An angle-swept high-frequency electron paramagnetic resonance (HFEPR) technique is described that facilitates efficient in situ alignment of single-crystal samples containing low-symmetry magnetic species such as single-molecule magnets (SMMs). This cavity-based technique involves recording HFEPR spectra at fixed frequency and field, while sweeping the applied field orientation. The method is applied to the study of a low-symmetry Jahn-Teller variant of the extensively studied spin $S=10$ ${\text{Mn}}_{12}$ SMMs (e.g., ${\text{Mn}}_{12}$-acetate). The low-symmetry complex also exhibits SMM behavior, but with a significantly reduced effective barrier to magnetization reversal $\left(U_{\text{eff}}\approx 43\text{K}\right)$ and, hence, faster relaxation at low temperature in comparison with the higher-symmetry species. ${\text{Mn}}_{12}$ complexes that crystallize in lower symmetry structures exhibit a tendency for one or more of the Jahn-Teller axes associated with the ${\text{Mn}}^{\text{III}}$ atoms to be abnormally oriented, which is believed to be the cause of the faster relaxation. An extensive multi-high-frequency angle-swept and field-swept electron paramagnetic resonance study of $\left[{\text{Mn}}_{12}{\text{O}}_{12}{\left({\text{O}}_2{\text{CCH}}_2{\text{Bu}}^t\right)}_{16}{\left({\text{H}}_2\text{O}\right)}_4\right]\cdot {\text{CH}}_2{\text{Cl}}_2\cdot {\text{MeNO}}_2$ is presented in order to examine the influence of the abnormally oriented Jahn-Teller axis on the effective barrier to magnetization reversal. The reduction in the axial anisotropy, $D$, is found to be insufficient to account for the nearly 40% reduction in $U_{\text{eff}}$. However, the reduced symmetry of the ${\text{Mn}}_{12}$ core gives rise to a very significant second-order transverse (rhombic) zero-field-splitting anisotropy, $E\approx D/6$. This, in turn, causes a significant mixing of spin projection states well below the top of the classical anisotropy barrier. Thus, magnetic quantum tunneling is the dominant factor contributing to the effective barrier reduction in fast relaxing ${\text{Mn}}_{12}$ SMMs.

The symmetry ordering of the valence bands in ZnO is derived from high-resolution magneto-optical measurements of bound excitons. We report on the experimental observation of a hole state related fine splitting for bound excitons in the Voigt configuration. This splitting is related to a nonzero Landé $g$ value $g_{\perp }$ for hole states from the $A$ valence band. Based on theoretical considerations, the symmetry of the uppermost valence band is doubtlessly identified as ${\mathbf{\Gamma }}_{\mathbf{7}}$. This attribution is confirmed by polarization and angular resolved magnetophotoluminescence spectroscopy.

We classify all possible 36 gap-opening instabilities in graphenelike structures in two dimensions, i.e., masses of Dirac Hamiltonian when the spin, valley, and superconducting channels are included. These 36 order parameters break up into 56 possible quintuplets of masses that add in quadrature and hence do not compete and thus can coexist. There is additionally a sixth competing mass, the one added by Haldane to obtain the quantum Hall effect in graphene without magnetic fields, which breaks time-reversal symmetry and competes with all other masses in any of the quintuplets. Topological defects in these five-dimensional order parameters can generically bind excitations with fractionalized quantum numbers. The problem simplifies greatly if we consider spin-rotation invariant systems without superconductivity. In such simplified systems, the possible masses are only 4 and correspond to the Kekulé dimerization pattern, the staggered chemical potential, and the Haldane mass. Vortices in the Kekulé pattern are topological defects that have Abelian fractional statistics in the presence of the Haldane term. We calculate the statistical angle by integrating out the massive fermions and constructing the effective field theory for the system. Finally, we discuss how one can have generically non-Landau-Ginzburg-type transitions with direct transitions between phases characterized by distinct order parameters.

