We study the magnetic-field dependence of the longitudinal spin relaxation time $T_1$ of donor-bound electrons placed in the middle of an 8-nm CdTe quantum well with different doping concentrations in the range from $1\times {10}^9$ to $2.9\times {10}^{11}$ $\mathrm{c}{\mathrm{m}}^{-2}$ and at low temperature. We use an extended photoinduced Faraday rotation technique, which expands the usual domain of the measured decays from tens of ns to $\mu \mathrm{s}$. As in high-purity bulk semiconductors, a maximum relaxation time of around $T_1\sim 10\mu \mathrm{s}$ is observed for a residually doped sample at low magnetic field of $B=0.08\mathrm{T}$. For higher doping concentrations, the magnetic-field dependence of $T_1$ shows a nonmonotonic behavior: first a rapid increase, followed by a plateau or a decrease of $T_1$. The fast increase of $T_1$ at low magnetic fields is explained by the inhibition of the mechanisms identified at zero field—hyperfine and anisotropic exchange interactions—while the behavior at high magnetic field can be succesfully explained by a mechanism proposed by Lyubinskiy and associated to electron hops [I. S. Lyubinskiy, JETP Lett. 88, 814 (2008)]. A good agreement between experiment and theory is found for samples below the metal-insulator transition, when Dresselhaus terms of spin-orbit coupling are considered to be the dominant ones in the Hamiltonian describing the system.

We report the experimental study of the temperature dependence of the longitudinal spin relaxation time $T_1$ of single nitrogen-vacancy (NV) centers hosted in nanodiamonds. To determine the relaxation mechanisms at stake, measurements of the $T_1$ relaxation time are performed for a set of individual NV centers both at room and cryogenic temperatures. The results are consistant with a temperature-dependent relaxation process, which is attributed to a thermally activated magnetic noise produced by paramagnetic impurities lying on the nanodiamond surface. These results confirm the existence of surface-induced spin relaxation processes occurring in nanodiamonds, which are relevant for future developments of sensitive nanoscale NV-based quantum sensors.

In quantum spin systems, a phase transition is studied from the perspective of magnetization curve and a magnetic susceptibility. We propose a new method for studying the anomaly of magnetic susceptibility $\chi$ that indicates a phase transition. In addition, we introduce the fourth derivative $A$ of the lowest-energy eigenvalue per site with respect to magnetization, i.e., the second derivative of ${\chi }^{-1}$. To verify the validity of this method, we apply it to an $S=1/2XXZ$ antiferromagnetic chain. The lowest energy of the chain is calculated by numerical diagonalization. As a result, the anomalies of $\chi$ and $A$ exist at zero magnetization. The anomaly of $A$ is easier to observe than that of $\chi$, indicating that the observation of $A$ is a more efficient method of evaluating an anomaly than that of $\chi$. The observation of $A$ reveals an anomaly that is different from the Kosterlitz-Thouless (KT) transition. Our method is useful in analyzing critical phenomena.

Spin-phonon coupling, the interaction of spins with a surrounding lattice, is a key parameter to understand the underlying physics of multiferroics and to engineer their magnetization dynamics. Elementary excitations in multiferroic materials are strongly influenced by spin-phonon interaction, making Raman spectroscopy a unique tool to probe these coupling(s). Recently, it has been suggested that the dielectric and magnetic properties of 15R-type hexagonal $\mathrm{BaMn}{\mathrm{O}}_3$ are correlated through the spin-lattice coupling. Here, we report the observation of an extensive renormalization of the Raman spectrum of $15R\text{−}\mathrm{BaMn}{\mathrm{O}}_3$ at 230, 280, and 330 K. Magnetic measurements reveal the presence of a long-range and a short-range magnetic ordering in $15R\text{−}\mathrm{BaMn}{\mathrm{O}}_3$ at 230 and 330 K, respectively. The Raman spectrum shows the appearance of new Raman modes in the magnetically ordered phases. Furthermore, an additional Raman phonon appears below ∼ 280 K, possibly arising from a local lattice distortion due to the displacement of Mn ions, that exhibits an anomalous shift with temperature. The origin of the observed renormalization and phonon anomalies in Raman spectra are discussed based on the evidence from temperature- and magnetic-field-dependent Raman spectra, temperature-dependent x-ray diffraction, magnetization, and specific-heat measurements. Our results indicate the presence of magnetostriction and spin-phonon coupling in $15R\text{−}\mathrm{BaMn}{\mathrm{O}}_3$, thus suggesting that the optical phonons are strongly correlated to its magnetoelectric properties.

