The effects of cerium substitution on the structural and magnetic properties of the $\mathrm{L}{\mathrm{a}}_{1-x}\mathrm{C}{\mathrm{e}}_x\mathrm{F}{\mathrm{e}}_{12}{\mathrm{B}}_6$ ($0\le x\le 0.175$) series of compounds have been studied. All of the compounds exhibit an antiferromagnetic ground state below the Néel temperature $T_{\mathrm{N}}\approx 36\mathrm{K}$. Both antiferromagnetic and paramagnetic states can be transformed into the ferromagnetic state irreversibly and reversibly depending on the magnitude of the applied magnetic field, the temperature, and the direction of their changes. Of particular interest is the low-temperature magnetization process. This process is discontinuous and evolves unexpected huge metamagnetic transitions consisting of a succession of sharp magnetization steps separated by plateaus, giving rise to an unusual avalanchelike behavior. At constant temperature and magnetic field, the evolution with time of the magnetization displays a spectacular spontaneous jump after a long incubation time. $\mathrm{L}{\mathrm{a}}_{1-x}\mathrm{C}{\mathrm{e}}_x\mathrm{F}{\mathrm{e}}_{12}{\mathrm{B}}_6$ compounds exhibit a unique combination of exceptional features like large thermal hysteresis, giant magnetization jumps, and remarkably huge magnetic hysteresis for the field-induced first-order metamagnetic transition.

We measure the response of a rotating sample of superfluid ${}^3\mathrm{He}-\mathrm{B}$ to spin-down to rest in the zero-temperature limit. Deviations from perfect cylindrical symmetry in the flow environment cause the initial response to become turbulent. The remaining high polarization of vortices along the rotation axis suppresses the turbulent behavior and leads to laminar late-time response. We determine the dissipation during laminar decay at $(0.13–0.22)T_{\mathrm{c}}$ from the precession frequency of the remnant vortex cluster. We extract the mutual friction parameter $\alpha$ and confirm that its dependence on temperature and pressure agrees with theoretical predictions. We find that the zero-temperature extrapolation of $\alpha$ has pressure-independent value $\alpha (T=0)\sim 5\times {10}^{-4}$, which we attribute to a process where Kelvin waves, excited at surfaces of the container, propagate into the bulk and enhance energy dissipation via overheating vortex core-bound fermions.

Temperature-dependent spin Seebeck effect data on Pt|yttrium iron garnet (YIG) (${\mathrm{Y}}_3\mathrm{F}{\mathrm{e}}_5{\mathrm{O}}_{12}$)|gallium gadolinium garnet ($\mathrm{G}{\mathrm{d}}_3\mathrm{G}{\mathrm{a}}_5{\mathrm{O}}_{12}$) are reported for YIG films of various thicknesses. The effect is reported as a spin Seebeck resistivity (SSR), the inverse spin-Hall field divided by the heat flux, to circumvent uncertainties about temperature gradients inside the films. The SSR is a nonmonotonic function of YIG thickness. A diffusive model for magnon transport demonstrates how these data give evidence for the existence of two distinct length scales in thermal spin transport, a spin-diffusion length, and a magnon energy relaxation length.

We have performed parallel measurements of dc-magnetization and ac-magnetic susceptibility for a ferromagnetic superconductor, ${\mathrm{UGe}}_2$, in the ferromagnetic-superconducting phase. dc-magnetization measurements revealed that adequate demagnetizing of the sample allows for the preparation of various magnetized states with different zero-field residual magnetization. We observed that these states exhibit varying ac superconducting response at large ac-field amplitudes. The amount of ac flux penetration is less in the demagnetized state involving many domain walls. This result seems to contradict the theory that considers the domain walls as weak links. Moreover, the ferromagnetic domain walls enforce the shielding capability of superconductivity. This observation sheds light on the role of the domain walls on superconductivity, which has been a controversial issue for several decades. Two possible scenarios are presented to explain the enhancement of the shielding capability by the domain walls.

