To capture the high-field magnetization process of herbertsmithite ${[\mathrm{ZnCu}}_3{\left(\mathrm{OH}\right)}_6{\mathrm{Cl}}_2]$, Faraday rotation (FR) measurements were carried out on a single crystal in magnetic fields of up to 190 T. The magnetization data evaluated from the FR angle exhibited a saturation behavior above 150 T at low temperatures, which was attributed to the 1/3 magnetization plateau. The overall behavior of the magnetization process was reproduced by theoretical models based on the nearest-neighbor Heisenberg model. This suggests that herbertsmithite is a proximate kagome antiferromagnet hosting an ideal quantum spin liquid in the ground state. A distinguishing feature is the superlinear magnetization increase, which is in contrast to the Brillouin function-type increase observed by conventional magnetization measurements and indicates a reduced contribution from free spins located at the Zn sites to the FR signal.

We propose a mathematical model for describing radially propagating spin waves emitted from the core region in a magnetic patch with $n$ vertices in a magnetic vortex state. The azimuthal anisotropic propagation of surface spin waves (SSWs) into the domain, and confined spin waves (or Winter's magnons) in domain walls, increases the complexity of the magnonic landscape. In order to understand the spin wave propagation in these systems, we first use an approach based on geometrical curves called “hippopedes”; however, it provides no insight into the underlying physics. Analytical models rely on generalized expressions from the dispersion relation of SSWs with an arbitrary angle between magnetization M and wave number k. The derived algebraic expression for the azimuthal dispersion is found to be equivalent to that of the hippopede curves. The fitting curves from the model yield a spin wave wavelength for any given azimuthal direction, number of patch vertices, and excitation frequency, showing a connection with fundamental physics of exchange-dominated surface spin waves. Analytical results show good agreement with micromagnetic simulations and can be easily extrapolated to any $n$-corner patch geometry.

We present a neutron-scattering study of phonons in single crystals of ${\left({\mathrm{Pb}}_{0.5}{\mathrm{Sn}}_{0.5}\right)}_{1-x}{\mathrm{In}}_x\mathrm{Te}$ with $x=0$ (metallic, but nonsuperconducting) and $x=0.2$ (nonmetallic normal state, but superconducting). We map the phonon dispersions (more completely for $x=0$) and find general consistency with theoretical calculations, except for the transverse and longitudinal optical (LO) modes at the Brillouin-zone center. At low temperature, both modes are strongly damped but sit at a finite energy ($\approx 4\mathrm{meV}$ in both samples), shifting to higher energy at room temperature. These modes are soft due to a proximate structural instability driven by the sensitivity of Pb-Te and Sn-Te $p$-orbital hybridization to off-center displacements of the metal atoms. The impact of the soft optical modes on the low-energy acoustic modes is inferred from the low thermal conductivity, especially at low temperature. Given that the strongest electron-phonon coupling is predicted for the LO mode, which should be similar for both studied compositions, it is intriguing that only the In-doped crystal is superconducting. In addition, we observe elastic diffuse (Huang) scattering that is qualitatively explained by the difference in Pb-Te and Sn-Te bond lengths within the lattice of randomly distributed Pb and Sn sites. We also confirm the presence of anomalous diffuse low-energy atomic vibrations that we speculatively attribute to local fluctuations of individual Pb atoms between off-center sites.

Doping is generally understood as a strategy for including additional positive or negative charge carriers in a semiconductor, thereby tuning the Fermi level and changing its electronic properties in the equilibrium limit. However, because dopants also couple to all of the microscopic degrees of freedom in the host, they may also alter the nonequilibrium dynamical properties of the parent material, especially at large dopant concentrations. Here, we show how substitutional doping by tungsten at the 1 at. % level modifies the complex electronic and lattice dynamics of the phase-change material vanadium dioxide. Using femtosecond broadband spectroscopy, we compare dynamics in epitaxial thin films of pristine and tungsten-doped $\mathrm{V}{\mathrm{O}}_2$ over the broadest wavelength and temporal ranges yet reported. We demonstrate that coupling of tungsten atoms to the host lattice modifies the early electron-phonon dynamics on a femtosecond timescale, altering in a counterintuitive way the ps-to-ns optical signatures of the phase transition. Density functional theory correctly captures the enthalpy difference between pristine and W-doped $\mathrm{V}{\mathrm{O}}_2$ and shows how the dopant softens critical V-V phonon modes while introducing new phononic modes due to W-V bonds. While substitutional doping provides a powerful method to control the switching threshold and contrast of phase-change materials, determining how the dopant dynamically changes the broadband optical response is equally important for optoelectronics.

