We report systematic low-temperature measurements of the dc magnetization, ac susceptibility, and heat capacity of dysprosium pyrogermanate $\left({\text{Dy}}_2{\text{Ge}}_2{\text{O}}_7\right)$ single crystal and powder samples. Our results confirm that ${\text{Dy}}_2{\text{Ge}}_2{\text{O}}_7$ is an anisotropic antiferromagnet. The isothermal field dependent magnetization and the integrated magnetic entropy both indicate that the ${\text{Dy}}^{3+}$ ions behave as Ising-like spins, analogous to those in the pyrochlore spin ice materials. Both single-spin and collective spin-relaxation phenomena appear to lead to spin freezing in this material, again in analogy to observations in the spin ice materials, suggesting that such phenomena may be generic to a broader class of magnetic materials.

Single crystals of ${\text{BaRh}}_2{\text{As}}_2$ have been synthesized from a Pb flux. We present the room-temperature crystal structure, single-crystal x-ray diffraction measurements as a function of temperature $T$, anisotropic magnetic susceptibility $\chi$ versus $T$, electrical resistivity in the $ab$ plane $\rho$ versus $T$, Hall coefficient versus $T$ and magnetic field $H$, and heat capacity $C$ versus $T$ measurements on the crystals. The single-crystal structure determination confirms that ${\text{BaRh}}_2{\text{As}}_2$ forms in the tetragonal ${\text{ThCr}}_2{\text{Si}}_2$-type structure (space group $I4/mmm$) with lattice parameters $a=b=4.0564\left(6\right)\text{Å}$ and $c=12.797\left(4\right)\text{Å}$. Band-structure calculations show that ${\text{BaRh}}_2{\text{As}}_2$ should be metallic with a small density of states at the Fermi energy $N\left(E_F\right)=3.49\text{states}/\text{eV}\text{f}\text{.u}\text{.}$ (where $\text{f}.\text{u}.\equiv \text{formula}$ unit) for both spin directions. $\rho \left(T\right)$ data in the $ab$ plane confirm that the material is indeed metallic with a residual resistivity $\rho \left(2\text{K}\right)=29\mu \Omega \text{cm}$ and with a residual resistivity ratio $\rho \left(310\text{K}\right)/\rho \left(2\text{K}\right)=5.3$. The observed $\chi \left(T\right)$ is small $\left(\sim {10}^{-5}{\text{cm}}^3/\text{mol}\right)$ and weakly anisotropic with ${\chi }_{ab}/{\chi }_c\approx 2$. The $C\left(T\right)$ data indicate a small density of states at the Fermi energy with the low-temperature Sommerfeld coefficient $\gamma =4.7\left(9\right)\text{mJ}/\text{mol}{\text{K}}^2$. There are no indications of superconductivity, spin-density wave, or structural transitions between 2 and 300 K. We compare the calculated density of states versus energy of ${\text{BaRh}}_2{\text{As}}_2$ with that of ${\text{BaFe}}_2{\text{As}}_2$.

We propose to generate terahertz surface plasmon polaritons (SPPs) on a metal wire by launching electron bunches onto a tapered end of the wire. To show the potential of this method, we solve Maxwell’s equations for the appropriate boundary conditions. The metal wire tip is modeled by a perfectly conducting semi-infinite cone. It is shown that the SPPs can be recovered from the idealized fields by well-known perturbation techniques. The emitted radiation is strongly concentrated into a narrow solid angle near the cone boundary for cones with a small opening angle. We calculate that, using currently available technology, subpicosecond SPPs with peak electric fields of the order of MV/cm on a 1 mm diameter wire can be obtained.

Apparently, some form of local superconducting pairing persists up to temperatures well above the maximum observed $T_c$ in underdoped cuprates; i.e., $T_c$ is suppressed due to the small phase stiffness. With this in mind, we consider the following question: Given a system with a high pairing scale ${\Delta }_0$ but with $T_c$ reduced by phase fluctuations, can one design a composite system in which $T_c$ approaches its mean-field value, $T_c\to T_{\text{MF}}\approx {\Delta }_0/2$? Here, we study a simple two-component model in which a “metallic layer” with ${\Delta }_0=0$ is coupled by single-particle tunneling to a “pairing layer” with ${\Delta }_0>0$ but zero phase stiffness. We show that in the limit where the bandwidth of the metal is much larger than ${\Delta }_0$, the $T_c$ of the composite system can reach the upper limit $T_c\approx {\Delta }_0/2$.

