Carrier dynamics in metal-halide perovskites, in particular diffusion, has typically been investigated on micrometer-length scales and above. However, surfaces and interfaces modify the carrier dynamics, e.g., by interface band bending and recombination. To investigate the carrier dynamics and diffusion at the surface, we use time-resolved photoelectron spectroscopy which is sensitive to the photoexcited carriers in the topmost few nm of the material. We extract electron mobilities in well-ordered ${\mathrm{CsPbBr}}_3$(001) and ${\mathrm{CsSnBr}}_3$(001) films at room temperature of $31\pm 6$ and $13\pm 1{\mathrm{cm}}^2{\mathrm{V}}^{-1}{\mathrm{s}}^{-1}$, respectively. The mobility in ${\mathrm{CsPbBr}}_3$(001) increases to $200\pm 8{\mathrm{cm}}^2{\mathrm{V}}^{-1}{\mathrm{s}}^{-1}$ at 90 K, indicating strong electron-phonon coupling for electrons at the conduction band minimum. Temperature-dependent photoemission experiments find different phonon coupling mechanisms for high-energetic electrons and holes at the valence band maximum and in the main valence bands.

We analyze the heat transfer between two metals separated by a vacuum gap in the extreme near-field regime. In this crossover regime between conduction and radiation, heat exchanges are mediated by photon, phonon, and electron tunneling. We quantify the relative contribution of these carriers with respect to both the separation distance between the two bodies and the applied bias voltage. In the presence of a weak bias ($V_{\mathrm{b}}<100\mathrm{mV}$), electrons and phonons can contribute equally to the heat transfer near contact, while the contribution of photons becomes negligible. On the other hand, for larger bias voltages, electrons play a dominant role. Moreover, we demonstrate that depending on the magnitude of this bias, electrons can either cool down or heat up the hot body by the Nottingham effect. Our results emphasize some inconsistencies in recent experimental results about heat exchanges in the extreme near-field regime and set a road map for future experiments.

Spin qubits in semiconductor quantum dots are one of the promising devices to realize a quantum processor. A better knowledge of the noise sources affecting the coherence of such a qubit is therefore of prime importance. In this paper, we study the effect of telegraphic noise induced by the fluctuation of a single electric charge. We simulate as realistically as possible a hole spin qubit in a quantum dot defined electrostatically by a set of gates along a silicon nanowire channel. Calculations combining Poisson and time-dependent Schrödinger equations allow us to simulate the relaxation and the dephasing of the hole spin as a function of time for a classical random telegraph signal. We show that dephasing time $T_2$ is well given by a two-level model in a wide range of frequencies. Remarkably, in the most realistic configuration of a low-frequency fluctuator, the system has a non-Gaussian behavior in which the phase coherence is lost as soon as the fluctuator has changed state. The Gaussian description becomes valid only beyond a threshold frequency ${\omega }_{th}$, when the two-level system reacts to the statistical distribution of the fluctuator states. We show that the dephasing time $T_2\left({\omega }_{th}\right)$ at this threshold frequency can be considerably increased by playing on the orientation of the magnetic field and the gate potentials, by running the qubit along “sweet” lines. However, $T_2\left({\omega }_{th}\right)$ remains bounded due to dephasing induced by the nondiagonal terms of the stochastic perturbation Hamiltonian. On the other hand, our simulations reveal that the spin relaxation, usually characterized by the time $T_1$, cannot be described cleanly in the two-level model because the coupling to higher-energy hole levels impacts very strongly the spin decoherence. This result suggests that multilevel simulations including the coupling to phonons should be necessary to describe the relaxation phenomenon in this type of qubit.

We consider a quantum Hall (QH) region in contact with a superconductor (SC), i.e., a QH-SC junction. Due to successive Andreev reflections, the QH-SC interface hosts hybridized electron and hole edge states called chiral Andreev edge states (CAES). We theoretically study the transport properties of these CAES by using a microscopic, tight-binding model. We find that the transport properties strongly depend on the contact geometry and the value of the filling factor. We notice that it is necessary to add local barriers at the corners of the junction in order to reproduce such properties when using effective one-dimensional models.