We report on the investigation of the quasiparticle local density of states and superconducting gap in the iron chalcogenide superconductor ${\text{Fe}}_{1+\delta }{\text{Se}}_{1-x}{\text{Te}}_x\left(T_{\text{c}}\sim 14\text{K}\right)$. The surface of a cleaved crystal revealed an atomic square lattice, superimposed on the inhomogeneous background, with a lattice constant of $\sim 3.8\text{Å}$ without any reconstruction. Tunneling spectra measured at 4.2 K exhibit the superconducting gap, which completely disappears at 18 K, with a magnitude of $\sim 2.3\text{meV}$, corresponding to $2\Delta /k_{\text{B}}{T}_{\text{c}}=3.8$. In stark contrast to the cuprate superconductors, the value of the observed superconducting gap is relatively homogeneous, following a sharp distribution with a small standard deviation of 0.23 meV. Conversely, the normal-state local density of states observed above $T_{\text{c}}$ shows spatial variation over a wide energy range of more than 1 eV, probably due to the excess iron present in the crystal.

This paper presents an efficient algorithm for computing the transition probability in auxiliary field quantum Monte Carlo simulations of strongly correlated electron systems using a Hubbard model. This algorithm is based on a low rank updating of the underlying linear algebra problem, and results in significant computational savings. The computational complexity of computing the transition probability and Green’s function update reduces to $\mathcal{O}\left(k^2\right)$ during the $k\text{th}$ step, where $k$ is the number of accepted spin flips, and results in an algorithm that is faster than the competing delayed update algorithm. Moreover, this algorithm is orders of magnitude faster than traditional algorithms that use naive updating of the Green’s function matrix.

We calculate, within a self-consistent Hartree-Fock approximation, the local density of states for different electron crystals in graphene subject to a strong magnetic field. We investigate both the Wigner crystal and bubble crystals with $M_{\text{e}}$ electrons per lattice site. The total density of states consists of several pronounced peaks, the number of which in the negative energy range coincides with the number of electrons $M_{\text{e}}$ per lattice site, as for the case of electron-solid phases in the conventional two-dimensional electron gas. Analyzing the local density of states at the peak energies, we find particular scaling properties of the density patterns if one fixes the ratio ${\nu }_N/M_{\text{e}}$ between the filling factor ${\nu }_N$ of the last partially filled Landau level and the number of electrons per bubble. Although the total density profile depends explicitly on $M_{\text{e}}$, the local density of states of the lowest peaks turns out to be identical regardless the number of electrons $M_{\text{e}}$. Whereas these electron-solid phases are reminiscent of those expected in the conventional two-dimensional electron gas in GaAs heterostructures in the quantum Hall regime, the local density of states and the scaling relations we highlight in this paper may be, in graphene, directly measured by spectroscopic means, such as, e.g., scanning tunneling microscopy.

We study two-dimensional Heisenberg antiferromagnets with additional multispin interactions which can drive the system into a valence-bond-solid state. For standard SU(2) spins, we consider both four- and six-spin interactions. We find continuous quantum phase transitions with the same critical exponents. Extending the symmetry to $\text{SU}\left(N\right)$, we also find continuous transitions for $N=3$ and 4. In addition, we also study quantitatively the crossover of the order-parameter symmetry from $Z_4$ deep inside the valence-bond-solid phase to $U\left(1\right)$ as the phase transition is approached.

In a GaMnAs/AlGaAs resonant tunneling diode (RTD) structure, we observe that both the magnitude and polarity of magnetoresistance are bias dependent when tunneling from a three-dimensional GaMnAs layer through a two-dimensional GaMnAs quantum well. This magnetoresistance behavior results from a shift of negative differential resistance features to higher bias as the relative alignment of the GaMnAs layer magnetizations is changed from parallel to antiparallel. Our observations agree with recent predictions from a theoretical analysis of a similar $n$-type structure by Ertler and Fabian, and our results suggest that further investigation into ferromagnetic RTD structures may result in significantly enhanced magnetoresistance.