Samples of ${\text{Nd}}_{2-x}{\text{Ce}}_x{\text{CuO}}_4$, an electron-doped high-temperature superconducting cuprate, near optimal doping at $x=0.155$ were measured via angle-resolved photoemission. We report a renormalization feature in the self-energy (“kink”) in the band dispersion at $\approx 50–60\text{meV}$ present in nodal and antinodal cuts across the Fermi surface. Specifically, while the kink had been seen in the antinodal region, it is now observed also in the nodal region, reminiscent of what has been observed in hole-doped cuprates.

We examine the problem of quantum vortex tunneling in the Gross-Pitaevskii model. The effect of gapless phonons is to produce a super-Ohmic quantum dissipation that renormalizes the effective tunneling parameters but does not destroy quantum coherence. The renormalization of the instanton frequency is computed within a specific model tunneling potential.

We analyze the problem of eliminating finite-size errors from quantum Monte Carlo (QMC) energy data. We demonstrate that both (i) adding a recently proposed [S. Chiesa et al., Phys. Rev. Lett. 97, 076404 (2006)] finite-size correction to the Ewald energy and (ii) using the model periodic Coulomb (MPC) interaction [L. M. Fraser et al., Phys. Rev. B 53, 1814 (1996); P. R. C. Kent et al., Phys. Rev. B 59, 1917 (1999); A. J. Williamson et al., Phys. Rev. B 55, R4851 (1997)] are good solutions to the problem of removing finite-size effects from the interaction energy in cubic systems provided the exchange-correlation (XC) hole has converged with respect to system size. However, we find that the MPC interaction distorts the XC hole in finite systems, implying that the Ewald interaction should be used to generate the configuration distribution. The finite-size correction of Chiesa et al. [Phys. Rev. Lett. 97, 076404 (2006)] is shown to be incomplete in systems of low symmetry. Beyond-leading-order corrections to the kinetic energy are found to be necessary at intermediate and high densities; we investigate the effect of adding such corrections to QMC data for the homogeneous electron gas. We analyze finite-size errors in two-dimensional systems and show that the leading-order behavior differs from that which has hitherto been supposed. We compare the efficiencies of different twist-averaging methods for reducing single-particle finite-size errors and we examine the performance of various finite-size extrapolation formulas. Finally, we investigate the system-size scaling of biases in diffusion QMC.

We calculate the magnetic-field and temperature dependence of all quantum corrections to the ensemble-averaged conductance of a network of quantum dots. We consider the limit that the dimensionless conductance of the network is large, so that the quantum corrections are small in comparison to the leading, classical contribution to the conductance. For a quantum dot network the conductance and its quantum corrections can be expressed solely in terms of the conductances and form factors of the contacts and the capacitances of the quantum dots. In particular, we calculate the temperature dependence of the weak localization correction and show that it is described by an effective dephasing rate proportional to temperature.

We observe oscillatory magnetoresistance in double quantum wells under microwave irradiation. The results are explained in terms of the influence of subband coupling on the frequency dependent photoinduced part of the electron distribution function. As a consequence, the magnetoresistance demonstrates the interference of magnetointersubband oscillations and conventional microwave induced resistance oscillations.

We introduce a class of exactly solvable $SO\left(n\right)$ symmetric Hamiltonians with matrix product ground states. For an odd $n\ge 3$ case, the ground state is a translational invariant Haldane gap spin liquid state; while for an even $n\ge 4$ case, the ground state is a spontaneously dimerized state with twofold degeneracy. In the matrix product ground states for both cases, we identify a hidden antiferromagnetic order, which is characterized by nonlocal string order parameters. The ground-state phase diagram of a generalized $SO\left(n\right)$ symmetric bilinear-biquadratic model is discussed.

A simple model is developed to predict the complex mechanical properties of carbon nanotube sheets (buckypaper) [L. J. Hall et al., Science 320, 504 (2008)]. Fabricated using a similar method to that deployed for making writing paper, these buckypapers can have in-plane Poisson’s ratios changed from positive to negative, becoming auxetic, as multiwalled carbon nanotubes are increasingly mixed with single-walled carbon nanotubes. Essential structural features of the buckypapers are incorporated into the model: isotropic in-plane mechanical properties, nanotubes preferentially oriented in the sheet plane, and freedom to undergo stress-induced elongation by both angle and length changes. The expressions derived for the Poisson’s ratios enabled quantitative prediction of both observed properties and remarkable new properties obtainable by structural modification.