Recent investigations have observed superdiffusion in integrable classical and quantum spin chains. An intriguing connection between these spin chains and the Kardar-Parisi-Zhang (KPZ) universality class has emerged. Theoretical developments (e.g., generalized hydrodynamics) have highlighted the role of integrability as well as spin symmetry in KPZ behavior. However, understanding their precise role on superdiffusive transport still remains a challenging task. The widely used quantum spin chain platform comes with severe numerical limitations. To circumvent this barrier, we focus on a classical integrable spin chain which was shown to have a deep analogy with the quantum spin-$\frac{1}{2}$ Heisenberg chain. Remarkably, we find that KPZ behavior prevails even when one considers integrability-breaking but spin-symmetry preserving terms, strongly indicating that spin symmetry plays a central role even in the nonperturbative regime. On the other hand, in the nonperturbative regime, we find that energy correlations exhibit clear diffusive behavior. We also study the classical analog of the out-of-time-ordered correlator and Lyapunov exponents. We find a significant presence of chaos for the integrability-broken cases even though KPZ behavior remains robust. The robustness of KPZ behavior is demonstrated for a wide class of spin-symmetry preserving integrability-breaking terms.

Structural transition from a cubic to a tetragonal phase in magnetically frustrated ${AB}_2{\mathrm{O}}_4$ spinel compounds is generally found indispensable to enable long-range magnetic ordering. Intriguingly, ${\mathrm{MnCr}}_2{\mathrm{O}}_4$ and ${\mathrm{CoCr}}_2{\mathrm{O}}_4$, constitute exceptions to the above general rule, as they seem to undergo long-range ferrimagnetic (FIM) ordering without any structural involvement in their corresponding cubic phases. We find that ${\mathrm{MnCr}}_2{\mathrm{O}}_4$ (and ${\mathrm{CoCr}}_2{\mathrm{O}}_4$) undergo a hitherto undetected partial glassy magnetic ordering of the spiral-spin components at a temperature (${\mathrm{T}}_{SP}$) higher than the long-range FIM transition. The spin-glass transition at ${\mathrm{T}}_{SP}$ triggers the onset of structural modifications that helps to reduce the geometric-magnetic-frustration and thereby enable the long-range FIM ordering within an overall cubic phase. In the absence of the higher-temperature glassy transition in doped ${\mathrm{MnCr}}_2{\mathrm{O}}_4$, the corresponding magnetic ordering is found to be glassy instead of being a long-ranged FIM ordering. In magnetic spinel oxides, like ${\mathrm{CdCr}}_2{\mathrm{O}}_4$ and ${\mathrm{FeCr}}_2{\mathrm{O}}_4$, where cubic to tetragonal structural transition occurs from either a spin-Jahn-Teller effect or through Jahn-Teller distortions, such a higher-temperature glassy magnetic transition (i.e., above the long-range magnetic ordering) is absent. Our results, thus, clearly elucidate that the structural modifications associated with the glassy magnetic ordering at ${\mathrm{T}}_{SP}$, aided with presence of magnetic $A$-site ions, play a pivotal role in releasing the geometric-magnetic-frustration in ${\mathrm{MnCr}}_2{\mathrm{O}}_4$ (and also ${\mathrm{CoCr}}_2{\mathrm{O}}_4$) to enable long-range FIM ordering in these systems.

Using a combination of the mean-field Bogoliubov–de Gennes approach and the density matrix renormalization group method, we discover first-order topological transitions between topological superconducting and trivial insulating phases in a sawtooth lattice of intersite attractive fermions. The topological characterization of different phases is achieved in terms of winding numbers, Majorana edge modes, and entanglement spectra. By studying the effect of disorder on first-order topological phase transitions, we establish disorder-induced topological phase coexistence as a mechanism for generating a finite density of Majorana particles.

