The magnetic susceptibility, crystal and magnetic structures, and electronic structure of the double perovskite ${\mathrm{Sr}}_2{\mathrm{ScOsO}}_6$ are reported. Using both neutron and x-ray powder diffraction we find that the crystal structure is monoclinic $P{2}_1/n$ from 3.5 to 300 K. Magnetization measurements indicate an antiferromagnetic transition at $T_{\mathrm{N}}=92\mathrm{K}$, one of the highest transition temperatures of any double perovskite hosting only one magnetic ion. Type I antiferromagnetic order is determined by neutron powder diffraction, with an Os moment of only $1.6\left(1\right){\mu }_{\mathrm{B}}$, close to half the spin-only value for a crystal field split $5d$ electron state with a $t_{2g}^3$ ground state. Density functional calculations show that this reduction is largely the result of strong Os-O hybridization, with spin-orbit coupling responsible for only a $\sim 0.1{\mu }_{\mathrm{B}}$ reduction in the moment.

We study the phase diagram of the spin-1 quantum bilinear-biquadratic antiferromagnet on the kagome lattice using exact diagonalization and the density matrix renormalization group algorithm. The $\mathrm{SU}\left(3\right)$-symmetric point of this model Hamiltonian is a spontaneously trimerized state whose qualitative nature persists even at the Heisenberg point, a finding that contrasts previous proposals. We report the ground state energy per site of the Heisenberg model to be $-1.410\left(2\right)$ and establish the presence of a spin gap.

The pressure-induced phase transition of the multiferroic manganese tungstate ${\mathrm{MnWO}}_4$ is studied on single crystals using synchrotron x-ray diffraction and Raman spectroscopy. We observe the monoclinic $P2/c$ to triclinic $P\overline{1}$ phase transition at 20.1 GPa and get insight on the phase transition mechanism from the appearance of tilted triclinic domains. Selective Raman spectroscopy experiments with single crystals have shown that the onset of the phase transition occurs 5 GPa below the previously reported pressure obtained from experiments performed with powder samples.

We investigate the magnetic-field-induced $(H\parallel b)$ transition in the reference improper multiferroic $\mathrm{MnW}{\mathrm{O}}_4$ using superspace formalism. The distinct superspace group symmetries are determined from single-crystal neutron diffraction. The study of the full magnetic symmetry of the incommensurate magnetic phases under field provides the keys for understanding its magnetic and ferroelectric behavior up to 12 T. At moderate fields, below the transition, the magnetic modulations of the two Mn sites in the simple cell are independent. At the field-induced transition between multiferroic phases, the electric polarization undergoes a flip from the monoclinic $b$ axis to the $ac$ plane. An additional component in the polarization is predicted along the $c$ axis, due to a point symmetry change from $2_y{1}^{\prime }$ to $m_y{1}^{\prime }$. In contrast with the low-field phase, above the transition the influence of the magnetic field results in the coupling between the magnetic modulations of the two Mn sites of the original monoclinic cell.

We investigate numerically the time dependence of “window” overlaps in a three-dimensional Ising spin glass below its transition temperature after a rapid quench. Using an efficient GPU implementation, we are able to study large systems up to lateral length $L=128$ and up to long times of $t={10}^8$ sweeps. We find that the data scales according to the ratio of the window size $W$ to the nonequilibrium coherence length $\xi \left(t\right)$. We also show a substantial change in behavior if the system is run for long enough that it globally equilibrates, i.e., $\xi \left(t\right)\approx L/2$, where $L$ is the lattice size. This indicates that the local behavior of a spin glass depends on the spin configurations (and presumably also the bonds) far away. We compare with similar simulations for the Ising ferromagnet. Based on these results, we speculate on a connection between the nonequilibrium dynamics discussed here and averages computed theoretically using the “metastate.”

