We report measurements of Rabi oscillations and spectroscopic coherence times in an $\text{Al}/{\text{AlO}}_x/\text{Al}$ and three $\text{Nb}/{\text{AlO}}_x/\text{Nb}$ dc SQUID phase qubits. One junction of the SQUID acts as a phase qubit and the other junction acts as a current-controlled nonlinear isolating inductor, allowing us to change the coupling to the current bias leads in situ by an order of magnitude. We found that for the Al qubit, a spectroscopic coherence time $T_2^{\ast }$ varied from 3 to 7 ns and the decay envelope of Rabi oscillations had a time constant $T^{\prime }=25\text{ns}$ on average at 80 mK. The three Nb devices also showed $T_2^{\ast }$ in the range of 3–8 ns, but $T^{\prime }$ was 9–15 ns, just about 1/2 the value we found in the Al device. For all the devices, the time constants were roughly independent of the isolation from the bias lines, implying that noise and dissipation from the bias leads were not the principal sources of dephasing and inhomogeneous broadening.

The magnetotransport properties of antidot lattices containing artificially designed grain boundaries have been measured. We find that the grain boundaries broaden the commensurability resonances and displace them anisotropically. These phenomena are unexpectedly weak but differ characteristically from isotropic Gaussian disorder in the antidot positions. The observations are interpreted in terms of semiclassical trajectories, which tend to localize along the grain boundaries within certain magnetic-field intervals. Furthermore, our results indicate how the transport through superlattices generated by self-organizing templates may get influenced by grain boundaries.

State of the art nanomechanical resonators present quality factors $Q\sim {10}^3–{10}^5$, which are much lower than those that can be naively extrapolated from the behavior of micromechanical resonators. We analyze the dissipation mechanism that arises in nanomechanical beam structures due to the tunneling of mesoscopic phonons between the beam and its supports (known as clamping losses). We derive the environmental force spectral density that determines the quantum Brownian motion of a given resonance. Our treatment is valid for low frequencies and provides the leading contribution in the aspect ratio. This yields fundamental limits for the $Q$ values, which are described by simple scaling laws and are relevant for state of the art experimental structures. In this context, for resonant frequencies in the $0.1–1\mathrm{GHz}$ range, while this dissipation mechanism can limit flexural resonators, it is found to be negligible for torsional ones. In the case of structureless three-dimensional supports, the corresponding environmental spectral densities are Ohmic for flexural resonators and super-Ohmic for torsional ones, while for two-dimensional slab supports, they yield $1∕f$ noise. Furthermore, analogous results are established for the case of suspended semiconducting single-walled carbon nanotubes. Finally, we provide a general expression for the spectral density that allows us to extend our treatment to other geometries and illustrate its use by applying it to a microtoroid. Our analysis is relevant for applications in high precision measurements and for the prospects of probing quantum effects in a macroscopic mechanical degree of freedom.

Following the discovery of the Fe-pnictide superconductors, local-density approximation (LDA) band structure calculations showed that the dominant contributions to the spectral weight near the Fermi energy came from the $\text{Fe}3d$ orbitals. The Fermi surface is characterized by two hole surfaces around the $\Gamma$ point and two electron surfaces around the $M$ point of the two Fe/cell Brillouin zone. Here, we describe a two-band model that reproduces the topology of the LDA Fermi surface and exhibits both ferromagnetic and $q=\left(\pi ,0\right)$ spin-density wave fluctuations. We argue that this minimal model contains the essential low energy physics of these materials.

