We present a systematic angle-resolved photoemission spectroscopic study of the high-$\mathrm{T}c$ superconductor class ${\left(\text{Sr}/\text{Ba}\right)}_{1-x}{\text{K}}_x{\text{Fe}}_2{\text{As}}_2$. By utilizing a photon-energy-modulation contrast and scattering geometry we report the Fermi surface and the momentum dependence of the superconducting gap, $\Delta \left(\vec{k}\right)$. A prominent quasiparticle dispersion kink reflecting strong scattering processes is observed in a binding-energy range of 25–55 meV in the superconducting state, and the coherence length or the extent of the Cooper pair wave function is found to be about $20\text{Å}$, which is uncharacteristic of a superconducting phase realized by the BCS-phonon-retardation mechanism. The observed $40\pm 15\text{meV}$ kink likely reflects contributions from the frustrated spin excitations in a $J_1\text{−}{J}_2$ magnetic background and scattering from the soft phonons. Results taken collectively provide direct clues to the nature of the pairing potential including an internal phase-shift factor in the superconducting order parameter which leads to a Brillouin zone node in a strong-coupling setting.

Multiband superconductivity, involving resonant pair scattering between different bands, has emerged as a possible explanation of some of the main characteristics of the recently discovered iron pnictides. A key feature of such interband pairing mechanism is that it can generate or enhance superconductivity irrespective of whether it is attractive or repulsive. The latter case typically leads to the superconducting gap switching its sign among different sections of the Fermi surface. In iron pnictides, the natural scenario is that the gap changes sign between the hole and the electron Fermi surfaces. However, the macroscopic symmetry of such an extended $s^{\prime }$-wave state still belongs to the general $s$-wave category, raising the question of how to distinguish it from an ordinary $s$ wave. In such a quest, it is essential to use experimental techniques that have a momentum space resolution and can probe momenta of order $\left(\pi ,\pi \right)$: the wave vector that separates the hole and the electron Fermi surfaces in the Brillouin zone. Here we study experimental signatures in the spin fluctuation dynamics of the fully gapped $s$- and $s^{\prime }$-wave superconducting states, as well as those of the nodal $d$ and $p$ wave states. The coupling between spin fluctuations of the incipient nearly nested spin-density wave (SDW) and the Bogoliubov-de Gennes quasiparticles of the superconducting state leads to the Landau-type damping of the former. The intrinsic structure of the superconducting gap leaves a distinctive signature in the form of this damping, allowing it to be used to diagnose the nature of iron-based superconductivity in neutron scattering and other experiments sensitive to spin fluctuations in momentum space. We also discuss the coexistence between superconductivity and SDW order.

We use interlayer tunneling to study bilayer two-dimensional electron systems at ${\nu }_T=1$ over a wide range of charge-density imbalance $\Delta \nu ={\nu }_1-{\nu }_2$ between the two layers. We find that the strongly enhanced tunneling associated with the coherent excitonic ${\nu }_T=1$ phase at small layer separation can survive at least up to an imbalance of $\Delta \nu =0.5$, i.e., $\left({\nu }_1,{\nu }_2\right)=\left(3/4,1/4\right)$. Phase transitions between the excitonic ${\nu }_T=1$ state and bilayer states which lack significant interlayer correlations can be induced in three different ways: by increasing the effective interlayer spacing $d/\ell$, the temperature $T$, or the charge imbalance $\Delta \nu$. We observe that close to the phase boundary the coherent ${\nu }_T=1$ phase can be absent at $\Delta \nu =0$, present at intermediate $\Delta \nu$, and then absent again at large $\Delta \nu$, thus indicating an intricate phase competition between it and incoherent quasi-independent layer states. At zero imbalance, the critical $d/\ell$ shifts linearly with temperature, while at $\Delta \nu =1/3$ the critical $d/\ell$ is only weakly dependent on $T$. At $\Delta \nu =1/3$ we report on an observation of a direct phase transition between the coherent excitonic ${\nu }_T=1$ bilayer integer quantum Hall phase and the pair of single-layer fractional quantized Hall states at ${\nu }_1=2/3$ and ${\nu }_2=1/3$.

Intrinsic Josephson-junction stacks realized in high-temperature superconductors provide a very attractive base for developing coherent sources of electromagnetic radiation in the terahertz frequency range. A promising way to synchronize phase oscillations in all the junctions is to excite an internal cavity resonance. We demonstrate that this resonance promotes the formation of an alternating coherent state, in which the system spontaneously splits into two subsystems with different phase-oscillation patterns. There is a static phase shift between the oscillations in the two subsystems, which changes from 0 to $2\pi$ in a narrow region near the stack center. The oscillating electric and magnetic fields are almost homogeneous in all the junctions. The formation of this state promotes efficient pumping of the energy into the cavity resonance leading to strong resonance features in the current-voltage dependence.