We demonstrate that a low-remanent sample magnetization $M_{\mathrm{R}}$ is crucial for all-optical magnetic switching (AOS) in ferrimagnets and ferrimagnet heterostructures. $M_{\mathrm{R}}$ may be devised by the composition of the material. However, it can also be controlled in situ by changing the sample temperature because it affects $M_{\mathrm{R}}$. We show that increasing the lattice temperature by laser pulses or a simple heating resistor enables AOS in a ferrimagnetic Tb-Fe film, which does not exhibit AOS at room temperature. We reconcile earlier contradicting results for AOS in heated and cooled magnetic films by applying the low-remanence criterion. It also applies to other existing rare-earth transition-metal (RE-TM) alloys, such as GdFeCo or Tb-Co, to ferrimagnet heterostructures, and to RE-free synthetic ferrimagnets.

Critical point scaling in a field $H$ applies for the limits $t\to 0$ (where $t=T/T_c-1)$ and $H\to 0$ but with the ratio $R=t/H^{2/\Delta }$ finite. $\Delta$ is a critical exponent of the zero-field transition. We study the replicon correlation length $\xi$ and from it the crossover scaling function $f\left(R\right)$ defined via $1/\left(\xi H^{4/(d+2-\eta )}\right)\sim f\left(R\right)$. We have calculated analytically $f\left(R\right)$ for the mean-field limit of the Sherrington-Kirkpatrick model. In dimension $d=3$, we have determined the exponents and the critical scaling function $f\left(R\right)$ within two versions of the Migdal-Kadanoff (MK) renormalization group procedure. One of the MK versions gives results for $f\left(R\right)$ in $d=3$ in reasonable agreement with those of the Monte Carlo simulations at the values of $R$ for which they can be compared. If there were a de Almeida-Thouless (AT) line for $d\le 6$, it would appear as a zero of the function $f\left(R\right)$ at some negative value of $R$, but there is no evidence for such behavior. This is consistent with the arguments that there should be no AT line for $d\le 6$, which we review.

The magnetic moment of a single-domain nanoparticle can be effectively switched on an ultrashort time scale by means of oscillating (microwave) magnetic fields. This switching technique can be further improved by using fields with time-dependent frequency (autoresonance). Here, we provide a full theoretical framework for the autoresonant switching technique, by exploiting the analogy between the magnetization state of an isolated nanoparticle and a two-level quantum system, whereby the switching process can be interpreted as a population transfer. We derive analytical expressions for the threshold amplitude of the microwave field, with and without damping, and consider the effect of thermal fluctuations. Comparisons with numerical simulations show excellent agreement.

We study a short multichannel superconducting junction subject to dc and ac phase biases. The ac modulation changes the occupation of the Andreev bound states formed at the constriction by transitions between bound states and the continuum. In a short junction, the nonequilibrium Andreev bound-state population may relax through processes that conserve parity of the occupation number on the same bound state and processes that do not conserve it. We argue that the parity-conserving processes occur on a much faster time scale. In this case, even a weak driving may lead to a large nonequilibrium quasiparticle population scaling with the number of channels and results in a large deviation of the supercurrent from its equilibrium value. We show that this effect is accompanied by a quasiparticle current which may lead to a measurable charge imbalance in the vicinity of the junction. Furthermore, we study the time evolution of the supercurrent after switching off the ac drive. On a time scale where parity relaxation is negligible, the supercurrent relaxes to a stationary nonequilibrium state. Finally, we briefly outline the regime of ultraweak driving where the ac-induced processes occur on a time scale comparable to that of parity relaxation.

We report results of zero-field muon spin relaxation experiments on the filled-skutterudite superconductors ${\mathrm{Pr}}_{1-x}{\mathrm{Ce}}_x{\mathrm{Pt}}_4{\mathrm{Ge}}_{12},x=0$, 0.07, 0.1, and 0.2, to investigate the effect of Ce doping on broken time-reversal symmetry (TRS) in the superconducting state. In these alloys broken TRS is signaled by the onset of a spontaneous static local magnetic field $B_s$ below the superconducting transition temperature. We find that $B_s$ decreases linearly with $x$ and $\to 0$ at $x\approx 0.4$, close to the concentration above which superconductivity is no longer observed. The ${\text{(Pr,Ce)Pt}}_4{\mathrm{Ge}}_{12}$ and isostructural ${\text{(Pr,La)Os}}_4{\mathrm{Sb}}_{12}$ alloy series both exhibit superconductivity with broken TRS, and in both the decrease of $B_s$ is proportional to the decrease of Pr concentration. This suggests that Pr-Pr intersite interactions are responsible for the broken TRS. The two alloy series differ in that the La-doped alloys are superconducting for all La concentrations, suggesting that in ${\text{(Pr,Ce)Pt}}_4{\mathrm{Ge}}_{12}$ pair-breaking by Ce doping suppresses superconductivity. For all $x$ the dynamic muon spin relaxation rate decreases somewhat in the superconducting state. This may be due to Korringa relaxation by conduction electrons, which is reduced by the opening of the superconducting energy gap.

