The magnetic field and temperature dependences of the thermoelectric power (TEP) of the antiferromagnetically ordered heavy fermion compound YbAgGe are measured across the field-induced quantum critical point. These TEP measurements reproduce the earlier $H\text{−}T$ phase diagram and identify an additional domelike phase between $\sim 45$ and $\sim 70\text{kOe}$. On the low-field side of this region, $H>H_c\sim 45\text{kOe}$, the sign of the TEP changes from negative to positive; on the high-field side of this region, $H\approx 70\text{kOe}$, a non-Fermi-liquid state is evidenced as the logarithmic temperature dependence of $S\left(T\right)/T$, in agreement with previous specific heat results $C\left(T\right)/T\propto -\text{log}\left(T\right)$. For higher fields, $H>70\text{kOe}$, the observed large value of $\alpha$, $S\left(T\right)=\alpha T$, is indicative of the heavy fermion state and shows a correlation with $C\left(T\right)/T$.

We have conducted structure measurements of ${\text{SiO}}_2$ glass under high pressure up to 100 GPa by an energy-dispersive x-ray diffraction method with synchrotron white x rays. X-ray diffraction data indicate that ${\text{SiO}}_2$ glass transforms from a fourfold- to a sixfold-coordinated structure at pressures between 20 and 35 GPa, and then behaves as an amorphous polymorph having a sixfold-coordinated structure at least up to 100 GPa. These results are consistent with those of density measurements by an x-ray absorption method with monochromatic x rays, except for the slight difference in the pressure range where the transformation from a fourfold- to a sixfold-coordinated structure is completed. They are also mostly consistent with those of structure measurements by an x-ray diffraction method with monochromatic x rays but some aspects are quantitatively different. It is presumed that the irradiation of white x rays may have relaxed the structure and deviatoric stresses of ${\text{SiO}}_2$ glass in this study.

We derive effective Hamiltonians for lattice bosons with strong geometrical frustration of the kinetic energy by projecting the interactions on the flat lowest Bloch band. Specifically, we consider the Bose Hubbard model on the one-dimensional sawtooth lattice and the two-dimensional kagome lattice. Starting from a strictly local interaction the projection gives rise to effective long-range terms stabilizing a supersolid phase at densities above ${\nu }_c=1/9$ of the kagome lattice. In the sawtooth lattice on the other hand we show that the solid order, which exists at the magic filling ${\nu }_c=1/4$, is unstable to further doping. The universal low-energy properties at filling $1/4+\delta \nu$ are described by the well-known commensurate-incommensurate transition. We support the analytic results by detailed numerical calculations using the density-matrix renormalization group and exact diagonalization. Finally, we discuss possible realizations of the models using ultracold atoms as well as frustrated quantum magnets in high magnetic fields. We compute the momentum distribution and the noise correlations, that can be extracted from time of flight experiments or neutron scattering, and point to signatures of the unique supersolid phase of the kagome lattice.

A detailed knowledge of the gap structure for the Fe-pnictide superconductors is still rather rudimentary with several conflicting reports of either nodes, deep gap minima, or fully isotropic gaps on the Fermi-surface sheets, both in the $k_x\text{−}{k}_y$ plane and along the $c$ axis. In this Rapid Communication, we present considerations for electronic Raman scattering which can help clarify the gap structure and topology using different light-scattering geometries. Using density-functional calculations for the Raman vertices, it is shown that the location of the gap minima may occur on loops stretching over a portion of the $c$ axis in $\text{Ba}{\left({\text{Fe}}_{0.94}{\text{Co}}_{0.06}\right)}_2{\text{As}}_2$.

It is a theoretical conjecture that 5/2 fractional quantum Hall state charge e/4 excitations may obey non-Abelian statistics. In edge state interference these purported non-Abelian quasiparticles should display period e/4 Aharonov-Bohm oscillations if the interfering quasiparticle encircles an even number of localized e/4 charges but suppression of oscillations if an odd number is encircled. To test this hypothesis, here we perform swept area interference measurements at 5/2. We observe an alternating pattern of e/4 and e/2 period oscillations in resistance for a large change in the interferometer area, with the area sweep changing the number of enclosed localized e/4 quasiparticles. This aperiodic alternation is consistent with proposed non-Abelian properties: the e/4 oscillations occur for encircling an even number of localized quasiparticles, and the lower amplitude e/2 oscillations are observed when encircling an odd number. The large amplitude e/4 oscillations dominate the measurement when the localized quasiparticle number is even, but at odd number they are suppressed, allowing observation of the smaller e/2 oscillations that may be persistent throughout the measurement. The aperiodic alternation corresponds to the area sweep sampling an expected aperiodic spatial distribution of localized quasiparticles. Importantly, adding localized quasiparticles to the encircled area by changing magnetic field can change the parity of the enclosed quasiparticle number and should induce interchange of the expressed e/4 and e/2 periods: just such interchange of the e/4 and e/2 oscillation periods is observed in measurements presented here, with agreement between the measured and calculated values of $B$ field necessary to add a single quasiparticle to the interferometer area. These results are specifically consistent with proposed non-Abelian e/4 quasiparticle properties.