We report on pressure tuning of superconductivity in a strongly coupled superconductor ${\mathrm{Mo}}_8{\mathrm{Ga}}_{41}$ with a critical temperature $T_c\sim 9.8\mathrm{K}$. Resistance measurements show that $T_c$ is suppressed to 3.6 K at 19 GPa followed by a nonmonotonic variation at higher pressures. Synchrotron x-ray diffraction experiments reveal an amorphization taking place at the critical pressure $P_c\sim 19\mathrm{GPa}$. As comparing with $\mathrm{Mg}{\mathrm{B}}_2$, the ${\mathrm{Mo}}_8{\mathrm{Ga}}_{41}$ shows a relative low decreasing rate of $T_c$, which might be attributed to its unique vertex-sharing type of cluster architecture. Survival of superconductivity across the crystalline to amorphous transition indicates that, although long-range structural order of endohedral Ga clusters is destroyed, short-range-ordered intracluster structure still exists and allows for local Cooper pairing. Due to presence of disorders in the amorphous phase, establishment of Cooper pair coherence is shifted to lower temperatures, which causes unusual transport behaviors near $T_c$, e.g., the anomalous resistance peak and dip. The amorphous superconducting phase is able to be retained to ambient pressure and therefore provides an ideal platform to study disorder effects on superconductivity.

We performed numerical simulations of decaying quantum turbulence by using a generalized Gross-Pitaevskii equation that includes a beyond mean field correction and a nonlocal interaction potential. The nonlocal potential is chosen in order to mimic He II by introducing a roton minimum in the excitation spectrum. We observe that at large scales the statistical behavior of the flow is independent of the interaction potential, but at scales smaller than the intervortex distance a Kelvin wave cascade is enhanced in the generalized model. In this range, the incompressible kinetic energy spectrum obeys the weak wave turbulence prediction for Kelvin wave cascade not only for the scaling with wave numbers but also for the energy flux and the intervortex distance.

An experimentally feasible energy-storage concept is formulated based on vorticity (hydro)dynamics within an easy-plane insulating magnet. The free energy associated with the magnetic winding texture is built up in a circular easy-plane magnetic structure by injecting a vorticity flow in the radial direction. The latter is accomplished by electrically induced spin-transfer torque, which pumps energy into the magnetic system in proportion to the vortex flux. The resultant magnetic metastable state with a finite winding number can be maintained indefinitely because the process of its relaxation via phase slips is exponentially suppressed when the temperature is brought well below the Curie temperature. We characterize the vorticity-current interaction underlying the energy-loading mechanism through its contribution to the effective electric inductance in the rf response. Our proposal may open an avenue for naturally powering spintronic circuits and nontraditional magnet-based neuromorphic networks.

We study a disordered ensemble of quantum emitters collectively coupled to a lossless cavity mode. The latter is found to modify the localization properties of the “dark” eigenstates, which exhibit a character of being localized on multiple noncontiguous sites. We denote such states as semilocalized and characterize them by means of standard localization measures. We show that those states can very efficiently contribute to coherent energy transport. Our paper underlines the important role of dark states in systems with strong light-matter coupling.