We consider the quantum corrections to the conductivity of fermions interacting via a Chern–Simons gauge field and concentrate on the Hartree-type contributions. The first-order Hartree approximation is only valid in the limit of weak coupling $\lambda ⪡g^{-1/2}$ to the gauge field ($g⪢1$ is the dimensionless conductance) and results in an antilocalizing conductivity correction $\sim {\lambda }^{2}g{\text{ln}}^{2}T$. In the case of strong coupling, an infinite summation of higher-order terms is necessary, which includes both the virtual (renormalization of the frequency) and real (dephasing) processes. At intermediate temperatures, $T_0⪡T⪡g{T}_0$, where $T_0\sim 1/g^2\tau$ and $\tau$ is the elastic scattering time, the $T$ dependence of the conductivity is determined by the Hartree correction, $\delta {\sigma }^H\left(T\right)-\delta {\sigma }^H\left(g{T}_0\right)\propto g^{1/2}-{\left(T/T_0\right)}^{1/2}\left[1+\text{ln}{\left(g{T}_0/T\right)}^{1/2}\right]$, so that $\sigma \left(T\right)$ increases with lowering $T$. At low temperatures, $T⪡T_0$, the temperature-dependent part of the Hartree correction assumes a logarithmic form with a coefficient of order unity, $\delta {\sigma }^H\propto \text{ln}\left(1/T\right)$. As a result, the negative exchange contribution $\delta {\sigma }^{\text{ex}}\propto -\text{ln}g\text{ln}\left(1/T\right)$ becomes dominant, which yields localization in the limit of $T\to 0$. We further discuss dephasing at strong coupling and show that the dephasing rates are of the order of $T$, owing to the interplay of inelastic scattering and renormalization. On the other hand, the dephasing length is anomalously short, $L_{\phi }⪡L_T$, where $L_T$ is the thermal length. For the case of composite fermions with long-range Coulomb interaction, the gauge-field propagator is less singular. The resulting Hartree correction has the usual sign and temperature dependence, $\delta {\sigma }^H\propto \text{ln}g\text{ln}\left(1/T\right)$, and for realistic $g$ is overcompensated by the negative exchange contribution due to the gauge-boson and scalar parts of the interaction. In this case, the dephasing length $L_{\phi }$ is of the order of $L_T$ for not too low temperatures and exceeds $L_T$ for $T\lesssim g{T}_0$.

We study triangular clusters of three spin-1/2 Kondo or Anderson impurities that are coupled to two conduction leads. In the case of Kondo impurities, the model takes the form of an antiferromagnetic Heisenberg ring with Kondo-type exchange coupling to continuum electrons. We show that this model exhibits many types of the behavior found in various simpler one- and two-impurity models, thereby enabling the study of crossovers between a number of Fermi-liquid (FL) and non-Fermi-liquid (NFL) fixed points. In particular, we explore a direct crossover between the two-impurity Kondo-model NFL fixed point and the two-channel Kondo-model NFL fixed point. We show that the concept of the two-stage Kondo effect applies even in the case when the first-stage Kondo state is of NFL type. In the case of Anderson impurities, we consider the transport properties of three coupled quantum dots. This class of models includes, as limiting cases, the familiar serial double quantum dot and triple quantum dot nanostructures. By extracting the quasiparticle scattering phase shifts, we compute the low-temperature conductance as a function of the interimpurity tunneling coupling. We point out that due to the existence of exponentially low-temperature scales, there is a parameter range where the stable “zero-temperature” fixed point is essentially never reached (not even in numerical renormalization group calculations). The zero-temperature conductance is then of no interest and it may only be meaningful to compute the conductance at finite temperature. This illustrates the perils of studying the conductance in the ground state and considering thermal fluctuations only as a small correction.

We formulate a bosonic dynamical mean-field theory (B-DMFT) which provides a comprehensive, thermodynamically consistent framework for the theoretical investigation of correlated lattice bosons. The B-DMFT is applicable for arbitrary values of the coupling parameters and temperature and becomes exact in the limit of high spatial dimensions $d$ or coordination number $Z$ of the lattice. In contrast to its fermionic counterpart, the construction of the B-DMFT requires different scalings of the hopping amplitudes with $Z$ depending on whether the bosons are in their normal state or in the Bose-Einstein condensate (BEC). A detailed discussion of how this conceptual problem can be overcome by performing the scaling in the action rather than in the Hamiltonian itself is presented. The B-DMFT treats normal and condensed bosons on equal footing and thus includes the effects caused by their dynamic coupling. It reproduces all previously investigated limits in parameter space, such as the Beliaev-Popov and Hartree-Fock-Bogoliubov approximations, and generalizes the existing mean-field theories of interacting bosons. The self-consistency equations of the B-DMFT are those of a bosonic single-impurity coupled to two reservoirs corresponding to bosons in the condensate and in the normal state, respectively. We employ the B-DMFT to solve a model of itinerant and localized, interacting bosons analytically. The local correlations are found to enhance the condensate density and the BEC transition temperature $T_{\text{BEC}}$. This effect may be used experimentally to increase $T_{\text{BEC}}$ of bosonic atoms in optical lattices.