We report the direct measurement of spin polarization in epitaxial ${\text{CoFe}}_2{\text{O}}_4$ tunnel barriers using the Meservey-Tedrow technique. By observing an asymmetry in the Al quasiparticle density of states in $\text{Pt}\left(111\right)/{\text{CoFe}}_2{\text{O}}_4\left(111\right)/\gamma {\text{-Al}}_2{\text{O}}_3\left(111\right)/\text{Al}$ tunnel junctions, we prove the existence of spin filtering in our ${\text{CoFe}}_2{\text{O}}_4$ tunnel barriers. We further analyze the effect of oxidation conditions during film growth on the polarization of the tunneling current, revealing an important role played by oxygen vacancies in the spin-filter efficiency of this material.

We investigated the magnetism under a magnetic field in the quasi-two-dimensional organic Mott insulator $\kappa \text{−}{\left(\text{BEDT-TTF}\right)}_2\text{Cu}\left[\text{N}{\left(\text{CN}\right)}_2\right]\text{Cl}$ through magnetization and ${}^{13}\text{C}\text{-NMR}$ measurements. We found that in the nominally paramagnetic phase (i.e., above Néel temperature) the field-induced local moments have a staggered component perpendicular to the applied field. As a result, the antiferromagnetic transition well defined at a zero field becomes crossover under a finite field. This unconventional behavior is qualitatively reproduced by the molecular-field calculation for Hamiltonian including the exchange, Dzyaloshinsky-Moriya (DM), and Zeeman interactions. This calculation also explains other unconventional magnetic features in $\kappa \text{−}{\left(\text{BEDT-TTF}\right)}_2\text{Cu}\left[\text{N}{\left(\text{CN}\right)}_2\right]\text{Cl}$ reported in the literature. The present results highlight the importance of the DM interaction in field-induced magnetism in a nominally paramagnetic phase, especially in low-dimensional spin systems.

Very recently, the electric dipole spin resonance (EDSR) of single electrons in quantum dots was discovered by three independent experimental groups. Remarkably, these observations revealed three different mechanisms of EDSR: coupling of electron spin to its momentum (spin-orbit), to the operator of its position (inhomogeneous Zeeman coupling), and to the hyperfine Overhauser field of nuclear spins. In this paper, I present a unified microscopic theory of these resonances in quantum dots. A mean field theory, derived for all three mechanisms and based on retaining only two-spin correlators, justifies applying macroscopic description of nuclear polarization to the EDSR theory. In the framework of the mean field theory, a fundamental difference in the time dependence of EDSR inherent of these mechanisms is revealed; it changes from the Rabi-type oscillations to a nearly monotonic growth. The theory provides a regular procedure to account for the higher nuclear-spin correlators that become of importance for a wider time span. For the hyperfine mechanism, they change the asymptotic behavior of EDSR dramatically. The theory also allows evaluating the effect of electron-spin dynamics on the effective coupling between nuclear spins.

We demonstrate reliable manipulation of the magnetization dynamics of a precessing nanomagnet by precisely controlling the spin transfer torque on the subnanosecond time scale. Using a simple pulse shaping scheme consisting of two ultrafast spin torque pulses with variable amplitudes and delay, we demonstrate coherent control over the precessional orbits and the ability to tune the switching probability of a nanomagnet at room temperature and 77 K. Our measurements suggest that appropriately shaped spin transfer can be used to efficiently manipulate the orientation of a free layer nanomagnet, thus providing an alternative for spin torque driven spintronic devices.

We report on a study of electronic transport in semimetallic ${\text{Bi}}_{0.96}{\text{Sb}}_{0.04}$. At zero field, the system is a very dilute Fermi liquid displaying a $T^2$ resistivity with an enhanced prefactor. Quantum oscillations in resistivity as well as in Hall, Nernst, and Seebeck responses of the system are detectable and their period quantifies the shrinking of the Fermi surface with antimony doping. For a field along the trigonal axis, the quantum limit was found to occur at a field as low as 3 T. An ultraquantum anomaly at twice this field was detected in both charge transport and Nernst response. Its origin appears to lie beyond the one-particle picture and linked to unidentified many-body effects.

Dynamical correlations of various local operators are studied in the spin-half two-channel Kondo (2CK) model in the presence of channel anisotropy or external magnetic field. A conformal field theory–based scaling approach is used to predict the analytic properties of various spectral functions in the vicinity of the two-channel Kondo fixed point. These analytical results compare well with highly accurate density-matrix numerical renormalization-group results. The universal crossover functions interpolating between channel-anisotropy- or magnetic-field-induced Fermi-liquid regimes and the two-channel Kondo, non-Fermi-liquid regimes are determined numerically. The boundaries of the real 2CK scaling regime are found to be rather restricted and to depend both on the type of the perturbation and on the specific operator whose correlation function is studied. In a small magnetic field, a universal resonance is observed in the local fermion’s spectral function. The dominant superconducting instability appears in the composite superconducting channel.

