We apply a recently developed nonequilibrium real-time renormalization group (RG) method in frequency space to describe nonlinear quantum transport through a small fermionic quantum system coupled weakly to several reservoirs via spin and/or orbital fluctuations. Within a weak-coupling two-loop analysis, we derive analytic formulas for the nonlinear conductance and the kernel determining the time evolution of the reduced density matrix. A consistent formalism is presented how the RG flow is cut off by relaxation and dephasing rates. We apply the general formalism to the nonequilibrium anisotropic Kondo model at finite magnetic field. We consider the weak-coupling regime, where the maximum of voltage and bare magnetic field is larger than the Kondo temperature. In this regime, we calculate the nonlinear conductance, the magnetic susceptibility, the renormalized spin relaxation and dephasing rates, and the renormalized $g$ factor. All quantities are considered up to the first logarithmic correction beyond leading order at resonance. Up to a redefinition of the Kondo temperature, we confirm previous results for the conductance and the magnetic susceptibility in the isotropic case. In addition, we present a consistent calculation of the resonant line shapes, including the determination whether the spin relaxation or dephasing rate cuts off the logarithmic divergence. Furthermore, we calculate quantities characterizing the exponential decay of the time evolution of the magnetization. In contrast to the conductance, we find that the derivative of the spin relaxation (dephasing) rate with respect to the magnetic field is logarithmically enhanced (suppressed) for voltages smaller (larger) than the renormalized magnetic field, and that the logarithmic divergence is cut off by the opposite rate. The renormalized $g$ factor is predicted to show a symmetric logarithmic suppression at resonance, which is cut off by the spin relaxation rate. We propose a three-terminal setup to measure the suppression at resonance. For all quantities, we analyze also the anisotropic case and find additional nonequilibrium effects at resonance.

An interplay between the increase in the number of carriers and the decrease in the scattering time is expected to result in a saturation of the in-plane resistivity, ${\rho }_{ab}$, in graphite above room temperature. Contrary to this expectation, we observe a pronounced increase in ${\rho }_{ab}$ in the interval between 300 and 900 K. We provide a theory of this effect based on intervalley scattering of charge carriers by high-frequency, graphenelike optical phonons.

This is an in situ measurement of the full stress tensor and its evolution in a growing deformation twin and, simultaneously, in the grain where the twin forms. The combined information provides a detailed picture of the grain-twin interaction. The three-dimensional x-ray diffraction method using 80.7 keV synchrotron x rays allows us to in situ investigate a grain within the bulk of a magnesium alloy (AZ31) sample that is compressed to activate the $\left\{10\overline{1}2\right\}⟨\overline{1}011⟩$ tensile twin system. We observe that the stress state of the twin is drastically different from the one of the grain in which it is embedded. We analyze such result in terms of the shear transformation associated with twinning and the dimensional constraints imposed by the surrounding aggregate.

We have carried out a comprehensive investigation of magnetic properties of ${\text{LuFe}}_2{\text{O}}_{4+\delta }$, measuring ac susceptibility, dc magnetization, and specific heat. A magnetic phase transition around 236 K is identified as a paramagnetic to ferrimagnetic transition in accordance with previous studies. Upon further cooling below this temperature, highly relaxational magnetic behavior is observed: the dc magnetization exhibits history and time dependence, and the real and the imaginary parts of the ac susceptibility show large frequency dependence. Dynamic scaling of the ac susceptibility data suggests that this low-temperature phase can be described as a spin-glass phase. We also discuss the magnetic field dependence of the spin-glass transition and aging, memory, and rejuvenation effect below the glass transition temperature around 229 K.

Real- and momentum-space spectrally resolved images of microcavity polariton emission in the regime of condensation are investigated under nonresonant excitation using a laser source with reduced intensity fluctuations on the time scale of the exciton lifetime. We observe that the polariton emission consists of many macroscopically occupied modes. Lower-energy modes are strongly localized by the spatial polaritonic potential disorder on a scale of a few microns. Higher-energy modes have finite $k$ vectors and are delocalized over $10–15\mu \text{m}$. All the modes exhibit long-range spatial coherence comparable to their size. We provide a theoretical model describing the behavior of the system with the results of the simulations in good agreement with the experimental observations. We show that the multimode emission of the polariton condensate is a result of its nonequilibrium character, the interaction with the local polaritonic potential, and the reduced intensity fluctuations of the excitation laser.