We report time and frequency domain studies of spin-torque-driven vortex self-oscillations at zero magnetic field. We observe two types of abrupt fluctuations in the frequency and amplitude, with very long random mean lifetimes ($\sim {10}^2$ to $\sim {10}^4$ oscillation cycles). First, we observe fluctuations between two center frequencies separated by 10s of MHz that we identify with switching between quasiuniform and vortex states of the magnetic polarizing layer. Second, we resolve much smaller, discrete frequency fluctuations that lead to a fine structure of the oscillations. We find that this fine structure plays a key role in determining the long-time average linewidths and we suggest a possible physical origin.

We have studied the quantum oscillations of the conductance for arrays of connected mesoscopic metallic rings, in the presence of an external magnetic field. Several geometries have been considered: a linear array of rings connected with short or long wires compared to the phase coherence length, square networks, and hollow cylinders. Compared to the well-known case of the isolated ring, we show that for connected rings, the winding of the Brownian trajectories around the rings is modified, leading to a different harmonics content of the quantum oscillations. We relate this harmonics content to the distribution of winding numbers. We consider the limits where coherence length $L_{\phi }$ is small or large compared to the perimeter $L$ of each ring constituting the network. In the latter case, the coherent diffusive trajectories explore a region larger than $L$, whence a network-dependent harmonics content. Our analysis is based on the calculation of the spectral determinant of the diffusion equation for which we have a simple expression on any network. It is also based on the hypothesis that the time dependence of the dephasing between diffusive trajectories can be described by an exponential decay with a single characteristic time ${\tau }_{\phi }$ (model A). At low temperature, decoherence is limited by electron-electron interaction, and can be modeled in a one-electron picture by the fluctuating electric field created by other electrons (model B). It is described by a functional of the trajectories and thus the dependence on geometry is crucial. Expressions for the magnetoconductance oscillations are derived within this model and compared to the results of model A. It is shown that they involve several temperature-dependent length scales.

We have measured a strong increase of the low-temperature resistivity ${\rho }_{xx}$ and a zero-value plateau in the Hall conductivity ${\sigma }_{xy}$ at the charge neutrality point in graphene subjected to high magnetic fields up to 30 T. We explain our results by a simple model involving a field dependent splitting of the lowest Landau level of the order of a few Kelvin, as extracted from activated transport measurements. The model reproduces both the increase in ${\rho }_{xx}$ and the anomalous $\nu =0$ plateau in ${\sigma }_{xy}$ in terms of coexisting electrons and holes in the same split zero-energy Landau level.

Through a systematic high-resolution angle-resolved photoemission study of the iron pnictide compounds $\left(\text{Ba},\text{Sr}\right){\text{Fe}}_2{\text{As}}_2$, we show that the electronic structures of these compounds are significantly reconstructed across the spin density wave transition, which cannot be described by a simple folding scenario of conventional density wave ordering. Moreover, we find that LDA calculations with an incorporated suppressed magnetic moment of $0.5{\mu }_{\text{B}}$ can match well the details in the reconstructed electronic structure, suggesting that the nature of magnetism in the pnictides is more itinerant than local, while the origin of suppressed magnetic moment remains an important issue for future investigations.

We introduce the complex band structure and a medium-dependent (Green’s function) quantum-optics formalism to study the enhanced spontaneous emission factors and Lamb shifts from a quantum dot or atom near the surface of a slow-light metamaterial waveguide. Using a realistic loss factor of $\gamma /2\pi =2\text{THz}$, Purcell factors of approximately 250 and 100 are found at optical frequencies for $p$-polarized and $s$-polarized dipoles, respectively, placed 28 nm $\left(0.02{\lambda }_0\right)$ above the slab surface. For smaller loss values, we demonstrate that the slow-light regime of odd metamaterial waveguide propagation modes can be observed and related to distinct resonances in the Purcell factors. Correspondingly, we predict unusually large and rich Lamb shifts of approximately $-1$ to $-6\text{GHz}$ for a dipole moment of 50 Debye. We also make a direct calculation of the far-field-emission spectra which contains direct measurable access to these enhanced Purcell factors and Lamb shifts.