We show that Coulomb drag in bilayer systems in the regime of electron hydrodynamics leads to additional viscosity terms in the hydrodynamic equations, the drag, and drag-Hall viscosities, besides the well-known kinematic and Hall viscosities. These additional viscosity terms arise from a change of the stress tensor due to the interlayer Coulomb interactions. All four viscosity terms are tunable by varying the applied magnetic field and the electron densities in the two layers. At certain ratios between the electron densities in the two layers, the drag viscosity dramatically changes the longitudinal transport resulting in a negative drag conductivity.

Hyperferroelectricity (HyFE) is an intriguing phenomenon in which spontaneous polarization persists under an open-circuit boundary condition (OCBC) despite the existence of a depolarization field. However, small polarization and shallow well depth have been found so far in most hyperferroelectric materials, which severely limits their applications. Using first-principles density functional theory, we report the discovery of a giant hyperferroelectricity in LiZbSb. The HyFE spontaneous polarization is shown to be remarkably large with $\mathrm{P}=0.282\mathrm{C}/{\mathrm{m}}^2$ under OCBC, which is one order of magnitude greater than the HyFE polarization in ${\mathrm{LiNbO}}_3$. Furthermore, HyFE in LiZnSb is found to be exceptionally stable with a well depth of electric free energy $\mathrm{\Delta }F=-332\mathrm{meV}$, which makes LiZnSb a possible hyperferroelectric solid at room temperature. The origin of the giant hyperferroelectricity in LiZnSb is identified and is attributed to the large mode effective charge of the soft longitudinal-optic phonon and the large high-frequency dielectric constant.

Perpendicular magnetic anisotropy (PMA) at the Fe/MgO interface has been widely studied for memory applications, and further improving PMA has been one of the central challenges. In this study we fabricate epitaxial Fe/alkali halide (LiF, NaCl, or CsI)/MgO multilayers using molecular beam epitaxy and investigate how the alkali halide layer insertion can enhance PMA at the Fe/MgO interface. We find that insertion of an ultrathin LiF layer (0.1–0.4 nm) enhances interfacial PMA, but thicker LiF layer insertion (0.6–1 nm) weakens it. For the CsI and NaCl cases, interfacial PMA energy decreases monotonically with CsI or NaCl thickness. The present results indicate that well-defined Fe-anion orbital hybridization achieved by good lattice matching between the Fe and insulator layers is essential to have strong PMA. It is also suggested that anion atoms with high electronegativities, such as F, are beneficial for interfacial PMA, probably because of the weaker Fe-F hybridization and stronger electron localization at the interface. Our study shall serve as a guiding principle for designing a new dielectric layer to achieve stronger PMA in ultrathin Fe films.

The intermetallic compound ${\mathrm{Nd}}_2{\mathrm{Co}}_{0.85}{\mathrm{Si}}_{2.88}$ having a triangular lattice could be synthesized in single phase only with defect crystal structure. Investigation through different experimental techniques indicate the presence of two magnetic transitions in the system. As verified experimentally and theoretically, the high-temperature transition $T_{\mathrm{H}}\sim$ 140 K is associated with the development of ferromagnetic interaction between itinerant Co moments, whereas the low-temperature transition at $T_{\mathrm{L}}\sim$ 6.5 K is due to the coupling among $\text{Nd}\text{−}4f$ and $\mathrm{Co}\text{−}3d$ moments, which is antiferromagnetic in nature. Detailed studies of temperature-dependent dc magnetic susceptibility, field dependence of isothermal magnetization, nonequilibrium dynamical behavior, viz., magnetic relaxation, aging effect, magnetic-memory effect, and temperature dependence of heat capacity, along with density functional theory (DFT) calculations, suggest that the ground state is magnetically frustrated spin glass in nature, having competing magnetic interactions of equivalent energies. DFT results further reveal that the $3d/5d$-conduction carriers are blocked in the system and act as a barrier for the $4f\text{−}4f$ RKKY interactions, resulting in spin frustration. Presence of vacancy defects in the crystal are also conducive to the spin frustration. This is an unique mechanism of magnetic frustration, not emphasized so far in any of the ternary $R_{2}T{X}_3$ ($R$ = rare earth, $T$ = transition elements, and $X$ = Si, Ge, In) type compounds. Due to the competing character of the itinerant $3d$ and localized $4f$ moments, the compound exhibits anomalous field dependence of magnetic coercivity. The system also exhibits a considerable magnetic entropy change of $-\mathrm{\Delta }{S}_{\mathrm{M}}\sim$ 13.3 J/kg K with a relative cooling power (RCP) of 220 J/kg and adiabatic temperature change $\mathrm{\Delta }{T}_{\mathrm{ad}}$ of 6 K for magnetic field change of 70 kOe.