In a high-mobility two-dimensional hole gas (2DHG) in C-doped (001) $\text{GaAs}/{\text{Al}}_{0.4}{\text{Ga}}_{0.6}\text{As}$ quantum wells, we observe sharp features in the differential magnetoresistance, which we interpret as the Zener-tunneling peak and valley associated with the commensuration transition of Landau orbits. Comparison of data with that in $\text{GaAs}/{\text{Al}}_{0.3}{\text{Ga}}_{0.7}\text{As}$ 2D electron gas suggests that the amplitude of Zener oscillations in a 2DHG is strongly damped. The data indicate the significant role of band structures in nonlinear transport in 2D electronic systems.

We consider the contribution of superconducting fluctuations in the mesoscopic persistent current (PC) of an ensemble of normal metallic rings, made of a superconducting material whose low bare transition temperature $T_c^0$ is much smaller than the Thouless energy $E_c$. The effect of pair breaking is introduced via the example of magnetic impurities. We find that over a rather broad range of pair-breaking strength $\hslash /{\tau }_s$, such that $T_c^0\lesssim \hslash /{\tau }_s\lesssim E_c$, the superconducting transition temperature is normalized down to minute values or zero while the PC is hardly affected. This may provide an explanation for the magnitude of the average PCs in copper and gold as well as a way to determine their $T_c^0$’s. The dependence of the current and the dominant superconducting fluctuations on $E_c{\tau }_s$ and on the ratio between $E_c$ and the temperature is analyzed. The measured PCs in copper (gold) correspond to $T_c^0$ of a few (a fraction of) mK.

Hf distributions in as-grown and annealed ${\left({\text{HfO}}_2\right)}_{0.25}{\left({\text{SiO}}_2\right)}_{0.75}$ films with thicknesses in the range of 4–19 nm were investigated by high-resolution transmission electron microscopy, high angle annular dark field scanning transmission electron microscopy and angle-resolved x-ray photoelectron spectroscopy. Composition waves normal to the film surface were observed providing evidence for surface-directed spinodal decomposition in the pseudobinary ${\left({\text{HfO}}_2\right)}_x{\left({\text{SiO}}_2\right)}_{1-x}$ alloy system.

Linear-response conductance of a two-terminal Aharonov-Bohm (AB) interferometer is an even function of magnetic field. This phase symmetry is not expected to hold beyond the linear-response regime and in simple AB rings the phase of the oscillations changes smoothly (almost linearly) with voltage bias. However, in an interferometer with a quantum dot in its arm, tuned to the Coulomb blockade regime, experiments indicate that phase symmetry seems to persist even in the nonlinear regime. In this paper we discuss the processes that break AB phase symmetry and show that breaking of phase symmetry in such an interferometer is possible only after the onset of inelastic cotunneling, i.e., when the voltage bias is larger than the excitation energy in the dot. The asymmetric component of AB oscillations is significant only when the contributions of different levels to the symmetric component nearly cancel out (e.g., due to different parity of these levels), which explains the sharp changes in the AB phase. We show that our theoretical results are consistent with experimental findings.

An experiment is presented which allows the observation of charge-carrier pair formation that precedes electronic transitions such as spin-dependent recombination or spin-dependent transport. It is based on an electrically detected magnetic-resonance-induced rotary echo sequence. The experimental demonstration is performed on precursor (spin) pairs of electrons in the emitter layer of crystalline silicon/amorphous silicon heterostructures. Precursor pair-generation-rate coefficients extracted from these measurements are studied as a function of light intensity and are found to show only a minor dependence on the illumination level indicating that the pair generation is not determined by charge-carrier densities.

We explore the collective electromagnetic response in atomic clusters of various sizes and geometries. Our aim is to understand, and hence to control, their dielectric response based on a fully quantum-mechanical description which captures accurately their relevant collective modes. The electronic energy levels and wave functions, calculated within the tight-binding model, are used to determine the nonlocal dielectric response function. It is found that the system shape, the electron filling, and the driving frequency of the external electric field strongly control the resonance properties of the collective excitations in the frequency and spatial domains. Furthermore, it is shown that one can design spatially localized collective excitations by properly tailoring the nanostructure geometry.

We study electron transport in quasi-one-dimensional wires at relatively weak electrostatic confinements, where the Coulomb interaction distorts the ground state, leading to the bifurcation of the electronic system into two rows. Evidence of finite coupling between the rows, resulting in bonding and antibonding states, is observed. At high dc source-drain bias, a structure is observed at $0.5\left(2e^2/h\right)$ due to parallel double-row transport, along with a structure at $0.25\left(2e^2/h\right)$, providing further evidence of coupling between the two rows.

Using x-ray absorption (XAS) and resonant inelastic x-ray scattering (RIXS), charge dynamics at and near the $\text{Fe}L$ edges is investigated in Fe-pnictide materials and contrasted to that measured in other Fe compounds. It is shown that the XAS and RIXS spectra for 122 and 1111 Fe pnictides are each qualitatively similar to Fe metal. Cluster diagonalization, multiplet, and density-functional calculations show that Coulomb correlations are much smaller than in the cuprates, highlighting the role of Fe metallicity and strong covalency in these materials. The best agreement with experiment is obtained using Hubbard parameters $U\lesssim 2\text{eV}$ and $J\approx 0.8\text{eV}$.