We present heat-capacity measurements of the upper critical fields of single-crystal ${\text{NdFeAsO}}_{1-x}{\text{F}}_x$. In zero-magnetic field a clear step in the heat capacity is observed at $T_c\approx 47\text{K}$. In fields applied perpendicular to the FeAs layers the step broadens significantly whereas for the in-plane orientation the field effects are small. This behavior is reminiscent of the ${\text{CuO}}_2$-high-$T_c$ superconductors and is a manifestation of pronounced fluctuation effects. Using an entropy conserving construction we determine the transition temperatures in applied fields and the upper critical-field slopes of $\partial H_{c2}^c/\partial T=-0.72\text{T}/\text{K}$ and $\partial H_{c2}^{ab}/\partial T=-3.1\text{T}/\text{K}$. Zero-temperature coherence lengths of ${\xi }_{ab}\approx 3.7\text{nm}$ and ${\xi }_c\approx 0.9\text{nm}$ and a modest superconducting anisotropy of $\lambda \sim 4$ can be deduced in a single-band model.

In strongly interacting electron systems with low density the thermodynamic density of states is negative at low temperatures. This creates difficulties with understanding of the Einstein relation between conductivity and diffusion coefficient. Using the expression for electrochemical potential that takes into account the long-range part of the Coulomb interaction it is shown that at negative density of states the Einstein relation gives a negative sign of the diffusion coefficient $D$, but under this condition there is no thermodynamic limitation on the sign of $D$. That happens because the unipolar relaxation of inhomogeneous electron density is not described by the diffusion equation. The relaxation goes much faster due to electric forces caused by the inhomogeneous electron density. The diffusion coefficient is irrelevant in this case and it is not necessarily positive because the diffusion process does not contribute to the positive production of entropy. In the case of bipolar diffusion, negative $D$ results in a global absolute instability that leads to formation of neutral excitons. Graphene is considered as an example of a system where the density relaxation is expected to be due to electric force rather than diffusion. It may also have a negative density of states.

We report the experimental observation of subband formation in graphene nanoribbons, where conductance becomes quantized due to the lateral quantum confinement. We show that this quantization in graphene nanoribbons can be observed at temperatures as high as 80 K and channel lengths as long as $1.7\mu \text{m}$. The observed quantization is in agreement with that predicted by theoretical calculations.

We have measured the longitudinal and transverse sound velocities of ${\text{La}}_{2-x}{\text{Ca}}_x{\text{CoO}}_4$ $\left(0.4\le x\le 0.7\right)$ propagating along [100], [110], and [001] from 200 K down to 3 K with a frequency of 10 MHz. It was found that $c_{66}$ shows a remarkable softening below ${\text{Co}}^{2+}$ spin-ordering temperature $T_N$ in the whole doping range, while $\frac{c_{11}-c_{12}}{2}$ exhibits a clear hardening below ${\text{Co}}^{3+}$ spin-ordering temperature $T_{N2}$ only when $x=0.5$. To explain the observed anomaly, the Landau free energy for the ${\text{La}}_{2-x}{\text{Ca}}_x{\text{CoO}}_4$ system has been constructed, including the coupling energy between elastic strains and a two-dimensional order parameter $\left(Q_1,Q_2\right)$, having a spatial modulation along [110] or $\left[\overline{1}10\right]$. We argued that this represents orbital order, and it is this order that leads to the softening of $c_{66}$.

We investigate theoretically the phase diagram of a classical Heisenberg antiferromagnet on the pyrochlore lattice perturbed by a weak second-neighbor interaction $J_2$. The huge ground-state degeneracy of the nearest-neighbor Heisenberg spins is lifted by $J_2$ and a magnetically ordered ground state sets in upon approaching zero temperature. We have found a new, partially ordered phase with collinear spins at finite temperatures for a ferromagnetic $J_2$. In addition to a large nematic order parameter, this intermediate phase also exhibits a layered structure and a bond order that breaks the sublattice symmetry. Thermodynamic phase boundaries separating it from the fully disordered and magnetically ordered states scale as $1.87J_2{S}^2$ and $0.26J_2{S}^2$ in the limit of small $J_2$. The phase transitions are discontinuous. We analytically examine the local stability of the collinear state and obtain a boundary $T\sim J_2^2/J_1$ in agreement with Monte Carlo simulations.

