manipulation at nanoscales. One of the most widely used computational approaches to consider them and design effective optical devices is the Fourier modal method (FMM) based on Fourier decomposition of electromagnetic fields. Nevertheless, calculating periodic structures with small inclusions is often a difficult task since they induce lots of high-$k_{\parallel }$ harmonics that should be taken into account. In this paper, we consider small-particle lattices with bases (complex unit cells) and construct their scattering matrices via discrete dipole approximation. Afterwards, these matrices are implemented in FMM for consideration of complicated layered structures. We show the performance of the proposed hybrid approach by its application to a lattice, which routes left and right circularly polarized incident light to guided modes propagating in opposite directions. We also demonstrate its precision by spectra comparison with finite-element method calculations. The high speed and precision of this approach enable the calculation of angle-dependent spectra with very high resolution in a reasonable time, which allows resolving narrow lines unobservable by other methods.

A basic result on the dynamics of spinless quantum systems is that the Maryland model exhibits dynamical localization in any dimension. Here we implement mathematical spectral theory and numerical experiments to show that this result does not hold when the two-dimensional Maryland model is endowed with spin 1/2—hereafter dubbed spin-Maryland (SM) model. Instead, in a family of SM models, tuning the (effective) Planck constant drives dynamical localization-delocalization transitions of topological nature. These transitions are triggered by the self-duality, a symmetry generated by some transformation in the parameter—the inverse Planck constant—space. This provides significant insights into new dynamical phenomena such as what occur in the spinful quantum kicked rotor.

We investigate the dynamics of two-dimensional quantum spin systems under the combined effect of random unitary gates and local projective measurements. When considering steady states, a measurement-induced transition occurs between two distinct dynamical phases, one characterized by a volume-law scaling of entanglement entropy, the other by an area law. Employing stabilizer states and Clifford random unitary gates, we numerically investigate square lattices of linear dimension up to $L=48$ for two distinct measurement protocols. For both protocols, we observe a transition point where the dominant contribution in the entanglement entropy displays multiplicative logarithmic violations to the area law. We obtain estimates of the correlation length critical exponent at the percent level; these estimates suggest universal behavior and are incompatible with the universality class of 3D percolation.

In this study, we report the observation of ultralow Gilbert damping in epitaxial ${\mathrm{CrO}}_2$ films. The dynamic properties of (100)- and (110)-oriented ${\mathrm{CrO}}_2$ epitaxial films grown on ${\mathrm{TiO}}_2$ substrates were studied using ferromagnetic resonance measurements based on a resonant cavity and a coplanar waveguide in a large frequency range. The Lande $g$ factor was found to be 1.98, and it was independent of film orientation and thickness. The effective damping constant rapidly increased with film thickness when the film thickness was smaller than 50 nm, which might be attributed to magnon scattering. Extremely low damping was observed in the (110)-oriented ${\mathrm{CrO}}_2$ film with a thickness of 364 nm, and the damping constant was obtained as $\left(6.2\pm 0.4\right)\times {10}^{-4}$. This value is about half an order of magnitude lower than that of ultralow-damping CoFe systems $[\left(1.3\text{–}2.0\right)\times {10}^{-3}]$ and is comparable with the lowest value observed recently in some Heusler alloy systems. The extremely low damping behavior in the ${\mathrm{CrO}}_2$ system is strongly correlated with its half-metallic nature.

We implement the Monte Carlo renormalization group approach in the continuous-time Monte Carlo simulation of a quantum system. We demonstrate numerically the emergent isotropy between space and time at large distances for the systems that exhibit Lorentz invariance at quantum criticality. This allows us to estimate accurately the sound velocity for these quantum systems. $Q$-state Potts models in one and two space dimensions are used to illustrate the method.