We apply a low-field ($<100$ G) technique to measure the critical pair momentum $p_c$ in thin underdoped films of ${\mathrm{Y}}_{0.7}{\mathrm{Ca}}_{0.3}{\mathrm{Ba}}_2{\mathrm{Cu}}_3{\mathrm{O}}_{7-\delta }$ and ${\mathrm{Bi}}_2{\mathrm{Sr}}_2{\mathrm{CaCu}}_2{\mathrm{O}}_{8+\delta }$ reflecting a wide range of hole doping. We observe that $p_c\propto \hslash /\xi$ scales with $T_c$ and therefore superfluid density $n_s(T\to 0)$ in our two-dimensional cuprate films. This relationship was famously predicted by a universal model of the cuprates with a doping-independent superconducting gap, but has not been observed by high field $(>{10}^4\mathrm{G})$ measurements of the coherence length $\xi$ because high-field cuprate phenomena obscure the simple scaling we observe at much lower fields.

The electronic structure of ${\mathrm{Ce}}_3{\mathrm{Pd}}_{20}{X}_6(X=\text{Si,}\text{Ge})$ has been studied using detailed density functional theory (DFT) calculations and high-resolution photoelectron spectroscopy (PES) measurements. The orbital decomposition of the electronic structure by DFT calculations indicates that Ce atoms at the (8c) site surrounded by 16 Pd atoms have a tendency to be magnetic. Ce atoms at the (4a) site surrounded by 12 Pd and 6 $X$ atoms, on the other hand, are more localized and paramagnetic. The $4d\text{−}4f$ resonance PES measurements clearly indicate the Ce $4f$ contribution in the valence band in these compounds. The spectral weight of Ce $4f^0$ is stronger than that of Ce $4f^1$, indicating the localized nature of Ce $4f$ electrons. Near the Fermi level, the Ce $4f^1$ weight of ${\mathrm{Ce}}_3{\mathrm{Pd}}_{20}{\mathrm{Si}}_6$ is stronger than that of ${\mathrm{Ce}}_3{\mathrm{Pd}}_{20}{\mathrm{Ge}}_6$, suggesting stronger $c\text{−}f$ hybridization in the former.

The effects of mass imbalance in a three-component Hubbard model are studied by the dynamical mean-field theory combined with exact diagonalization. The model describes a fermion-fermion mixture of two different particle species with a mass imbalance. One species is two-component fermion particles, and the other is single-component ones. The local interaction between particle species is considered isotropically. It is found that the mass imbalance can drive the mixture from insulator to metal. The insulator-metal transition is a species-selective-like transition of lighter mass particles and occurs only at commensurate particle densities and moderate local interactions. For weak and strong local interactions the mass imbalance does not change the ground state of the mixture.