The topological insulator/ferromagnetic metal (TI/FMM) bilayer thin films emerged as promising topological surface-state-based spintronic devices, most notably in their efficiency of current-induced spin torque. Using a lattice model, we reveal that the surface-state Dirac cone of the TI can gradually merge into or be highly intertwined with the FMM bulk bands, and the surface states percolate into the FMM and eventually hybridize with the quantum well states therein. The magnetization can distort the spin-momentum locking of the surface states and yield an asymmetric band structure, which causes a laminar flow of room-temperature persistent charge current. Moreover, the proximity to the FMM also promotes a persistent laminar spin current. Through a linear response theory, we elaborate that both the surface state and the FMM bulk bands contribute to the current-induced spin torque, and their real wave functions render the spin torque predominantly fieldlike, with a magnitude highly influenced by the degree of the percolation of the surface states. On the other hand, impurities can generate a dampinglike torque, produce a torque even when the magnetization points in plane and orthogonal to the current direction, and moreover strongly suppress the Edelstein effect at the surface of the topological insulator.

Layered structural materials have been a fertile playground to investigate mechanisms of fundamental physics and explore potential applications. Here, we report investigations on ferromagnetic van der Waals $\mathrm{Cr}\mathrm{Si}{\mathrm{Te}}_3$ via high-pressure synchrotron x-ray diffraction, electrical resistance, and magnetoresistance measurements. Under compression, $\mathrm{Cr}\mathrm{Si}{\mathrm{Te}}_3$ undergoes an insulator-metal transition and a structural transition at $\sim 7.5$ GPa. Concomitantly with the structural transition, the magnetoresistance changes sign, the negative Hall coefficient increases dramatically, and superconductivity emerges at 3 K. The superconductivity persists up to the highest measured pressure of 47.1 GPa with a maximum $T_c\approx$ 4.5 K at $\sim 30$ GPa. Our results suggest that $\mathrm{Cr}\mathrm{Si}{\mathrm{Te}}_3$ is paramagnetic in the pressure range of superconductivity. The discoveries of superconductivity and magnetic transition in ferromagnetic $\mathrm{Cr}\mathrm{Si}{\mathrm{Te}}_3$ under pressure provide new perspectives to explore the interplay between superconductivity and magnetism in Cr-based two-dimensional van der Waals materials.

We study the single- and many-particle properties of a two-leg ladder model threaded by a flux with the legs coupled by a spatially varying term. Although a priori unrelated to twisted bilayer graphene (TBG), the model is found to have striking similarities: a quasi-flat low-energy band emerges with characteristics similar to that of magic angle TBG. We study the effect of interparticle interaction in our model using the density matrix renormalization group and find that when the band is quasi-flat, the ground state is a ferromagnetic Mott insulator. As the band becomes more dispersive, the system undergoes a ferromagnetic to antiferromagnetic transition. We discuss how our model is relevant not only to magic-angle physics in TBG, but also in the larger context of one-dimensional correlations and magnetism.

Smoothly confined graphene quantum dots (GQDs) localize Dirac electrons with conserved spin and valley degrees of freedom. Recent experimental realization of such structures using a combination of magnetic fields and a scanning tunneling microscope tip showcased their potential in locally probing and adjusting the valley degree of freedom. The present work models the influence of lattice defects on the level structure of GQDs. We study both the adiabatic level spacing “landscape”—orbital splitting and valley splitting—as well as transition dynamics between GQD states. The system is modeled using a tight-binding approach with on-site and hopping parameters in the vicinity of the defect region extracted from density functional theory via Wannier orbitals while time propagation is done using Magnus operators. Different defect types, such as double vacancy, Stone-Wales, flower, and Si substitution, are considered. We predict tunable valley splittings of the order of 2–20 meV. The level structure can thus be tailored at will by engineering appropriate defect geometries.