The dynamics of single electron and nuclear spins in a diamond lattice with different ${}^{13}\text{C}$ nuclear spin concentration is investigated. It is shown that coherent control of up to three individual nuclei in a dense nuclear spin cluster is feasible. The free-induction decays of nuclear spin Bell states and single nuclear coherences among ${}^{13}\text{C}$ nuclear spins are compared and analyzed. Reduction in a free-induction-decay time $T_2^{\ast }$ and a coherence time $T_2$ upon increase in nuclear spin concentration has been found. For pure diamond, $T_2^{\ast }$ as long as $30\mu \text{s}$ and $T_2$ of up to 0.65 ms for the electron spin has been observed. The ${}^{13}\text{C}$ concentration dependence of $T_2^{\ast }$ is explained by Fermi contact and dipolar interactions with nuclei in the lattice. It has been found that $T_2$ decreases approximately as $1/n$, where $n$ is ${}^{13}\text{C}$ concentration, which corresponds to the reported theoretical line of $T_2$ for an electron spin interacting with a nuclear spin bath.

We report an estimate $\nu =2.593$ [2.587,2.598] of the critical exponent of the Chalker-Coddington model of the integer quantum Hall effect that is significantly larger than previous numerical estimates and in disagreement with experiment. This suggests that models of noninteracting electrons cannot explain the critical phenomena of the integer quantum Hall effect.

Cluster expansions have proven to be a valuable tool in alloy theory and other problems in materials science but the generation of cluster expansions can be a computationally expensive and time-consuming process. We present a Bayesian framework for developing cluster expansions that explicitly incorporates physical insight into the fitting procedure. We demonstrate how existing methods fit within this framework and use the framework to develop methods that significantly improve the predictive power of cluster expansions for a given training set size. The key to the methods is to apply physical insight and cross validation to develop physically meaningful prior probability distributions for the cluster expansion coefficients. We use the Bayesian approach to develop an efficient method for generating cluster expansions for low-symmetry systems such as surfaces and nanoparticles.

Direct analytic continuation of the self-energy is used to determine the effect of antiferromagnetic ordering on the spectral function and optical conductivity of a Mott insulator. Comparison of several methods shows that the most robust estimation of the gap value is obtained by use of the real part of the continued self-energy in the quasiparticle equation within the single-site dynamical mean-field theory of the two-dimensional square lattice Hubbard model, where, for a $U$ that is slightly greater than the Mott critical value, antiferromagnetism increases the gap by about 80%.

Ab initio electronic structure calculations on potassium at high pressure reveal that the orthorhombic $oP8$ structure, found recently in sodium, has a lower enthalpy in potassium than do other candidate crystal structures recently reported to be stable at high pressure [Y. Ma, A. R. Oganov, and Y. Xie, Phys. Rev. B 78, 014102 (2008)]. Subsequent powder-diffraction studies to above 100 GPa confirm $oP8$ to be the stable phase of potassium from 54(2) to 90(2) GPa, where it transforms to the tetragonal $tI4$ structure, which in turn transforms to the orthorhombic $oC16$ structure at 96(3) GPa.

We study electron transport in a double quantum dot in the Pauli spin blockade regime in the presence of strong spin-orbit coupling. The effect of spin-orbit coupling is incorporated into a modified interdot tunnel coupling. We elucidate the role of the external magnetic field, the nuclear fields in the dots, and the spin relaxation. We find qualitative agreement with experimental observations, and we propose a way to extend the range of magnetic fields in which blockade can be observed.

We have studied the frequency and voltage dependences of voltage-induced torsional strains in orthorhombic ${\text{TaS}}_3$ [V. Y. Pokrovskii, S. G. Zybtsev, and I. G. Gorlova, Phys. Rev. Lett. 98, 206404 (2007)] by measuring the modulation of the resonant frequency of an rf cavity containing the sample. The strain has an onset voltage below the charge-density-wave (CDW) threshold voltages associated with changes in shear compliance and resistance, suggesting that the strain is associated with polarization of the CDW rather than CDW current. Measurements with square-wave voltages show that the strain is very sluggish, not even reaching its dc value at a frequency of 0.1 Hz, but the dynamics appears to be very sample dependent. By applying oscillating torque while biasing the sample with a dc current, we have also looked for strain-induced voltage in the sample; none is observed at the low biases where the voltage-induced strains first occur, but an induced voltage is observed at higher biases, probably associated with a strain-dependent CDW conductance.