Magnetic anisotropy plays an essential role in information technology applications of magnetic materials, providing a means to retain the long-term stability of a magnetic state in the presence of thermal fluctuations. Anisotropy consists of a single-ion contribution stemming from the crystal structure and two-ion terms attributed to the exchange interactions between magnetic atoms. A lack of robust theory crucially limits the understanding of the temperature dependence of the anisotropy in pure two-ion and mixed single-ion and two-ion systems. Here, we use Green's function theory and atomistic Monte Carlo simulations to determine the temperature scaling of the effective anisotropy in ferromagnets in these pure and mixed cases, from saturated to vanishing magnetization. At low temperature, we find that the pure two-ion anisotropy scales with the reduced magnetization as $k\left(m\right)\sim m^{2.28}$, while the mixed scenario describes the diversity of the temperature dependence of the anisotropy observed in real materials. The deviation of the scaling exponent of the mixed anisotropy from previous mean-field results is ascribed to correlated thermal spin fluctuations, and its value determined here is expected to considerably contribute to the understanding and the control of the thermal properties of magnetic materials.

The Nd magnet, ${\mathrm{Nd}}_2{\mathrm{Fe}}_{14}\mathrm{B}$, is an important material because of its high coercivity applied in modern technologies. However, the microscopic mechanism of the coercivity has not been well understood. We study the magnetization reversal of a single grain of the magnet at a finite temperature by a real-time stochastic Landau-Lifshitz-Gilbert simulation of an atomistic model, which enables us to analyze dynamical properties reflecting the atomic-scale magnetic structure. There exist difficulties to estimate long relaxation times of the reversal quantitatively, i.e., the limitation of simulation time and also dependence on the damping factor $\alpha$. Here we develop a statistical method to estimate precisely long relaxation times in the stochastic region, by which one can identify an initial transient process and a long-time regular relaxation process. The relaxation time is found to largely depend on $\alpha$ especially in the stochastic region. However, it is found that a sharp increase of the relaxation time with lowering an external magnetic field causes a close location of the threshold fields for different values of $\alpha$. By making use of this fact, we quantitatively estimate the coercive field at which the relaxation time is 1 s.

Space-time modulation has recently attracted considerable attention as it enables new possibilities in wave manipulation and control. Here we propose a tunable unidirectional acoustic amplifier based on a space-time modulated membrane system. The transfer matrix method is applied to design this system and optimize the modulation parameters for the realization of unidirectional amplification. Efficient one-way amplification of acoustic waves is demonstrated theoretically and numerically within a frequency range from 2260 to 2520 Hz, with overall dimension being less than $1/3$ of the corresponding wavelength. Across this operation band, the amplification factor in the positive direction varies smoothly from 3.0 to 5.8, while the transmitted wave energy in the negative direction does not change significantly. Our work identifies a feasible approach to realize a tunable unidirectional acoustic amplifier via space-time modulated membranes, which offers a design platform for a number of applications in sensing, imaging, and communication.

In a quantum many-body system, dimensionality plays a critical role in determining the type of quantum phase transition. To study the quantum system during dimensional crossover, we studied the Bose-Hubbard model on a cubic lattice with anisotropic hopping by using the high-order symbolic strong-coupling expansion method. The analytic series expanded boundaries between the Mott insulator and the superfluid phase up to eighth order are calculated. The critical exponents are extracted by the Padé resummation method, which clearly shows dimensional crossover behavior. Meanwhile, the critical points at commensurate filling can also be obtained, and they match well with the prediction of renormalization group theory. Finally, the scaling of the gap energy and the whole phase diagram are given, and they can be taken as the benchmark for experiment and numerical simulations in future study.

Recently, a new transition-metal dichalcogenide (TMD) material $2M$-phase ${\mathrm{WS}}_2$ was synthesized. The material $2M\text{−}{\mathrm{WS}}_2$ not only exhibits superconductivity, with the highest $T_{\text{c}}$ being 8.8 K at ambient pressure among TMDs, but also hosts a topological surface state. Here we report the low-temperature thermal conductivity measurements on $2M\text{−}{\mathrm{WS}}_2$ single crystals to investigate its superconducting gap structure. A negligible residual linear term ${\kappa }_0/T$ in zero field shows that $2M\text{−}{\mathrm{WS}}_2$ has a fully superconducting gap with all electrons paired. The field dependence of ${\kappa }_0/T$ suggests an anisotropic superconducting gap or multiple nodeless superconducting gaps in $2M\text{−}{\mathrm{WS}}_2$. Such a fully gapped superconducting state is compatible with $2M\text{−}{\mathrm{WS}}_2$ being a topological superconductor candidate.