Scanning tunneling microscopy, Auger electron spectroscopy (AES), and low energy electron diffraction are used to investigate $0.5\mathrm{wt}\%$ Nb doped $\mathrm{Sr}\mathrm{Ti}{\mathrm{O}}_3$ (110) samples, which are annealed in ultrahigh vacuum. An $(n\times 1)$ family of reconstructions constituting $(3\times 1)$, $(4\times 1)$, and $(6\times 1)$ periodicities form on the surface at varying annealing temperatures. Wood’s notation of the reconstructions is defined such that the first integer represents the multiple of the bulk terminated periodicity in the [001] direction and the second digit shows the multiple of the periodicity in the [$1\overline{1}0$] direction. AES reveals that all the reconstructions are O deficient, but the $(4\times 1)$ is also Ti enriched and the $(6\times 1)$ is Sr enriched with respect to stoichiometry. The $(3\times 1)$ surface possesses step edges which are decorated with a $(1\times 4)$ reconstruction on the upper terrace. Similarly, step edges on the $(6\times 1)$ surface often possess small regions of $(1\times 2)$ reconstruction on the lower terrace. Step heights are always found to be equivalent to integer multiples of the distance between two similar {110} planes (the $d_{110}$ lattice parameter, $0.28\mathrm{nm}$).

In recent work, we argued that a particular algebraic spin liquid (ASL) may be the ground state of the $S=1/2$ kagome lattice Heisenberg antiferromagnet. Furthermore, this state, which lacks a spin gap, is appealing in light of recent experiments on herbertsmithite $\left[{\text{ZnCu}}_3{\left(\text{OH}\right)}_6{\text{Cl}}_2\right]$. Here, we study the properties of this ASL in more detail using both the low-energy effective field theory and Gutzwiller-projected wave functions of fermionic spinons. We identify the competing orders of the ASL, which are observables having slowly decaying power-law correlations—among them we find a set of magnetic orders lying at the $M$ points of the Brillouin zone, the familiar $\mathit{q}=0$ magnetic ordered state, the “Hastings” valence-bond solid (VBS) state, and a pattern of vector spin-chirality ordering corresponding to one of the Dzyaloshinskii–Moriya (DM) interaction terms present in herbertsmithite. Identification of some of these orders requires an understanding of the quantum numbers of magnetic monopole operators in the ASL. We discuss the detection of the magnetic and VBS competing orders in experiments. While we primarily focus on a clean system without DM interaction, we consider the effects of small DM interaction and argue that, surprisingly, it leads to spontaneously broken time-reversal symmetry (for DM interaction that preserves $XY$ spin rotation symmetry, there is also $XY$ magnetic order). Our analysis of the projected wave function provides an estimate of the “Fermi velocity” $v_F$ that characterizes all low-energy excitations of the ASL—this allows us to estimate the specific heat, which compares favorably with experiments. We also study the spin and bond correlations of the projected wave function and compare these results with those of the effective field theory. While the spin correlations in the effective field theory and wave function seem to match rather well (although not completely), the bond correlations are more puzzling. We conclude with a discussion of experiments in herbertsmithite and make several predictions.

We derive a semiclassical expression for the Green’s function in graphene, in which the presence of a semiclassical phase is made apparent. The relationship between this semiclassical phase and the adiabatic Berry phase, usually referred to in this context, is discussed. These phases coincide for the perfectly linear Dirac dispersion relation. They differ, however, when a gap is opened at the Dirac point. We furthermore present several applications of our semiclassical formalism. In particular, we provide, for various configurations, a semiclassical derivation of the electron’s Landau levels, illustrating the role of the semiclassical “Berry-like” phase.

Direct density measurements were made from shock-loaded aluminum and copper samples by combining plate-impact experiments with proton radiography at the Los Alamos Neutron Science Center. Flyer plates were accelerated using a 40 mm bore powder gun to create a shock wave in a sample. The sample material was then interrogated in real time using the proton radiography facility. The increase in density behind the shock front causes a measurable change in the transmission of protons through the sample, which can then be quantified as a density value in the material. Hugoniot values were calculated using more traditional techniques to evaluate the accuracy of the radiographically obtained density measurements.