A body-centered tetragonal carbon (bct-carbon) allotrope has been predicted to be a transparent carbon polymorph obtained under pressure. The structural transition pathways from graphite to diamond, M-carbon, and bct-carbon are simulated and the lowest activation barrier is found for the graphite-bct transition. Furthermore, bct-carbon has a higher shear strength than diamond due to its perpendicular graphenelike structure. Our results provide a possible explanation for the formation of a transparent carbon allotrope via the cold compression of graphite. We also verify that this allotrope is hard enough to crack diamond.

Using scanning gate microscopy (SGM), we probe the scattering between a beam of electrons and a two-dimensional electron gas (2DEG) as a function of the beam’s injection energy, and distance from the injection point. At low injection energies, we find electrons in the beam scatter by small angles, as has been previously observed. At high injection energies, we find a surprising result: placing the SGM tip where it backscatters electrons increases the differential conductance through the system. This effect is explained by a nonequilibrium distribution of electrons in a localized region of 2DEG near the injection point. Our data indicate that the spatial extent of this highly nonequilibrium distribution is within $\sim 1\mu \text{m}$ of the injection point. We approximate the nonequilibrium region as having an effective temperature that depends linearly upon injection energy.

We investigate the stability of quantum critical points (QCPs) in the presence of two competing phases. These phases near QCPs are assumed to be either classical or quantum and assumed to repulsively interact via square-square interactions. We find that for any dynamical exponents and for any dimensionality strong enough interaction renders QCPs unstable and drives transitions to become first order. We propose that this instability and the onset of first-order transitions lead to spatially inhomogeneous states in practical materials near putative QCPs. Our analysis also leads us to suggest that there is a breakdown of conformal field theory scaling in the Anti de Sitter models, and in fact these models contain first-order transitions in the strong-coupling limit.

Using density-functional theory (DFT) in combination with thermodynamic considerations the morphology change in $\text{Re}\left(11\overline{2}1\right)$ in the presence of a nitrogen atmosphere has been investigated. In agreement with experimental observations, we find that nitrogen adsorption causes two-sided ridges consisting of atomically rough $\text{Re}\left(13\overline{4}2\right)$ and $\left(31\overline{4}2\right)$ faces to become much more favorable than the initially planar $\text{Re}\left(11\overline{2}1\right)$. We show that this surface faceting is due to a strong preference of N to adsorb at specific surface sites on $\text{Re}\left(13\overline{4}2\right)$. Our studies suggest that it is possible to stabilize atomically rough surfaces with high density of steps and kinks by the adsorption of certain strongly interacting adsorbates.

We have fabricated a single Cooper-pair box (SCB) together with an on-chip lumped element resonator. By utilizing the quantum capacitance of the SCB, its state can be read out by detecting the phase of a radio-frequency signal reflected off the resonator. The resonator was optimized for fast readout. By studying quasiparticle tunneling events in the SCB, we have characterized the performance of the readout and found that we can perform a single-shot parity measurement, with a signal-to-noise ratio of 1, in approximately 50 ns. This is an order of magnitude faster than previously reported measurements.

We develop a theory of the magneto-optical and magnetoelectric properties of a topological insulator thin film in the presence of a quantizing external magnetic field. We find that low-frequency magneto-optical properties depend only on the sum of top and bottom surface Dirac-cone filling factors ${\nu }_{\text{T}}$ and ${\nu }_{\text{B}}$ whereas the low-frequency magnetoelectric response depends only on the difference. The Faraday rotation is quantized in integer multiples of the fine structure constant and the Kerr effect exhibits a $\pi /2$ rotation. Strongly enhanced cyclotron-resonance features appear at higher frequencies that are sensitive to the filling factors of both surfaces. When the product of the bulk conductivity and the film thickness in $e^2/h$ units is small compared to $\alpha$, magneto-optical properties are only weakly dependent on accidental doping in the interior of the film.

The reduction in symmetry associated with the onset of ferroelectric order in ${\text{PbZr}}_{0.2}{\text{Ti}}_{0.8}{\text{O}}_3$ (PZT) thin films leads to a pronounced difference at the $\text{Ti}{L}_{3,2}$ absorption edges between spectra measured with the x-ray linear polarization perpendicular and parallel to the ferroelectric polarization. We introduce a general method to analyze the observed difference spectra using atomic multiplet calculations. Moreover, we find experimental evidence for structural changes in PZT induced by the reversal of the ferroelectric polarization.