A newly developed fast molecular dynamics method is applied to ${\text{BaTiO}}_3$ ferroelectric thin-film capacitors with short-circuited electrodes or under applied voltage. The molecular dynamics simulations based on a first-principles effective Hamiltonian clarify that dead layers (or passive layers) between ferroelectrics and electrodes markedly affect the properties of capacitors, and predict that the system is unable to hop between a uniformly polarized ferroelectric structure and a striped ferroelectric domain structure at low temperatures. Simulations of hysteresis loops of thin-film capacitors are also performed, and their dependence on film thickness, epitaxial constraints, and electrodes are discussed.

We perform electrical transport measurements in graphene with several sample geometries. In particular, we design “invasive” probes crossing the whole graphene sheet as well as “external” probes connected through graphene side arms. The four-probe conductance measured between external probes varies linearly with charge density and is symmetric between electron and hole types of carriers. In contrast measurements with invasive probes give a strong electron-hole asymmetry and a sublinear conductance as a function of density. By comparing various geometries and types of contact metal, we show that these two observations are due to transport properties of the metal/graphene interface. The asymmetry originates from the pinning of the charge density below the metal, which thereby forms a $p\text{−}n$ or $p\text{−}p$ junction, depending on the polarity of the carriers in the bulk graphene sheet. Our results also explain part of the sublinearity observed in conductance as a function of density in a large number of experiments on graphene, which has generally been attributed to short-range scattering only.

Coherent magnons and phonons are excited by subpicosecond laser pulses in the weak ferromagnet ${\text{FeBO}}_3$. Impulsive stimulated Raman scattering (ISRS) is proven to be the microscopic mechanism of the excitation. It is shown that coherent magnons can be excited by both linearly and circularly polarized laser pulses where the efficiency of the process depends on the mutual orientation of the magnetic and crystallographic axes and the light propagation direction. The strong ellipticity of the ferromagnetic magnon mode is demonstrated, both experimentally and theoretically, to be essential for the excitation and observation of such coherent magnons. Because of this ellipticity, the amplitude of the coherent magnons excited by linearly polarized light may exceed by 2 orders of magnitude the amplitude of those excited by circularly polarized light. The primary difference between the excitation of coherent magnons by linearly polarized pulses via ISRS and via the earlier reported process of photoinduced magnetic anisotropy is discussed. Furthermore, the ISRS process is found to be responsible for the excitation of two optical phonon branches (8.4 and 12.1 THz) observed in our experiments. A coherent excitation, with a temperature-independent frequency of 0.7 THz, has also been observed in the magnetically ordered phase but could not be assigned to any optical phonon modes known in ${\text{FeBO}}_3$. The well-pronounced dependence of the amplitude of this mode on temperature suggests that this mode of nonmagnetic origin becomes Raman active only in the magnetically ordered phase and, therefore, can be excited and observed only below the Néel temperature.

Static disorder in a noninteracting gas of electrons confined to two dimensions can drive a continuous quantum (Anderson) transition between a metallic and an insulating state when time-reversal symmetry is preserved but spin-rotation symmetry is broken. The critical exponent $\nu$ that characterizes the diverging localization length and the bulk multifractal scaling exponents that characterize the amplitudes of the critical wave functions at the metal-insulator transition do not depend on the topological nature of the insulating state, i.e., whether it is topologically trivial (ordinary insulator) or nontrivial (a ${\mathbb{Z}}_2$ insulator supporting a quantum spin Hall effect). This is not true of the boundary multifractal scaling exponents, which we show (numerically) to depend on whether the insulating state is topologically trivial or not.

The appearance of long-range correlations near the Dirac point of a Dirac-type spinor model with random vector potential is studied. These correlations originate from a spontaneously broken symmetry and their corresponding Goldstone modes. Using a strong-disorder expansion, correlation functions and matrix elements are analyzed and compared with results from a weak-disorder expansion. The local density of states correlation and the overlap between states above and below the Dirac point are characterized by a long-range behavior. The correlation range decreases with the distance from the Dirac point. Transport is diffusive and the diffusion coefficient is proportional to the one-particle scattering time for any strength of disorder. A consequence of the special properties of particle-hole scattering is a constant microwave conductivity for weak, as well as for strong disorder, describing a deviation from conventional Drude-type transport. Some properties of the model can be linked to a kind of Kondo scale, which is generated by disorder. Finally, the properties of the wave functions at the Dirac point are characterized by their participation ratios, indicating a critical state at the Dirac point.