We derive and analyze the longitudinal and the anomalous Hall conductivity for a general momentum-block-diagonal two-band model. This model captures a broad spectrum of physically very different systems including Néel antiferromagnetic and spiral spin density waves as well as models that involve spin-orbit interaction and are known to show topological properties. We present a complete microscopic derivation for finite temperature and constant scattering rate $\mathrm{\Gamma }$ that is diagonal and equal, but arbitrarily large for both bands. We identify two criteria that allow for a unique and physically motivated decomposition of the conductivities. On the one hand, we distinguish intraband and interband contributions that are defined by the involved quasiparticle spectral functions. On the other hand, we distinguish symmetric and antisymmetric contributions that are defined by the symmetry under the exchange of the current and the electric field directions. The (symmetric) intraband contributions generalize the formula of standard Boltzmann transport theory, which is valid only in the clean limit (small $\mathrm{\Gamma }$), whereas the interband contributions capture interband coherence effects beyond independent quasiparticles. We show that the symmetric interband contribution is a correction only present for finite $\mathrm{\Gamma }$ and is controlled by the quantum metric. The antisymmetric interband contributions generalize the formula of the anomalous Hall conductivity in terms of the Berry curvature to finite $\mathrm{\Gamma }$. We study the clean (small $\mathrm{\Gamma }$) and dirty (large $\mathrm{\Gamma }$) limit analytically. The connection between the presented derivation and the Bastin and Streda formalism is given. We apply our results to a Chern insulator, a ferromagnetic multi-$d$-orbital, and a spiral spin density wave model.

$A{B}_2{\mathrm{O}}_4$ normal spinels with a magnetic $B$ site can host a variety of magnetic and orbital frustrations leading to spin-liquid phases and field-induced phase transitions. Here, we report the epitaxial growth of (111)-oriented $\mathrm{Mg}{\mathrm{Cr}}_2{\mathrm{O}}_4$ thin films. By characterizing the structural and electronic properties of films grown along the (001) and (111) directions, the influence of growth orientation has been studied. Despite distinctly different growth modes observed during deposition, the comprehensive characterization reveals no measurable disorder in the cation distribution nor multivalency issue for Cr ions in either orientation. Contrary to a naive expectation, the (111) stabilized films exhibit a smoother surface and a higher degree of crystallinity than (001)-oriented films. The preference in growth orientation is explained within the framework of heteroepitaxial stabilization in connection to a significantly lower (111) surface energy. These findings open broad opportunities in the fabrication of two-dimensional kagome-triangular heterostructures with emergent magnetic behavior inaccessible in bulk crystals.

We investigate localization properties of driven models which exhibit a subextensive number of extended states in the static setting. We consider instances where the extended modes are or are not protected by topological considerations. To this end, we contrast the strongly driven, disordered, lowest Landau level, which we refer to as the random Landau model (RLM), with the random dimer model (RDM); the RDM also has a subextensive set of delocalized modes in the middle of the spectrum whose origin is not topological. We map the driven models on to a higher-dimensional effective model and numerically compute the localization length as a function of disorder strength, drive amplitude, and frequency using the recursive Green's function method. Our numerical results indicate that, in the presence of a strong drive (low frequency and/or large drive amplitude), the topologically protected RLM continues to exhibit a spectrum with both localized and delocalized (or critical) modes, but the spectral range of delocalized modes is enhanced by the driving. This occurs due to an admixture of the localized modes with extended modes arising due to the topologically protected critical energy in the middle of the spectrum. On the other hand, in the RDM, a weak drive immediately localizes the entire spectrum. This occurs in contrast to the naive expectation from perturbation theory that mixing between localized and delocalized modes generically enhances the delocalization of all modes. Our work highlights the importance of the origin of the delocalized modes in the localization properties of the corresponding Floquet model.

The $5d$ transition-metal oxides have a unique electronic structure dominated by strong spin-orbit coupling and hence they can be an intriguing platform to explore spin-current physics. Here, we report on room-temperature generation of spin-orbit torque (SOT) from a conductive $5d$ iridium oxide, $\mathrm{Ir}{\mathrm{O}}_2$. By measuring second-harmonic Hall resistance of ${\mathrm{Ni}}_{81}{\mathrm{Fe}}_{19}/\mathrm{Ir}{\mathrm{O}}_2$ bilayers, we find both dampinglike and fieldlike SOTs. The former is larger than the latter, enabling easier control of magnetization. We also observe that the dampinglike SOT efficiency has a significant dependence on $\mathrm{Ir}{\mathrm{O}}_2$ thickness, which is well described by the drift-diffusion model based on the bulk spin Hall effect. We deduce the effective spin Hall angle of +0.093 ± 0.003 and the spin-diffusion length of 1.7 ± 0.2 nm. By comparison with control samples Pt and Ir, we show that the effective spin Hall angle of $\mathrm{Ir}{\mathrm{O}}_2$ is comparable to that of Pt and seven times higher than that of Ir. The fieldlike SOT efficiency has a negative sign without appreciable dependence on the thickness, in contrast to the dampinglike SOT. This suggests that the fieldlike SOT likely stems from the interface. These experimental findings suggest that the uniqueness of the electronic structure of $5d$ transition-metal oxides is crucial for highly efficient charge to spin-current conversion.