The experimental consequences of different order parameters in iron-based superconductors are theoretically analyzed. We consider both nodeless and nodal order parameters, with emphasis on the $\text{cos}\left(k_x\right)\text{cos}\left(k_y\right)$ nodeless order parameter recently derived by Seo et al. [arXiv:0805.2958, Phys. Rev. Lett. (to be published)]. We analyze the effect of this order parameter on the spectral function, density of states, tunneling differential conductance, penetration depth, and the NMR spin-relaxation time. This extended $s$-wave symmetry has line zeros in between the electron and hole pockets, but they do not intersect the two Fermi surfaces for moderate doping, and the superconductor is fully gapped. However, this suggests several quantitative tests: the exponential decay of the penetration depth weakens and the density of states reveals a smaller gap upon electron or hole doping. Moreover, the $\text{cos}\left(k_x\right)\text{cos}\left(k_y\right)$ superconducting gap is largest on the smallest (hole) Fermi surface. For the $1/T_1$ NMR spin-relaxation rate, the interband contribution is consistent with the current experimental results, including a (nonuniversal) $T^3$ behavior and the absence of a coherence peak. However, the intraband contribution is considerably larger than the interband contributions and still exhibits a small enhancement in the NMR spin-relaxation rate right below $T_c$ in the clean limit.

Time-resolved optical spectroscopy has been used to measure the shock pressure steadiness, emissivity, and temperature of liquid deuterium shocked to 22–90 GPa. The shock was produced using magnetically accelerated flyer plate impact, and spectra were acquired with a suite of four fiber-optic-coupled spectrometers with streak camera detectors. The shock pressure changes by an average of $-1.2\%$ over the 10–30 ns cell transit time, determined from the relative changes in the shock front self-emission with time. The shock front reflectivity was measured from $5140\text{Å}$ and $5320\text{Å}$ laser light reflected from the $D_2$ shock. The emissivity inferred from the reflectivity measurements was in reasonably good agreement with quantum molecular dynamics simulation predictions. The spectral radiance wavelength dependence was found to agree well (average normalized ${\chi }^2=1.6$) with a Planckian multiplied by the emissivity. The shock front temperature was determined from the emissivity and the wavelength-dependent shock self-emission. Thirty-seven temperature measurements spanning the 22–90 GPa range were accumulated. The large number of temperature measurements enables a comparison of the scatter in the data with expectations for a Gaussian distribution. This facilitates determination of uncertainties that incorporate both apparatus contributions and otherwise unquantified systematic effects that cause self-emission variations from one experiment to another. Agreement between temperatures determined from the absolute spectral radiance and from the relative shape of the spectrum further substantiates the absence of systematic biases. The weighted mean temperature uncertainties were as low as $\pm 3–4\%$, enabling the discrimination between competing models for the $D_2$ equation of state (EOS). The temperature results agree well with models that predict a maximum compression of $\sim 4.4$. Softer models that predict approximately sixfold compression are inconsistent with the data to a very high statistical confidence level. Previous analysis [D. Saumon and T. Guillot, Astrophys. J. 609, 1170 (2004)] of Jupiter’s internal structure has shown that the core mass is restricted to be less than approximately three times the mass of the Earth, if EOS models consistent with these temperature measurements are employed.

The origin of the spiral spin order in perovskite multiferroic manganites $R{\text{MnO}}_3$ ($R=\text{Tb}$ or Dy) is here investigated using a two $e_g$-orbital double-exchange model. Our main result is that the experimentally observed spiral phase can be stabilized by introducing a relatively weak next-nearest-neighbor superexchange coupling ($\sim 10\%$ of the nearest-neighbor superexchange). Moreover, the Jahn-Teller lattice distortion is also shown to be essential to obtain a realistic spiral period. Supporting our conclusions, the generic phase diagram of undoped perovskite manganites is obtained using Monte Carlo simulations, showing phase transitions from the $A$-type antiferromagnet, to the spiral phase, and finally to the $E$-type antiferromagnet, with decreasing size of the $R$ ions. These results are qualitatively explained by the enhanced relative intensity of the superexchanges.

Several states were proposed as candidates for the $\nu =5/2$ quantum Hall plateau. We suggest an experiment which can determine the physical state. The proposal involves transport measurements in the geometry with three quantum Hall edges connected by two quantum point contacts. In contrast to interference experiments, this approach can distinguish the Pfaffian and anti-Pfaffian states as well as different states with identical Pfaffian or anti-Pfaffian statistics. The transport is not sensitive to the fluctuations of the number of the quasiparticles trapped in the system.

We consider a model inspired by a metal break junction hypothetically caught at its breaking point, where the nonadiabatic center-of-mass motion of the bridging atom can be treated as a two-level system. By means of numerical renormalization group (NRG) we calculate the influence of the two-level system on the ballistic conductance across the bridge atom. The results are shown to be fully consistent with a conformal field theory treatment. We find that the conductance calculated by coupling Fermi-liquid theory to our NRG is always finite and fractional at zero temperature but drops quite fast as the temperature increases.