In this work, we quantify oxygen self-diffusion in monoclinic-phase zirconium oxide as a function of temperature and oxygen partial pressure. A migration barrier of each type of oxygen defect was obtained by first-principles calculations. Random walk theory was used to quantify the diffusivities of oxygen interstitials by using the calculated migration barriers. Kinetic Monte Carlo simulations were used to calculate diffusivities of oxygen vacancies by distinguishing the threefold- and fourfold-coordinated lattice oxygen. By combining the equilibrium defect concentrations obtained in our previous work together with the herein calculated diffusivity of each defect species, we present the resulting oxygen self-diffusion coefficients and the corresponding atomistically resolved transport mechanisms. The predicted effective migration barriers and diffusion prefactors are in reasonable agreement with the experimentally reported values. This work provides insights into oxygen diffusion engineering in $\mathrm{Zr}{\mathrm{O}}_2$-related devices and parametrization for continuum transport modeling.

An advanced nonequilibrium thermodynamic model for electron and phonon transport is formulated based on the steepest-entropy-ascent quantum thermodynamics framework. This framework, based on the principle of steepest entropy ascent (or the equivalent maximum entropy production principle), inherently satisfies the laws of thermodynamics and mechanics and is applicable at all temporal and spatial scales even in the far-from-equilibrium realm. Specifically, the model is proven to recover the Boltzmann transport equations in the near-equilibrium limit and the two-temperature model of electron-phonon coupling when no dispersion is assumed. The heat and mass transport at a temperature discontinuity across a homogeneous interface where the dispersion and coupling of electron and phonon transport are both considered are then modeled. Local nonequilibrium system evolution and nonquasiequilibrium interactions are predicted and the results discussed.

We analyze scaling behaviors of simulated annealing carried out on various classical systems with topological order, obtained as appropriate limits of the toric code in two and three dimensions. We first consider the three-dimensional ${\mathbb{Z}}_2$ (Ising) lattice gauge model, which exhibits a continuous topological phase transition at finite temperature. We show that a generalized Kibble-Zurek scaling ansatz applies to this transition, in spite of the absence of a local order parameter. We find perimeter-law scaling of the magnitude of a nonlocal order parameter (defined using Wilson loops) and a dynamic exponent $z=2.70\pm 0.03$, the latter in good agreement with previous results for the equilibrium dynamics (autocorrelations). We then study systems where (topological) order forms only at zero temperature—the Ising chain, the two-dimensional ${\mathbb{Z}}_2$ gauge model, and a three-dimensional star model (another variant of the ${\mathbb{Z}}_2$ gauge model). In these systems the correlation length diverges exponentially, in a way that is nonsmooth as a finite-size system approaches the zero temperature state. We show that the Kibble-Zurek theory does not apply in any of these systems. Instead, the dynamics can be understood in terms of diffusion and annihilation of topological defects, which we use to formulate a scaling theory in good agreement with our simulation results. We also discuss the effect of open boundaries where defect annihilation competes with a faster process of evaporation at the surface.

Josephson junctions containing two ferromagnetic layers are being considered for use in cryogenic memory. Our group recently demonstrated that the ground-state phase difference across such a junction with carefully chosen layer thicknesses could be controllably toggled between zero and $\pi$ by switching the relative magnetization directions of the two layers between the antiparallel and parallel configurations. However, several technological issues must be addressed before those junctions can be used in a large-scale memory. Many of these issues can be more easily studied in single junctions, rather than in the superconducting quantum interference device (SQUID) used for phase-sensitive measurements. In this work, we report a comprehensive study of spin-valve junctions containing a Ni layer with a fixed thickness of 2.0 nm and a NiFe layer of thickness varying between 1.1 and 1.8 nm in steps of 0.1 nm. We extract the field shift of the Fraunhofer patterns and the critical currents of the junctions in the parallel and antiparallel magnetic states, as well as the switching fields of both magnetic layers. We also report a partial study of similar junctions containing a slightly thinner Ni layer of 1.6 nm and the same range of NiFe thicknesses. These results represent the first step toward mapping out a “phase diagram” for phase-controllable spin-valve Josephson junctions as a function of the two magnetic layer thicknesses.