Two gapped quantum ground states in the same phase are connected by an adiabatic evolution which gives rise to a local unitary transformation that maps between the states. On the other hand, gapped ground states remain within the same phase under local unitary transformations. Therefore, local unitary transformations define an equivalence relation and the equivalence classes are the universality classes that define the different phases for gapped quantum systems. Since local unitary transformations can remove local entanglement, the above equivalence/universality classes correspond to pattern of long-range entanglement, which is the essence of topological order. The local unitary transformation also allows us to define a wave function renormalization scheme, under which a wave function can flow to a simpler one within the same equivalence/universality class. Using such a setup, we find conditions on the possible fixed-point wave functions where the local unitary transformations have finite dimensions. The solutions of the conditions allow us to classify this type of topological orders, which generalize the string-net classification of topological orders. We also describe an algorithm of wave function renormalization induced by local unitary transformations. The algorithm allows us to calculate the flow of tensor-product wave functions which are not at the fixed points. This will allow us to calculate topological orders as well as symmetry-breaking orders in a generic tensor-product state.

We present scanning-gate images of a single-layer graphene quantum dot which is coupled to source and drain via two constrictions. We image and locate conductance resonances of the quantum dot in the Coulomb-blockade regime as well as resonances of localized states in the constrictions in real space, which are interpreted in light of previous transport experiments. In addition our technique allows us to estimate the extent of these localized states, namely, radii of about 10–13 nm.

We report X-band electron paramagnetic resonance (EPR) and ${}^{125}\text{T}\text{e}$ and ${}^{77}\text{S}\text{e}$ NMR measurements on single-crystalline superconducting ${\text{FeSe}}_{0.42}{\text{Te}}_{0.58}$ $\left[T_c=11.5\left(1\right)\text{K}\right]$. The data provide indications for the coexistence of intrinsic localized and itinerant electronic states. In the normal state, localized moments couple to itinerant electrons in the Fe(Se,Te) layers and affect the local spin susceptibility and spin fluctuations. Below $T_c$, spin fluctuations become rapidly suppressed and an unconventional superconducting state emerges in which $1/T_1$ is reduced at a much faster rate than expected for conventional $s$- or $s_{\pm }$-wave symmetry. We suggest that the localized states arise from the strong electronic correlations within one of the Fe-derived bands. The multiband electronic structure together with the electronic correlations thus determine the normal and superconducting states of the ${\text{FeSe}}_{1-x}{\text{Te}}_x$ family, which appears much closer to other high-$T_c$ superconductors than previously anticipated.

We report optically detected nuclear magnetic resonance (ODNMR) measurements on small ensembles of nuclear spins in single GaAs quantum dots. Using ODNMR we make direct measurements of the inhomogeneous Knight field from a photoexcited electron which acts on the nuclei in the dot. The resulting shifts of the NMR peak can be optically controlled by varying the electron occupancy and its spin orientation, and lead to strongly asymmetric line shapes at high optical excitation. The all-optical control of the NMR line shape will enable position-selective control of small groups of nuclear spins inside a dot.

Scanning tunneling spectroscopies in the subkelvin temperature range were performed on superconducting silicon epilayers doped with boron in the atomic percent range. The resulting local differential conductance behaved as expected for a homogeneous superconductor, with an energy-gap dispersion below $\pm 10\mathrm{\%}$. The spectral shape, the amplitude, and temperature dependence of the superconductivity gap follow the BCS model, bringing support to the hypothesis of a hole pairing mechanism mediated by phonons in the weak-coupling limit.

We report measurements of the temperature-dependent components of the magnetic penetration depth $\lambda \left(T\right)$ in single-crystal samples of ${\text{YBa}}_2{\text{Cu}}_4{\text{O}}_8$ using a radio-frequency tunnel diode oscillator technique. We observe a downturn in $\lambda \left(T\right)$ at low temperatures for currents flowing along the $b$ and $c$ axes but not along the $a$ axis. The downturn in ${\lambda }_b$ is suppressed by a small dc field of $\sim 0.25\text{T}$. This and the zero field anisotropy of $\lambda \left(T\right)$ likely result from proximity induced superconducting on the CuO chains. However we also discuss the possibility that a significant part of the anisotropy might originate from the ${\text{CuO}}_2$ planes.

The dynamics of a strain-induced resistive phase transition in a ${\text{VO}}_2$ thin film was studied by using a low-frequency crystal bending stage. It was found that the resistance response of the film to strain excitation is determined by two time constants, interpreted as a nucleation lifetime, in the ${10}^{-6}–{10}^{-4}\text{s}$ range, and a domain growth lifetime of $\sim {10}^{-3}\text{s}$. The variation of the domain nucleation lifetime is discussed in terms of the barrier height separating the coexisting insulating and metallic phases in a ${\text{VO}}_2$ film. Below the transition temperature, the phase transition is nucleation limited. Above the transition temperature, the resistance change is strongly affected by the growth of metallic domains. Direct imaging of metallic domains by local-probe current mapping showed that the phase transition in a thin film is limited by the presence of grain boundaries.

We study the high-temperature regime within the glass phase of an elastic object with short-ranged disorder. In that regime we argue that the scaling functions of any observable are described by a continuum model with a $\delta$-correlated disorder and that they are universal up to only two parameters that can be explicitly computed. This is shown numerically on the roughness of directed polymer models and by dimensional and functional renormalization-group arguments. We discuss experimental consequences such as nonmonotonous behavior with temperature.