We present a first-principles-based model of electron-phonon scattering mechanisms and thermoelectric transport at the $L$ and $\mathrm{\Sigma }$ valleys in $p$-type PbTe, accounting for their thermally induced shifts. Our calculated values of all thermoelectric transport parameters at room temperature are in very good agreement with experiments for a wide range of doping concentrations. Scattering due to longitudinal optical phonons is the main scattering mechanism in $p$-type PbTe, while scattering due to transverse optical modes is the weakest. The $L$ valleys contribute most to thermoelectric transport at 300 K due to the sizable energy difference between the $L$ and $\mathrm{\Sigma }$ valleys. We show that both scattering between the $L$ and $\mathrm{\Sigma }$ valleys and additional transport channels of the $\mathrm{\Sigma }$ valleys are beneficial for the overall thermoelectric performance of $p$-type PbTe at 300 K. Our findings thus support the idea that materials with high valley degeneracy may be good thermoelectrics.

Experimental evidence of resistance anomalies in the high-mobility two-dimensional electron gas (2DEG) formed in the GaAs/AlGaAs heterostructure, in the integer and fractional quantized Hall regime, is shown. The data complement to a good approximation the semianalytic calculations used to describe the formation of integral and fractional incompressible strips. The widths of current-carrying channels were calculated by incorporating the screening properties of the 2DEG and the effect of a magnetic field in the perpendicular mode. The many-body effects of the composite fermions are taken into consideration for the energy gap for the fractional states. It is shown that incompressible strips at the edges for both integer and fractional filling factors coexist in their evanescent phase for a particular range of magnetic fields, resulting in overshoot effects at the Hall resistance. Specifically, anomalous Hall resistances were noticed for filling factors $\nu$ = $\frac{4}{3}$, $\frac{3}{2}$, $\frac{5}{3}$, $\frac{8}{3}$, 3, $\frac{10}{3}$, $\frac{7}{2}$, and 5. This effect is explained and discussed using the screening theory.

Future quantum information devices will most likely rely on the realization of coherent light-matter interactions in strong coupling regime and the maintaining of the coherence with minimized incoherent damping pathways. Here, utilizing film-coupled planar nanoparticle supporting unique plasmon-induced magnetic resonance (PIMR), we theoretically study a strong coupling of a single nanocube to a monolayer of two-dimensional atomic crystal. We demonstrate that the nanocube-based planar configuration with magnetic flux passing through the dielectric layer exhibits much higher in-plane field confinement with respect to traditional plasmon electric modes, leading to an efficient coherent coupling to only a few in-plane excitonic dipoles with a record of normal mode splitting over 280 meV. Importantly, a much reduced incoherent coupling strength down to 3 meV is obtained. We reveal that the underlying mechanism lies in two main facts: (i) the small radiative damping rate of the excitonic system giving very limited contribution to the linewidth broadening of the A-exciton resonance, and (ii) the excitation of magnetic resonance provides a net electric dipole moment orientating mainly normal to the film surface, thus greatly suppressing the exchange of photons between the two subsystems via the continuum reservoir. The numerical simulations and theoretical analysis quantitatively evaluate the dependence of both the coherent and incoherent coupling strength on the thickness of the dielectric layer, revealing the fact that coherent/incoherent coupling strength can be simultaneously enhanced/suppressed with the decrease/increase of the film thickness, which can be appropriately designed to achieve the optimal coherent coupling with minimized incoherent coupling strength. Such hybrid nanostructure with simple geometry and ease of fabrication may not only offer as an attractive platform to explore light-matter interaction in the strong coupling regime but also show potential applications in realizing novel quantum and nanophotonic optical devices.

We theoretically describe how fast electrons couple to polaritonic modes in uniaxial materials by analyzing the electron energy-loss spectra. We show that in the case of a uniaxial medium with hyperbolic dispersion, bulk and surface modes can be excited by a fast electron traveling through the volume or along an infinite interface between the material and vacuum. Interestingly, and in contrast to excitations in isotropic materials, bulk modes can be excited by fast electrons traveling outside the uniaxial medium. We demonstrate our findings with the representative uniaxial material hexagonal boron nitride. We show that the excitation of bulk and surface phonon polariton modes is strongly related to the electron velocity and highly dependent on the angle between the electron beam trajectory and the optical axis of the material. Our work provides a systematic study for understanding bulk and surface polaritons excited by a fast electron beam in hyperbolic materials and sets a way to steer and control the propagation of the polaritonic waves by changing the electron velocity and its direction.

We study a network model on the kagome lattice (NMKL). This model generalizes the Chalker-Coddington network model for the integer quantum Hall transition. Unlike random network models we studied earlier, the geometry of the kagome lattice is regular. Therefore, we expect that the critical behavior of the NMKL should be the same as that of the Chalker-Coddington model. We numerically compute the localization length index $\nu$ in the NKML. Our result $\nu =2.658\pm 0.046$ is close to Chalker-Coddington model values obtained in a number of recent papers. We also map the NMKL to the Dirac fermions in random potentials and in a fixed periodic curvature background. The background turns out irrelevant at long scales. Our numerical and analytical results confirm our expectation of the universality of critical behavior on regular network models.