Nanowires with helical surface states represent key prerequisites for observing and exploiting phase-coherent topological conductance phenomena, such as spin-momentum locked quantum transport or topological superconductivity. We demonstrate in a joint experimental and theoretical study that gated nanowires fabricated from high-mobility strained HgTe, known as a bulk topological insulator, indeed preserve the topological nature of the surface states, that moreover extend phase-coherently across the entire wire geometry. The phase-coherence lengths are enhanced up to $5\mu \mathrm{m}$ when tuning the wires into the bulk gap, so as to single out topological transport. The nanowires exhibit distinct conductance oscillations, both as a function of the flux due to an axial magnetic field and of a gate voltage. The observed $h/e$-periodic Aharonov-Bohm-type modulations indicate surface-mediated quasiballistic transport. Furthermore, an in-depth analysis of the scaling of the observed gate-dependent conductance oscillations reveals the topological nature of these surface states. To this end we combined numerical tight-binding calculations of the quantum magnetoconductance with simulations of the electrostatics, accounting for the gate-induced inhomogeneous charge carrier densities around the wires. We find that helical transport prevails even for strongly inhomogeneous gating and is governed by flux-sensitive high-angular momentum surface states that extend around the entire wire circumference.

We present a combined ab initio study of several gyrotropic effects in $p$-doped trigonal tellurium (effects that reverse direction with the handedness of the spiral chains in the atomic structure). The key ingredients in our study are the $k$-space Berry curvature and intrinsic orbital magnetic moment imparted on the Bloch states by the chirality of the crystal structure. We show that the observed sign reversal with temperature of the circular photogalvanic effect can be explained by the presence of Weyl points near the bottom of the conduction band acting as sources and sinks of Berry curvature. The passage of a current along the trigonal axis induces a rather small parallel magnetization, which can nevertheless be detected by optical means (Faraday rotation of transmitted light) due to the high transparency of the sample. In agreement with experiment, we find that when infrared light propagates antiparallel to the current at low doping the current-induced optical rotation enhances the natural optical rotation. According to our calculations the plane of polarization rotates in the opposite sense to the bonded atoms in the spiral chains, in agreement with a recent experiment that contradicts earlier reports.

We study the stability of composite fermion fractional quantum Hall states in Harper-Hofstadter bands with Chern number $\left|C\right|>1$. From composite fermion theory, states are predicted to be found at filling factors $\nu =r/\left(kr\right|C|+1),r\in \mathbb{Z}$, with $k=1$ for bosons and $k=2$ for fermions. Here, we closely analyze these series in both cases, with contact interactions for bosons and nearest-neighbor interactions for (spinless) fermions. In particular, we analyze how the many-body gap scales as the bands are tuned to the effective continuum limit of Chern number $\left|C\right|$ bands, realized near flux density $n_{\varphi }=1/\left|C\right|$. Near these points, the Hofstadter model requires large magnetic unit cells that yield bands with perfectly flat dispersion and Berry curvature. We exploit the known scaling of energies in the effective continuum limit in order to maintain a fixed square aspect ratio in finite-size calculations. Based on exact diagonalization calculations of the band-projected Hamiltonian for these lattice geometries, we show that for both bosons and fermions, the vast majority of finite-size spectra yield the ground-state degeneracy predicted by composite fermion theory. For the chosen interactions, we confirm that states with filling factor $\nu =1/\left(k\right|C|+1)$ are the most robust and yield a clear gap in the thermodynamic limit. For bosons with contact interactions in $\left|C\right|=2$ and $\left|C\right|=3$ bands, our data for the composite fermion states are compatible with a finite gap in the thermodynamic limit. We also report new evidence for gapped incompressible states stabilized for fermions with nearest-neighbor interactions in $\left|C\right|>1$ bands. For cases with a clear gap, we confirm that the thermodynamic limit commutes with the effective continuum limit within finite-size error bounds. We analyze the nature of the correlation functions for the Abelian composite fermion states and find that the correlation functions for $\left|C\right|>1$ states are smooth functions for positions separated by $\left|C\right|$ sites along both axes, giving rise to ${\left|C\right|}^2$ sheets; some of which can be related by inversion symmetry. We also comment on two cases which are associated with a bosonic integer quantum Hall effect (BIQHE): For $\nu =2$ in $\left|C\right|=1$ bands, we find a strong competing state with a higher ground-state degeneracy, so no clear BIQHE is found in the band-projected Hofstadter model; for $\nu =1$ in $\left|C\right|=2$ bands, we present additional data confirming the existence of a BIQHE state.