The ternary intermetallic compound, ${\text{Pr}}_6{\text{Ni}}_2{\text{Si}}_3$, is a member of a structure series of compounds based on a triangular structure where the number of Pr atoms in the prism cross section can be systematically varied. ${\text{Pr}}_6{\text{Ni}}_2{\text{Si}}_3$ contains two distinct Pr lattice sites which result in complex interactions between the magnetic ions. Extensive measurements of specific heat and magnetization on single-crystal samples indicate that ${\text{Pr}}_6{\text{Ni}}_2{\text{Si}}_3$ orders with both a ferromagnet and an antiferromagnet component, with ordering temperatures of 39.6 and $\sim 32\text{K}$, respectively. The ferromagnetic component $\parallel$ $c$ axis is accompanied by a large hysteresis, and the antiferromagnetic component $\perp$ $c$ axis is accompanied by a spin-flop-type transition. More detailed measurements, of the vector magnetization, indicate that the ferromagnetic and the antiferromagnetic order appear independent of each other. These results not only clarify the behavior of ${\text{Pr}}_6{\text{Ni}}_2{\text{Si}}_3$ itself but also of the other members of the structure series, ${\text{Pr}}_5{\text{Ni}}_2{\text{Si}}_3$ and ${\text{Pr}}_{15}{\text{Ni}}_7{\text{Si}}_{10}$.

In quasi-two-dimensional $\text{Ce}\left(\text{Ir},\text{Rh}\right){\text{In}}_5$ system, it has been suggested that the phase diagram contains two distinct domes with different heavy-fermion superconducting states. Here, we report the systematic pressure dependence of the electron transport properties in the normal state of ${\text{CeRh}}_{0.2}{\text{Ir}}_{0.8}{\text{In}}_5$ and ${\text{CeIrIn}}_5$, which locates in first and second superconducting domes, respectively. We observed non-Fermi liquid behavior at low temperatures in both compounds, including nonquadratic $T$ dependence of the resistivity, large enhancement of the Hall coefficient, and the violation of the Kohler rule in the magnetoresistance. We show that the cotangent of the Hall angle $\text{cot}{\Theta }_H$ varies as $T^2$, and the magnetoresistance is quite scaled well by the Hall angle as $\Delta {\rho }_{xx}/{\rho }_{xx}\propto {\text{tan}}^2{\Theta }_H$. The observed transport anomalies are common features of $\text{Ce}M{\text{In}}_5$ ($M=\text{Co}$, Rh, and Ir) and high-$T_c$ cuprates, which suggest that the anomalous transport properties observed in ${\text{CeIrIn}}_5$ are mainly governed by the antiferromagnetic spin fluctuations not by the Ce-valence fluctuations, which have been proposed to be the possible origin for the second superconducting dome.

Our measurements of the differential susceptibility $\partial M/\partial H$ of the frustrated magnetic molecules $\left\{{\text{Mo}}_{72}{\text{Fe}}_{30}\right\}$ and $\left\{{\text{Mo}}_{72}{\text{Cr}}_{30}\right\}$ reveal a pronounced dependence on magnetic field $\left(H\right)$ and temperature $\left(T\right)$ in the low $H$–low $T$ regime, contrary to the predictions of the existing models. Excellent agreement with experiment is achieved upon formulating a nearest-neighbor classical Heisenberg model where the 60 nearest-neighbor exchange interactions in each molecule, rather than being identical as has been assumed heretofore, are described by a two-parameter rectangular probability distribution of values of the exchange constant. We suggest that the probability distribution provides a convenient phenomenological platform for summarizing the combined effects of multiple microscopic mechanisms that disrupt the idealized picture of a Heisenberg model based on a single value of the nearest-neighbor exchange constant.

We investigate theoretically the formation of collective excitations in atomic scale quasi-one-dimensional metallic nanostructures. The optical response of the system is calculated within the linear-response theory and the random-phase approximation. For uniform nanostructures a smooth transition from quantum scaling of single-particle excitations to classical plasmon scaling is observed, depending on the system’s length and electron density. We find crucial differences in the scaling behavior for quasi-one-dimensional and three-dimensional nanostructures. The presence of an additional modulating on-site potential is shown to localize electrons, leading to the response of the system that is highly sensitive to the number of electrons at low fillings.