Structural, electronic, and magnetic properties of modified cubic spinel compound ${\mathrm{LiNi}}_{0.5}{\mathrm{Mn}}_{1.5}{\mathrm{O}}_4$ are studied via x-ray diffraction, resistivity, dc and ac magnetization, heat capacity, neutron diffraction, ${}^7\mathrm{Li}$ nuclear magnetic resonance, magnetocaloric effect, magnetic relaxation, and magnetic memory effect experiments. We stabilized this compound in a cubic structure with space group $P{4}_{3}32$. It exhibits semiconducting character with an electronic band gap of $\mathrm{\Delta }/k_{\mathrm{B}}\simeq 0.4$ eV. The interaction within each ${\mathrm{Mn}}^{4+}$ and ${\mathrm{Ni}}^{2+}$ sublattice and between ${\mathrm{Mn}}^{4+}$ and ${\mathrm{Ni}}^{2+}$ sublattices is found to be ferromagnetic (FM) and antiferromagnetic (AFM), respectively. This leads to the onset of a ferrimagnetic transition at $T_{\mathrm{C}}\simeq 125$ K. The reduced values of frustration parameter ($f$) and ordered moments reflect magnetic frustration due to competing FM and AFM interactions. From the ${}^7\mathrm{Li}$ nuclear magnetic resonance shift vs susceptibility plot, the average hyperfine coupling between ${}^7\mathrm{Li}$ nuclei and ${\mathrm{Ni}}^{2+}$ and ${\mathrm{Mn}}^{4+}$ spins is calculated to be $\sim 672.4$ Oe/${\mu }_{\mathrm{B}}$. A detailed critical behavior study is done in the vicinity of $T_{\mathrm{C}}$ using modified-Arrott plot, Kouvel-Fisher plot, and universal scaling of magnetization isotherms. The magnetic phase transition is found to be second order in nature and the estimated critical exponents correspond to the three-dimensional XY universality class. A large magnetocaloric effect is observed with a maximum value of isothermal change in entropy $\mathrm{\Delta }{S}_m\simeq -11.3$ J/Kg K and a maximum relative cooling power of $\text{RCP}\simeq 604$ J/Kg for 9 T magnetic field change. The imaginary part of the ac susceptibility depicts a strong frequency-dependent hump at $T=T_{\mathrm{f}2}$ well below the blocking temperature $T_{\mathrm{b}}\simeq 120$ K. The Arrhenius behavior of frequency dependent $T_{\mathrm{f}2}$ and the absence of zero-field-cooled memory confirm the existence of superparamagnetism in the ferrimagnetically ordered state.

An approach to account for the effect of thermal lattice vibrations when calculating exchange coupling parameters is presented on the basis of the Korringa-Kohn-Rostoker Green function method making use of the alloy analogy model. Using several representative systems, it is shown that depending on the material the effect of thermal lattice vibrations can have a significant impact on the isotropic exchange as well as the Dzyaloshinskii-Moriya interactions (DMI). This should lead in turn to an additional contribution to the temperature dependence of the magnetic properties of solids, which cannot be neglected in the general case. As an example, we discuss such an influence on the critical temperature of various magnetic phase transitions. In particular, in the case of skyrmion hosting materials, a strong impact of lattice vibrations on the DMI is an additional source for the temperature dependent skyrmion size and stability, which should be taken into consideration. The present approach gives also access to features of magnetic properties that are associated with static atomic displacements as, for example, in random high-entropy alloys.