Excited states of a single donor in bulk silicon have previously been studied extensively based on effective mass theory. However, proper theoretical descriptions of the excited states of a donor cluster are still scarce. Here we study the excitations of lines of defects within a single-valley spherical band approximation, thus mapping the problem to a scaled hydrogen atom array. A series of detailed time-dependent Hartree-Fock, time-dependent hybrid density-functional theory and full configuration-interaction calculations have been performed to understand linear clusters of up to 10 donors. Our studies illustrate the generic features of their excited states, addressing the competition between formation of interdonor ionic states and intradonor atomic excited states. At short interdonor distances, excited states of donor molecules are dominant, at intermediate distances ionic states play an important role, and at long distances the intradonor excitations are predominant as expected. The calculations presented here emphasize the importance of correlations between donor electrons, and are thus complementary to other recent approaches that include effective mass anisotropy and multivalley effects. The exchange splittings between relevant excited states have also been estimated for a donor pair and for three-donor arrays; the splittings are much larger than those in the ground state in the range of donor separations between 10 and 20 nm. This establishes a solid theoretical basis for the use of excited-state exchange interactions for controllable quantum gate operations in silicon.

We study the quantum multicritical point in a (2+1)-dimensional Dirac system between the semimetallic phase and two ordered phases that are characterized by anticommuting mass terms with $\mathrm{O}\left(N_1\right)$ and $\mathrm{O}\left(N_2\right)$ symmetries, respectively. Using $\epsilon$ expansion around the upper critical space-time dimension of four, we demonstrate the existence of a stable renormalization-group fixed point, enabling a direct and continuous transition between the two ordered phases directly at the multicritical point. This point is found to be characterized by an emergent $\mathrm{O}(N_1+N_2)$ symmetry for arbitrary values of $N_1$ and $N_2$ and fermion flavor numbers $N_{\mathrm{f}}$ as long as the corresponding representation of the Clifford algebra exists. Small $\mathrm{O}\left(N\right)$-breaking perturbations near the chiral $\mathrm{O}\left(N\right)$ fixed point are therefore irrelevant. This result can be traced back to the presence of gapless Dirac degrees of freedom at criticality, and it is in clear contrast to the purely bosonic $\mathrm{O}\left(N\right)$ fixed point, which is stable only when $N<3$. As a by-product, we obtain predictions for the critical behavior of the chiral $\mathrm{O}\left(N\right)$ universality classes for arbitrary $N$ and fermion flavor number $N_{\mathrm{f}}$. Implications for critical Weyl and Dirac systems in 3+1 dimensions are also briefly discussed.

By coupling on chip via a resonant circuit a carbon nanotube to a quantum noise detector, a superconductor-insulator-superconductor junction we measure the emission noise of a carbon nanotube quantum dot in the Kondo regime. The signature of the Kondo effect in the current noise is measured for different ratios of the Kondo temperature over the measured frequency and for different asymmetries of the coupling to the contacts and compared to finite frequency quantum noise calculations. Our results point towards the existence of a high frequency cut-off of the electronic emission noise associated with the Kondo resonance. This cut-off frequency is on the order of a few times the Kondo temperature when the electronic system is close to equilibrium, which is the case for a strongly asymmetric coupling. On the other hand, this cut-off is shifted to lower frequencies in a symmetric coupling situation where the bias voltage drives the Kondo state out of equilibrium. We then attribute the low frequency cut-off to voltage-induced spin relaxation.

Two-dimensional layered transition-metal-dichalcogenide (TMDC) materials often exhibit exotic quantum phases due to the delicate coupling and competitions of charge, lattice, orbital, and spin degrees of freedom. Surprisingly, we here present, based on first-principles density-functional theory calculations, the incorporation of all such degrees of freedom in a charge density wave (CDW) of monolayer (ML) TMDC $1T\text{−}{\mathrm{TaS}}_2$. We reveal that this CDW accompanying the lattice distortion to the “David-star” (DS) superstructure constituted of one cental, six nearest-neighbor, and six next-nearest-neighbor Ta atoms is driven by the formation of quasimolecular orbitals due to a strong hybridization of Ta $t_{2\mathrm{g}}$ orbitals. The resulting weakly overlapped nonbonding orbitals between the DS clusters form a narrow half-filled band at the middle of the CDW gap, leading to the Stoner-type magnetic instability caused by an intramolecular exchange interaction. It is thus demonstrated that the Stoner parameter $I$ corresponding to the effective on-site Coulomb interaction $U$ opens a Mott gap. Our finding of the intricate charge-lattice-orbital-spin coupling in ML $1T\text{−}{\mathrm{TaS}}_2$ provides a framework for the exploration of various CDW phases observed in few-layer or bulk $1T\text{−}{\mathrm{TaS}}_2$.