In one-dimensional chains of trapped Rydberg excitons in cuprous oxide semiconductor the topological spin phase has been recently predicted [Phys. Rev. Lett. 123, 126801 (2019)]. This phase is characterized by the diluted antiferromagnetic order of $p$-shell exciton angular momenta-1 and the edge states behaving akin to spin-$1/2$ fermions. Here we study the properties of the ground state in the finite chains and its fine structure resulting from the effective interaction of the edge spins. We demonstrate that these edge states can be detected optically via the enhancement of the circular polarization of the edge emission as compared with the emission from the bulk. We calculate the distribution of the exciton angular momentum vs. trap number in the chain numerically and analytically based on the variational ansatz.

Energy dispersion and spin orientation of the protected states at an interface between topological insulators and nontopological materials depend on the charge redistribution, strain, and atomic displacement at the interface. Knowledge of these properties is essential for applications of topological compounds, but direct access to them in the interface geometry is difficult. We show that conductance of a gated double junction at the surface of a topological insulator exhibits oscillations and a quasilinear decay as a function of gate voltage in different regimes. These give the values for the quasiparticle velocities along and normal to the junction in the interface region, and determine the symmetry of the topological interface states. The results are insensitive to the boundary conditions at the junction.

We use a combination of microwave fields and free precession to drive the spin of a nitrogen-vacancy (NV) center in diamond on different trajectories on the Bloch sphere and investigate the physical significance of the frame-dependent decomposition of the total phase into geometric and dynamic parts. The experiments are performed on a two-level subspace of the spin-1 ground state of the NV, where the Aharonov-Anandan geometric phase manifests itself as a global phase, and we use the third level of the NV ground-state triplet to detect it. We show that while the geometric Aharonov-Anandan phase retains its connection to the solid angle swept out by the evolving spin, it is generally accompanied by a dynamic phase that suppresses the geometric dependence of the system dynamics. These results offer insights into the physical significance of frame-dependent geometric phases.

We study the magnon-mediated pairing between fermions on the surface of a topological insulator (TI) coupled to a ferromagnetic insulator with a tilted mean field magnetization. Tilting the magnetization toward the interfacial plane reduces the magnetic band gap and leads to a shift in the effective TI dispersions. We derive and solve the self-consistency equation for the superconducting gap in two different situations, where we neglect or include the frequency dependence of the magnon propagator. Neglecting the frequency dependence results in $p$-wave Amperean solutions. We also find that tilting the magnetization into the interface plane favors Cooper pairs with center-of-mass momenta parallel to the magnetization vector, increasing $T_c$ compared to the out-of-plane case. Including the frequency dependence of the magnon propagator, and solving for a low number of Matsubara frequencies, we find that the eigenvectors of the Amperean solutions at the critical temperature are dominantly odd in frequency and even in momentum, thus opening the possibility for odd-frequency Amperean pairing.

In recent years, nonreciprocal transport measurement has emerged as a probe of spin orbit interaction, spin texture, superconductivity, and other fundamental properties of materials. Here, we study the nonlinear Hall effect (NLHE) in ${\mathrm{Bi}}_2{\mathrm{Se}}_3/\mathrm{CoFeB}$ heterostructures measured by second harmonic voltage method. Magneto-optical Kerr effect measurements demonstrate the origin of the NLHE to be the asymmetric magnon scattering mechanism. Moreover, a linear relation between the nonlinear Hall resistance and the electron phase coherence length of ${\mathrm{Bi}}_2{\mathrm{Se}}_3$ is observed between 5 and 50 K. We propose a phenomenological model that explains the enhancement of the NLHE below 50 K as a result of one-dimensional antilocalization of electrons from spin disorder, enabled by spin-momentum locking and conservation of angular momentum.

Using first-principles calculations and micromagnetic simulations, we systematically investigate the magnetic properties of two-dimensional Janus chromium dichalcogenides ($\mathrm{Cr}X\mathrm{Te}, X=\mathrm{S}$, Se) under strain. We find that the CrSTe monolayer has high Curie temperature ($T_c$) of 295 K and an out-of-plane magnetic anisotropy. The CrSeTe monolayer has large Dzyaloshinskii-Moriya interaction (DMI) which can host chiral Néel domain wall (DW), and under an external magnetic field, the skyrmion states can be induced. As tensile strain increases, ferromagnetic exchange coupling and perpendicular magnetic anisotropy of Janus $\mathrm{Cr}X\mathrm{Te}$ monolayers both increase significantly, and the magnitude of DMI is reduced, which results in the giant ferromagnetism enhancement. Interestingly, for CrSeTe monolayer, distinct spin textures from chiral DW to uniform ferromagnetic states are induced under tensile strain. Moreover, the diameter and density of skyrmions in CrSeTe can be tuned by the amplitude of external magnetic field and strain. These findings highlight that the Janus $\mathrm{Cr}X\mathrm{Te}$ monolayers as good candidates for spintronic nanodevices.