The cooling dynamics of glass-embedded noble metal nanoparticles with diameters ranging from 4 to 26 nm were studied using ultrafast pump-probe spectroscopy. Measurements were performed probing away from the surface plasmon resonance of the nanoparticles to avoid spurious effects due to glass heating around the particle. In these conditions, the time-domain data reflect the cooling kinetics of the nanoparticle. Cooling dynamics are shown to be controlled by both thermal resistance at the nanoparticle-glass interface and heat diffusion in the glass matrix. Moreover, the interface conductances are deduced from the experiments and found to be correlated with the acoustic impedance mismatch at the metal/glass interface.

We report on the experimental verification of the Zurek-Kibble scenario in an isolated superconducting ring over a wide parameter range. The probability of creating a single flux quantum spontaneously during the fast normal-superconducting phase transition of a wide Nb loop clearly follows a scaling relation on the quenching time ${\tau }_Q$, as one would expect if the transition took place as fast as causality permits. However, the observed Zurek-Kibble scaling exponent $\sigma =0.62\pm 0.15$ is two times larger than anticipated for large loops. Assuming Gaussian winding number densities we show that this doubling is well founded for small annuli.

The detailed optical properties of ${\text{BaFe}}_2{\text{As}}_2$ have been determined over a wide frequency range above and below the structural and magnetic transition at $T_N\simeq 138\text{K}$. A prominent in-plane infrared-active mode is observed at $253{\text{cm}}^{-1}$ (31.4 meV) at 295 K. The frequency of this vibration shifts discontinuously at $T_N$; for $T<T_N$ the frequency of this mode displays almost no temperature dependence, yet it nearly doubles in intensity. This anomalous behavior appears to be a consequence of orbital ordering in the Fe-As layers.

Picosecond Raman scattering is used to study the photoinduced ultrafast dynamics in Peierls distorted antimony. We find evidence for an ultrafast nonthermal reversible structural phase transition. Most surprisingly, we find evidence that this transition evolves toward a lower symmetry in contrast to the commonly accepted rhombohedral-to-simple cubic transition path. Our study demonstrates the feasibility of ultrafast Raman scattering symmetry analysis of photoinduced nonthermal transient phases.

Using general symmetry arguments and model calculations we analyze the superconducting gap in materials with multiple Fermi-surface pockets, with applications to iron pnictides. We show that the gap in the pnictides has an extended $s$-wave symmetry but is either nodeless or has nodes, depending on the interplay between intraband and interband interactions. We argue that the nodes in the gap emerge without a phase transition as the tendency toward a spin-density-wave order gets weaker. These findings provide a way to reconcile seemingly conflicting results of numerical and experimental studies of the pnictides.

We report on the kinetics of the inner ring in the exciton emission pattern. The formation time of the inner ring following the onset of the laser excitation is found to be about 30 ns. The inner ring is also found to disappear within 4 ns after the laser termination. The latter process is accompanied by a jump in the photoluminescence (PL) intensity. The spatial dependence of the PL jump indicates that the excitons outside of the region of laser excitation, including the inner ring region, are efficiently cooled to the lattice temperature even during the laser excitation. The ring formation and disappearance are explained in terms of exciton transport and cooling.

Element-specific x-ray magnetic circular dichroism measurements at the $\text{Ru}{L}_3$ absorption edge are used to search for the presence of a net Ru ferromagnetic moment in the superconducting state of ${\text{RuSr}}_2{\text{Eu}}_{1.5}{\text{Ce}}_{0.5}{\text{Cu}}_2{\text{O}}_{10}$. A net moment of $0.21{\mu }_B/\text{Ru}$ is observed in zero applied field. Together with a homogeneous Ru local structure probed by x-ray absorption fine-structure measurements, the results unequivocally demonstrate the coexistence of a ferromagnetic component in the magnetically ordered ${\text{RuO}}_2$ planes with superconductivity in the ${\text{CuO}}_2$ planes.