We study the dynamical response of the half-filled one-dimensional Hubbard model for a range of interaction strengths $U$ and temperatures $T$ by a combination of numerical and analytical techniques. Using time-dependent density matrix renormalization group computations we find that the single-particle spectral function undergoes a crossover to a spin-incoherent Luttinger liquid regime at temperatures $T\sim J=4t^2/U$ for sufficiently large $U>4t$. At smaller values of $U$ and elevated temperatures the spectral function is found to exhibit two thermally broadened bands of excitations, reminiscent of what is found in the Hubbard-I approximation. The dynamical density-density response function is shown to exhibit a finite-temperature resonance at low frequencies inside the Mott gap, with a physical origin similar to the Villain mode in gapped quantum spin chains. We complement our numerical computations by developing an analytic strong-coupling approach to the low-temperature dynamics in the spin-incoherent regime.

We present a robust and exact method for calculating the polarization function and plasmon dispersion of graphene, for an arbitrary (isotropic) nonequilibrium carrier distribution, within the random phase approximation. This is demonstrated for a range of carrier distributions, including hot-carrier distributions which occur within the femtoseconds following photoexcitation. We show that qualitatively different behavior from the equilibrium case can occur. As the polarization function determines dynamic screening, its calculation will be essential to quantifying carrier-carrier scattering channels for graphene far from equilibrium.

The valence band dispersion of the graphite $\pi$ band along the $k_z$ axis was measured using a sequence of photon energies between 60 and 230 eV. The interlayer interaction induces the well-known splitting of the $\pi$ band into two, a lower binding energy band ${\pi }_{\mathrm{L}}$ and a higher binding energy band ${\pi }_{\mathrm{H}}$. However, in the photoelectron spectra, the bands ${\pi }_{\mathrm{L}}$ and ${\pi }_{\mathrm{H}}$ appeared alternatively in the Brillouin zones along the $k_z$ direction. As a result, a $4\pi$-periodic oscillation in the binding energy as a function of photon energy was observed for the $\pi$ band dispersion. This is explained by considering the constructive and destructive interference of $p_z$ orbitals within the unit cell, called the photoelectron structure factor. We derived the analytic expression of the photoelectron structure factor for graphite. The resonance integrals of the $p_z$ orbitals were determined for the in-plane and interlayer. Furthermore, the inner potential of graphite from the vacuum level was determined to be 17.17 eV.

In addition to magnetic field and electric charge noise adversely affecting spin-qubit operations, performing single-qubit gates on one of multiple coupled singlet-triplet qubits presents a new challenge: crosstalk, which is inevitable (and must be minimized) in any multiqubit quantum computing architecture. We develop a set of dynamically corrected pulse sequences that are designed to cancel the effects of both types of noise (i.e., field and charge) as well as crosstalk to leading order, and provide parameters for these corrected sequences for all 24 of the single-qubit Clifford gates. We then provide an estimate of the error as a function of the noise and capacitive coupling to compare the fidelity of our corrected gates to their uncorrected versions. Dynamical error correction protocols presented in this work are important for the next generation of singlet-triplet qubit devices where coupling among many qubits will become relevant.

We theoretically demonstrate a non-Hermitian metamaterial exhibiting broadband and wide-angle nonreciprocity. The metamaterial consists of planar metal-dielectric layers with a parity-time $\left(\mathcal{PT}\right)$ symmetric distribution of loss and gain. With increasing loss and gain, the band structure and band gap are strongly modified; further, the $\mathcal{PT}$ potential leads to distinct internal field distributions when illuminated from different sides. Including nonlinearities arising from natural loss and gain saturation leads to nonreciprocal transmission in the visible over a 50-nm wavelength and $\pm {60}^{\circ }$ angular range.