We explain the recently observed linear temperature dependence of the nodal Fermi velocity $v_F\left(T\right)$ in nearly optimally doped cuprates. We argue that it originates from electron-electron interaction and it is a fundamental property of an arbitrary two-dimensional (2D) Fermi liquid. We consider a spin-fermion model with input parameters extracted from the data, and we show that the $T$ term is about 30% at 300 K, in agreement with the data. We show that the sub-leading term in $v_F\left(T\right)$ is a regular (and small) $T^2$ correction. We also show that at a $2k_F$ quantum-critical point, temperature corrections to the dispersion are singular.

Whereas double hcp (dhcp) La superconducts at ambient pressure with $T_c\simeq 5\text{K}$, the other trivalent $d$-electron metals Sc, Y, and Lu only superconduct if high pressures are applied. Earlier measurements of the pressure dependence of $T_c$ for Sc and Lu metal are here extended to much higher pressures. Whereas $T_c$ for Lu increases monotonically with pressure to 12.4 K at 174 GPa (1.74 Mbar), $T_c$ for Sc reaches 19.6 K at 107 GPa, the second highest value observed for any elemental superconductor. At higher pressures a phase transition occurs whereupon $T_c$ drops to 8.31 K at 111 GPa. The $T_c\left(P\right)$ dependences for Sc and Lu are compared with those of Y and La. An interesting correlation is pointed out between the value of $T_c$ and the fractional free volume available to the conduction electrons outside the ion cores, a quantity which is directly related to the number of $d$ electrons in the conduction band.

Developing a theoretical framework to access the quantum phases of itinerant bosons or fermions in two dimensions that exhibit singular structure along surfaces in momentum space but have no quasiparticle description remains a central challenge in the field of strongly correlated physics. In this paper we propose that distinctive signatures of such two-dimensional (2D) strongly correlated phases will be manifest in quasi-one-dimensional “$N$-leg ladder” systems. Characteristic of each parent 2D quantum liquid would be a precise pattern of one-dimensional (1D) gapless modes on the $N$-leg ladder. These signatures could be potentially exploited to approach the 2D phases from controlled numerical and analytical studies in quasi-one-dimension. As a first step we explore itinerant-boson models with a frustrating ring-exchange interaction on the two-leg ladder, searching for signatures of the recently proposed two-dimensional $d$-wave-correlated Bose liquid (DBL) phase. A combination of exact diagonalization, density-matrix renormalization-group, variational Monte Carlo, and bosonization analysis of a quasi-1D gauge theory provide compelling evidence for the existence of an unusual strong-coupling phase of bosons on the two-leg ladder, which can be understood as a descendant of the two-dimensional DBL. We suggest several generalizations to quantum spin and electron Hamiltonians on ladders, which could likewise reveal fingerprints of such 2D non-Fermi-liquid phases.

We investigate transport in weakly coupled metal nanoparticle arrays, focusing on the regime where tunneling is competing with strong single electron charging effects. This competition gives rise to an interplay between two types of charge transport. In sequential tunneling, transport is dominated by independent electron hops from a particle to its nearest neighbor along the current path. In inelastic cotunneling, transport is dominated by cooperative multielectron hops that each go to the nearest neighbor but are synchronized to move charge over distances of several particles. In order to test how the temperature-dependent cotunnel distance affects the current-voltage $\left(I\text{−}V\right)$ characteristics, we perform a series of systematic experiments on highly ordered close-packed nanoparticle arrays. The arrays consist of $\sim 5.5\text{nm}$ diameter gold nanocrystals with tight size dispersion, spaced $\sim 1.7\text{nm}$ apart by interdigitating shells of dodecanethiol ligands. We present $I\text{−}V$ data for monolayer, bilayer, trilayer, and tetralayer arrays. For stacks 2–4 layers thick we compare in-plane measurements with data for vertical transport perpendicular to the array plane. Our results support a picture whereby transport inside the Coulomb blockade regime occurs by inelastic cotunneling, while sequential tunneling takes over at large bias above the global Coulomb blockade threshold $V_t\left(T\right)$ and at high temperatures. For the smallest measurable voltages, our data was fitted well by recent predictions for the temperature dependence zero-bias conductance due to multiple cotunneling. At finite but small bias, the cotunnel distance is predicted to set the curvature of the nonlinear $I\text{−}V$ characteristics, in good agreement with our data. The absence of significant magnetic-field dependence up to 10 T in the measured $I\text{−}V$ characteristics further supports the picture of inelastic cotunneling events where individual electrons hop no further than the nearest neighbor. At large bias, above the global Coulomb blockade threshold, the $I\text{−}V$ characteristics follow power-law behavior with temperature-independent exponent close to two, predicted for sequential tunneling along branching paths that optimize the overall charging energy cost.