Equal-spin Cooper pairs are pivotal building blocks for superconducting spintronics devices. In recent experiments unusual behavior was observed in ferromagnet/ferromagnet/superconductor devices when a precession of the magnetization was induced by ferromagnetic resonance. By using a nonequilibrium Usadel Green's function formalism, we study spin transport for such a setup. We solve for spin-resolved distribution functions and demonstrate that the spin injection process in superconductors is governed by the inverse proximity effect in the superconducting layer. We find that equal-spin Cooper pairs, which are produced by the two misaligned ferromagnetic layers, transport spin inside the superconductor layer. This then results in an increase of the injected spin current below the superconducting critical temperature. Our calculations provide evidence of the essential role of equal-spin Cooper pairs in spin-transport properties of superconductor/ferromagnet devices and pave avenues for the design of superconducting spintronics devices.

Motivated by experiments on chains of superconducting qubits, we consider the dynamics of a classical Klein-Gordon chain coupled to coherent driving and subject to dissipation solely at its boundaries. As the strength of the boundary driving is increased, this minimal classical model recovers the main features of the dissipative phase transition seen experimentally. Between the transmitting and nontransmitting regimes on either side of this transition (which support ballistic and diffusive energy transport, respectively), we observe additional dynamical regimes of interest. These include a regime of superdiffusive energy transport at weaker driving strengths, together with a resonant nonlinear wave regime at stronger driving strengths, which is characterized by emergent translation symmetry, ballistic energy transport, and coherent oscillations of a nonlinear normal mode. We propose a nonlocal Lyapunov exponent as an experimentally measurable diagnostic of many-body chaos in this system, and more generally in open systems that are only coupled to an environment at their boundaries.

The method of moments is developed and employed to analyze the equilibrium correlation functions of the magnetization of ferromagnetic nanoparticles in the case of inertial magnetization dynamics. The method is based on the Taylor series expansion of the correlation functions and the estimation of the expansion coefficients. This method significantly reduces the complexity of analysis of equilibrium correlation functions. Analytical expressions are derived for the first three coefficients for the longitudinal and transverse correlation functions for the uniaxial magnetocrystalline anisotropy of ferromagnetic nanoparticles with a longitudinal magnetic field. The limiting cases of very strong and negligibly weak external longitudinal fields are considered. The Gordon sum rule for inertial magnetization dynamics is discussed. In addition, we show that finite analytic series can be used as a simple and satisfactory approximation for the numerical calculation of correlation functions at short times.

We present synthesis and ${}^{75}\mathrm{As}$ nuclear quadrupole resonance (NQR) measurements for the noncentrosymmetric superconductor CaPtAs with a superconducting transition temperature $T_c$ of $\sim 1.5$ K. We discovered two different forms of CaPtAs during synthesis; one is a high-temperature tetragonal form that was previously reported, and the other is a low-temperature form consistent with the orthorhombic structure of CaPtP. According to the ${}^{75}\mathrm{As}$ NQR measurement for superconducting tetragonal CaPtAs, the nuclear spin-lattice relaxation rate $1/T_1$ has an obvious coherence peak below $T_c$ and does not follow a simple exponential variation at low temperatures. These findings indicate that CaPtAs is a multigap superconductor and a large $s$-wave component.