We report on theoretical investigations of the spin-orbit coupling effects in fluorinated graphene. First-principles density functional calculations are performed for the dense and dilute adatom coverage limits. The dense limit is represented by the single-side semifluorinated graphene, which is a metal with spin-orbit splittings of about 10 meV. To simulate the effects of a single adatom, we also calculate the electronic structure of a $10\times 10$ supercell, with one fluorine atom in the top position. Since this dilute limit is useful to study spin transport and spin relaxation, we also introduce a realistic effective hopping Hamiltonian, based on symmetry considerations, which describes the supercell bands around the Fermi level. We provide the Hamiltonian parameters which are best fits to the first-principles data. We demonstrate that, unlike for the case of hydrogen adatoms, fluorine's own spin-orbit coupling is the principal cause of the giant induced local spin-orbit coupling in graphene. The $s{p}^3$ hybridization induced transfer of spin-orbit coupling from graphene's $\sigma$ bonds, important for hydrogenated graphene, contributes much less. Furthermore, the magnitude of the induced spin-orbit coupling due to fluorine adatoms is about 1000 times more than that of pristine graphene, and 10 times more than that of hydrogenated graphene. Also unlike hydrogen, the fluorine adatom is not a narrow resonant scatterer at the Dirac point. The resonant peak in the density of states of fluorinated graphene in the dilute limit lies 260 meV below the Dirac point. The peak is rather broad, about 300 meV, making the fluorine adatom only a weakly resonant scatterer.

Graphene nanoflakes are interesting because electrons are naturally confined in these quasi-zero-dimensional structures, whereas confinement in bulk graphene would require a band gap. Vacancies inside the graphene lattice lead to localized states and the spins of such localized states may be used for spintronics. We perform a tight-binding description of a nanoflake with two vacancies and include a perpendicular magnetic field via a Peierls phase. The tunnel coupling strength and from it the exchange coupling between the localized states can be obtained from the energy splitting between numerically calculated bonding and antibonding energy levels. This allows us to estimate the exchange coupling $J$, which governs the dynamics of coupled spins. We predict the possibility of switching in situ from $J>0$ to $J=0$ by tuning the magnetic field. In the former case, the ground state will be antiferromagnetic with Néel temperatures accessible by experiment.

We use spin-density-functional theory within an envelope function approach to calculate electronic states in a GaAs/InAs core-shell nanowire pierced by an axial magnetic field. Our fully three-dimensional quantum modeling includes explicitly a description of the realistic cross section and composition of the sample, and the electrostatic field induced by external gates in two different device geometries: gate-all-around and back-gate. At low magnetic fields, we investigate Aharonov-Bohm oscillations and signatures therein of the discrete symmetry of the electronic system, and we critically analyze recent magnetoconductance observations. At high magnetic fields, we find that several charge and spin transitions occur. We discuss the origin of these transitions in terms of different localization and Coulomb regimes, and we predict their signatures in magnetoconductance experiments.

We present electrical transport measurements of a van-der-Waals heterostructure consisting of a graphene nanoribbon separated by a thin boron nitride layer from a micron-sized graphene sheet. The interplay between the two layers is discussed in terms of screening or, alternatively, quantum capacitance. The ribbon can be tuned into the transport gap by applying gate voltages. Multiple sites of localized charge leading to Coulomb blockade are observed, in agreement with previous experiments. Due to the strong capacitive coupling between the ribbon and the graphene top layer sheet, the evolution of the Coulomb blockade peaks in gate voltages can be used to obtain the local density of states and therefore the quantum capacitance of the graphene top layer. Spatially varying density and doping are found, which are attributed to a spatial variation of the dielectric due to fabrication imperfections.

We demonstrate a clear anticrossing between two polarization-orthogonal exciton-polariton modes in ZnO microwires. The strong coupling between exciton polaritons with orthogonal polarization is evidenced by using angle-resolved photoluminescence spectroscopy. The evolution of the coupling strength with the detuning between the two coupled polariton modes is studied by scanning the excitation spot on a tapered ZnO microwire. Steady oscillations are established between the two differently polarized polariton modes with resonant wave vectors and frequencies, which is well described by the plane wave coupling model.

We measured the intrinsic ground-state exciton dephasing and population dynamics in colloidal quasi-two-dimensional (2D) CdSe nanoplatelets at low temperature (5–50 K) using transient resonant four-wave mixing in heterodyne detection. Our results indicate that below 20 K the exciton dephasing is lifetime limited, with the exciton population lifetime being as fast as 1 ps. This is consistent with an exciton lifetime given by a fast radiative decay due to the large in-plane coherence area of the exciton center-of-mass motion in these quasi-2D systems compared to spherical nanocrystals. The radiative rate in such 2D platelet systems can be controlled by the platelet area over orders of magnitude.