We have used a hybrid vertical-lateral double quantum dot to study the excitation energy spectrum for one- and two-electron states, which are effectively formed on top of a certain core shell. We have observed bonding and antibonding states, and exchange coupled singlet and triplet states for the one- and two-electron states, respectively, and derived the interdot tunnel coupling and exchange coupling energies for various parameters of interdot level detuning, central gate voltage and magnetic field. The obtained exchange coupling energy is consistently reproduced by calculation using the Hund-Mulliken method.

We have obtained the experimental evidence for the Pockels effect of water, which is placed in the electric double layer (EDL) on the interface between the bulk water and a transparent electrode. The electric-field induced energy shift was observed for the visible interference fringes in a 300-nm indium-tin-oxide (ITO) electrode layer. The signal was shown to originate from a negative (positive) refractive index change of water in the EDL for the positive (negative) bias. The refractive index change was as large as $\Delta n=-0.1$ for the 2-nm thick EDL. The Pockels constants of water in the EDL were determined to be $r_{13}=\left(2.0\pm 0.3\right)\times 100\text{pm}/\text{V}$ and $r_{33}=\left(2.5\pm 0.6\right)\times 100\text{pm}/\text{V}$. At the same time, the ITO showed blue shifts of both the band gap in the UV and the plasma frequency in the near infrared due to the band population effect in the space charge layer. Both shifts lead to a negative refractive index change in the visible region of the ITO, but their effect on the interference fringe shifts in the visible region was estimated to be much smaller than that of water in the EDL.

We study the effect of magnetic Mn ions on the two-band superconductor ${\text{MgB}}_2$, and compute both the total and spin-resolved scanning tunneling spectrum in the vicinity of the magnetic impurity. We show that when the internal structure of the Mn ion’s $d$-shell is taken into account, multiple Shiba states appear in the spectrum. The presence of these multiplets could alter significantly the overall interpretation of local tunneling spectra for a wide range of superconducting hosts and magnetic impurities.

We have analyzed various characteristic temperatures and energies of hole-doped high-$T_c$ cuprates as a function of a dimensionless hole-doping concentration $\left(p_u\right)$. Entirely based on the experimental grounds, we construct a unified electronic phase diagram (UEPD), where three characteristic temperatures ($T^{\ast }$’s) and their corresponding energies ($E^{\ast }$’s) converge as $p_u$ increases in the underdoped regime. $T^{\ast }$’s and $E^{\ast }$’s merge together with the $T_c$ curve and $3.5k_B{T}_c$ curve at $p_u\sim 1.1$ in the overdoped regime, respectively. They finally go to zero at $p_u\sim 1.3$. The UEPD follows an asymmetric half-dome-shaped $T_c$ curve, in which $T_c$ appears at $p_u\sim 0.4$, reaches a maximum at $p_u\sim 1$, and rapidly goes to zero at $p_u\sim 1.3$. The asymmetric half-dome-shaped $T_c$ curve is at odds with the well-known symmetric superconducting dome for ${\text{La}}_{2-x}{\text{Sr}}_x{\text{CuO}}_4$ (SrD-La214), in which two characteristic temperatures and energies converge as $p_u$ increases and merge together at $p_u\sim 1.6$, where $T_c$ goes to zero. The UEPD clearly shows that pseudogap phase precedes and coexists with high temperature superconductivity in the underdoped and overdoped regimes, respectively. It is also clearly seen that the upper limit of high-$T_c$ cuprate physics ends at a hole concentration that equals to 1.3 times the optimal doping concentration for almost all high-$T_c$ cuprate materials and 1.6 times the optimal doping concentration for the SrD-La214. Our analysis strongly suggests that pseudogap is a precursor of high-$T_c$ superconductivity, the observed quantum critical point inside the superconducting dome may be related to the end point of UEPD, and the normal state of the underdoped and overdoped high temperature superconductors cannot be regarded as a conventional Fermi liquid phase.

New terahertz data on the optical conductivity of Pb are presented, as well as a detailed Eliashberg analysis with particular emphasis on phonon-assisted processes not included in a BCS approach. Consideration of the optical self-energy instead of the conductivity itself helps highlight the differences with BCS predictions. Predicted coherence peaks are observed in the optical scattering rates. Impurities enhance the optical effective mass at zero frequency by an order of magnitude and induce a large peak at twice the gap in agreement with theory. This work illustrates the usefulness of the optical self-energy for the analysis of data.