Charge-to-spin conversion in inhomogeneous systems is studied theoretically. We consider free electrons subject to impurities with spin-orbit interaction and with spatially modulated distribution, and calculate spin accumulation and spin current induced by an external electric field. It is found that the spin accumulation is induced by the vorticity of electron flow through side-jump and skew-scattering processes, and the differences of the two processes are discussed. The results can be put in a form of a generalized spin diffusion equation with a spin source term given by the divergence of the spin Hall current. This spin source term reduces to the form of spin-vorticity coupling when the spin Hall angle is independent of impurity concentration. We also study the effects of long-range Coulomb interaction, which is indispensable when the process involves charge inhomogeneity and accumulation as in the present problem.

We investigate the influence of quenched disorder on the dynamic structure factor of Heisenberg bilayers on the square, triangular, and kagome lattice in the quantum paramagnetic phase. Perturbative continuous unitary transformations and white graphs are employed to calculate the one-triplon contribution up to high orders in perturbation about the dimer limit for bimodal and continuous disorder. For the square lattice we find that the lifetime of the gap mode is increased by stronger quantum correlations, while stronger disorder effects are observed for the triangular lattice due to geometric frustration. For intradimer disorder, in-band energy gaps are observed for both lattices which can be understood in terms of a level repulsion on dimers with low and high intradimer exchange that are close in energy at the momentum where the in-band gap opens. For the highly frustrated kagome lattice disorder even decreases the gap energy. In addition, the localization length of the low-energy flat band is increased up to order 7 in perturbation theory. The interplay of quenched disorder, geometric frustration, and strong correlations leads therefore to rich structures in the dynamical structure factor of two-dimensional quantum magnets.

${\left[{\mathrm{Ca}}_2{\mathrm{CoO}}_3\right]}_{0.62}\left[{\mathrm{CoO}}_2\right]$, a two dimensional misfit metallic compound, is famous for its rich phases accessed by temperature, i.e., high temperature spin-state transition, metal-insulator transition (MIT) at intermediate temperature ($\sim 100\mathrm{K}$), and low temperature spin density wave (SDW). It enters into a SDW phase below $T_{\text{MIT}}$ which becomes long range at 27 K. Information on the independent role of misfit layers (rocksalt/${\mathrm{Ca}}_2{\mathrm{CoO}}_3$ and triangular/${\mathrm{CoO}}_2$) in these phases is scarce. By combining a set of complementary macroscopic (DC magnetization and resistivity) and microscopic (neutron diffraction and x-ray absorption fine structure spectroscopy) measurements on pure (CCO) and Tb substituted in the rocksalt layer of CCO (CCO1), magnetic correlations in both subsystems of this misfit compound are unraveled. CCO is found to exhibit glassiness, as well as exchange bias (EB) effects, while CCO1 does not exhibit glassiness, albeit it shows weaker EB effect. By combining local structure investigations from extended x-ray absorption fine structure (EXAFS) spectroscopy and neutron diffraction results on CCO, we confirm that the SDW arises in the ${\mathrm{CoO}}_2$ layer. Our results show that the magnetocrystalline anisotropy associated with the rocksalt layer acts as a source of pinning, which is responsible for EB effect. Ferromagnetic clusters in the ${\mathrm{Ca}}_2{\mathrm{CoO}}_3$ layer affects the SDW in ${\mathrm{CoO}}_2$ and ultimately glassiness arises.

Systems with strong spin-orbit coupling have been a topic of fundamental interest in condensed matter physics due to the exotic topological phases and the unconventional phenomenon they exhibit. In this particular study, we have investigated the superconductivity in the transition-metal ternary noncentrosymmetric compounds $\mathrm{La}M\mathrm{Si}$ ($M=\mathrm{Ni}$, Pt) with different spin-orbit coupling strength, using muon-spin rotation and relaxation measurements. Transverse-field measurements made in the vortex state indicate that the superconductivity in both materials is fully gapped, with a conventional $s$-wave pairing symmetry and BCS-like magnitudes for the zero-temperature gap energies. Zero-field measurements suggest a time-reversal symmetry preserved superconductivity in both the systems, though a small increase in muon depolarization is observed upon decreasing temperature. However, this has been attributed to quasistatic electronic fluctuations.