The spin-dependent Kukkonen-Overhauser (KO) effective electron-electron interaction in electron gas with a deformable background is used to calculate the BCS superconducting parameters $\mu$, ${\mu }^{*}$, and $\lambda$. The density and spin local field factors are utilized to incorporate the quantitative effects of exchange and correlation. The repulsive parameter $\mu$ is compared to results using the historical Thomas-Fermi or nearly equivalent random phase approximation (RPA) interactions. The resulting $\mu$ using the KO interaction is $45\%$ larger at $r_s=1.65$; rising to $153\%$ larger at $r_s=5.62$. Retardation reduces the effective repulsion, but ${\mu }^{*}$ is still $20-25\%$ larger. The predicted superconducting transition temperature would be reduced by $5-20\%$ using the McMillan formula. The attractive superconducting parameter $\lambda$, which depends on the electron-test charge interaction and the phonon spectrum, is also calculated using simple Debye-based models for phonon dispersion. This leads to a larger value of $\lambda$ than the same calculation using Thomas-Fermi or RPA interactions. Modern calculations of the phonon dispersion relations are often done using density functional theory. With the proper exchange and correlation kernel, the self-consistency of the method should yield the correct phonon dispersion relation and electron-phonon matrix elements. If the Thomas-Fermi or RPA interactions are used at any stage, the results for $\lambda$ are probably quantitatively inaccurate.

Motivated by the experimental observation of a quantized 5/2 thermal conductance at filling $\nu =5/2$, a result incompatible with both the Pfaffian and the anti-Pfaffian states, we have pushed the expansion of the effective Hamiltonian of the $5/2$-quantized Hall state to third order in the parameter $\kappa =E_C/\hslash {\omega }_c\propto 1/\sqrt{B}$ controlling the Landau-level mixing, where $E_C$ is the Coulomb energy and ${\omega }_c$ is the cyclotron frequency. Exact diagonalizations of this effective Hamiltonian show that the difference in overlap with the Pfaffian state and the anti-Pfaffian state induced at second order is reduced by third-order corrections and disappears around $\kappa =0.4$, suggesting that these states are much closer in energy at smaller magnetic field than previously anticipated. Furthermore, we show that in this range of $\kappa$ the finite-size spectrum is typical of a quantum phase transition, with a strong reduction of the energy gap and with level crossings between excited states. These results point to the possibility of a quantum phase transition at smaller magnetic field into a phase with an emergent particle-hole symmetry that would explain the measured $5/2$ thermal conductance of the $5/2$-quantized Hall state.

Lattice thermal conductivity is a crucial parameter for thermoelectric (TE) applications. In pyrite-type metal dichalcogenides, it is observed that the type of metal atom has a significant influence in the phonon properties and lattice thermal conductivities, but the physical origin is not clear. In this paper, we show that, for the pyrite-type posttransition metal (such as Zn or Cd) dichalcogenides, because the fully occupied $d^{10}$ orbitals inside the valence bands are relatively localized, the enhanced symmetry-controlled $s\text{−}d$ coupling effects in the posttransition metal atoms when they vibrate away from the high-symmetry equilibrium positions can lead to soft phonon properties and strong anharmonicity. Thus, they behave as rattlinglike atoms, and these systems have extremely low lattice thermal conductivities. However, for some pyrite-type transition metal dichalcogenides, when the outermost $d$ electrons in transition metal atoms are only occupied by the $T_{2g}$-derived states, such as Fe with $d^6$ configuration, the symmetry-controlled $s\text{−}d$ coupling effects in the transition metal atoms are strongly cancelled by the varying crystal field splitting when symmetry is reduced during vibration. Therefore, these compounds exhibit hard phonon properties, weak anharmonicities, and high lattice thermal conductivities. In this paper, we reveal the electronic origin of the puzzling behaviors of pyrite-type dichalcogenides which show a wide range of thermal conductivity properties. The discussed mechanism can also be used to guide researchers in seeking promising TE materials with low lattice thermal conductivity.