We study the temperature evolution of the nonlinear oscillatory magnetoresistance in a high-mobility two-dimensional electron system subject to a strong dc electric field. We find that the decay of the oscillation amplitude with increasing temperature originates primarily from the increase in the quantum scattering rate entering the Dingle factor. We attribute this behavior to electron-electron interaction effects.

We investigate the Josephson current in a single wall carbon nanotube connected to superconducting electrodes. We focus on the parameter regime in which transport is dominated by Kondo physics. A sizeable supercurrent is observed for odd number of electrons on the nanotube when the Kondo temperature $T_K$ is sufficiently large compared to the superconducting gap. On the other hand when, in the center of the Kondo ridge, $T_K$ is slightly smaller than the superconducting gap, the supercurrent is found to be extremely sensitive to the gate voltage $V_{\text{BG}}$. Whereas it is largely suppressed at the center of the ridge, it shows a sharp increase at a finite value of $V_{\text{BG}}$. This increase can be attributed to a doublet-singlet transition of the spin state of the nanotube island leading to a $\pi$ shift in the current phase relation. This transition is very sensitive to the asymmetry of the contacts and is in good agreement with theoretical predictions.

We study junctions between superconductors mediated by the edge states of a quantum-spin-Hall insulator. We show that such junctions exhibit a fractional Josephson effect, in which the current phase relation has a $4\pi$ rather than a $2\pi$ periodicity. This effect is a consequence of the conservation of fermion parity—the number of electron $\text{mod}2$—in a superconducting junction and is closely related to the $Z_2$ topological structure of the quantum-spin-Hall insulator. Inelastic processes, which violate the conservation of fermion parity, lead to telegraph noise in the equilibrium supercurrent. We predict that the low-frequency noise due these processes diverges exponentially with temperature $T$ as $T\to 0$. Possible experiments on HgCdTe quantum wells will be discussed.

A combined study of Raman scattering, IR absorption, photoluminescence, and photoconductivity was performed in ZnO. Two shallow donors—hydrogen at the bond-centered lattice site, ${\text{H}}_{\text{BC}}$, and hydrogen bound in an oxygen vacancy, ${\text{H}}_{\text{O}}$—were identified. Donor ${\text{H}}_{\text{BC}}$ has an ionization energy of 53 meV. The recombination of an exciton bound to ${\text{H}}_{\text{BC}}$ gives rise to the $3360.1\pm 0.2\text{meV}$ photoluminescence line. A $1s\to 2p$ donor transition at $330{\text{cm}}^{-1}$ is detected in the Raman scattering and photoconductivity spectra. The stretch mode of the associated O-H bond is detected in IR absorption at $3611{\text{cm}}^{-1}$ with an effective charge of $\left(0.28\pm 0.03\right)e$. The concentration of ${\text{H}}_{\text{BC}}$ was determined from the frequency shift of the $E_1\left(\text{LO}\right)$ phonon-plasmon mode at $591{\text{cm}}^{-1}$. The ${\text{H}}_{\text{O}}$ donor in ZnO [A. Janotti and C. G. Van de Walle, Nat. Mater. 6, 44 (2007)] has an ionization energy of 47 meV. The excitonic recombination at ${\text{H}}_{\text{O}}$ leads to the previously labeled $I_4$ line at 3362.8 meV. Photoconductivity spectra reveal the $1s\to 2p$ donor transition at $265{\text{cm}}^{-1}$.

We have tuned in situ the proximity effect in a single graphene layer coupled to two Pt/Ta superconducting electrodes. An annealing current through the device changed the transmission coefficient of the electrode/graphene interface, increasing the probability of multiple Andreev reflections. Repeated annealing steps improved the contact sufficiently for a Josephson current to be induced in graphene.

The amorphous structure of ${\text{Ge}}_8{\text{Sb}}_2{\text{Te}}_{11}$, an alloy used in the Blu-ray Disc, the de facto successor to digital versatile disk (DVD) optical storage, has been characterized by large-scale (630 atoms, 0.4 ns) density-functional/molecular-dynamics simulations using the new PBEsol approximation for the exchange-correlation energy functional. The geometry and electronic structure agree well with available x-ray diffraction data and photoelectron measurements. The total coordination numbers are Ge: 4.0, Sb: 3.7, and Te: 2.9, and the Ge-Ge partial coordination number is 0.7. Most atoms (particularly Sb) prefer octahedral coordination but 42% of Ge atoms are “tetrahedral.” Structural details, including ring statistics, local coordination, and cavities (12% of total volume), are strikingly similar to those of ${\text{Ge}}_2{\text{Sb}}_2{\text{Te}}_5$, which is used in DVD–random access memory. The presence of less than 10% Sb atoms leads to significant changes from GeTe.

Brillouin light scattering (BLS) has been used to observe and confirm the existence of nonlinear three magnon splitting and confluence processes for propagating spin waves in the magnetostatic backward volume wave configuration. Wave vector and frequency selective BLS techniques were also used to provide a quantitative map of the wave vector make-up for the parametrically excited half-frequency dipole-exchange spin wave (DESW) split magnons and the confluence magnons that result from the recombination of these DESW modes. The experimental wave vector maps for the product splitting and confluence magnons matched nicely with those expected from spin-wave theory. The data were obtained with (1) a strip line excitation/detection transducer structure, (2) forward-scattering BLS optics, (3) a fixed magnetic field of 352 Oe applied along the propagation direction, (4) pumping frequencies from 2.5 down to 2.1 GHz, (5) and cw input powers from $200\mu \text{W}$ to 6 mW. The wave vector selective measurements utilized variable diameter circular diaphragms, rotatable slit apertures, and circular light blocks to access spin waves with wave numbers from about 100 to $3.6\times {10}^4\text{rad}/\text{cm}$ and the full $360°$ range of propagation angles.

We present measurements of temperature and magnetic-field dependence of the critical current and excess current in a carbon nanotube Josephson quantum dot junction. The junction is fabricated in a controlled environment which allows for extraction of the full critical current. The measurements are performed in the open quantum dot regime and fitted to theory with good qualitative agreement. We also show how to extract level spacing, level broadening, and charging energy of an open quantum dot from a bias spectroscopy plot.

We use quantum wires fabricated on undoped GaAs/AlGaAs heterostructures in which the average impurity separation is greater than the device size to compare the behavior of the zero-bias anomaly against predictions from Kondo and spin-polarization models. Both theories display shortcomings, the most dramatic of which is the linear electron-density dependence of the zero-bias anomaly spin splitting at fixed magnetic field $B$ and the suppression of the Zeeman effect at pinch off.

We present evidence for spin-polarized charge carriers in ${\text{In}}_2{\text{O}}_3$ films. Both ${\text{In}}_2{\text{O}}_3$ and Cr doped ${\text{In}}_2{\text{O}}_3$ films exhibit room-temperature ferromagnetism after vacuum annealing, with a saturation moment reaching approximately $0.5\text{emu}/{\text{cm}}^3$ for the Cr doped samples. We used point contact Andreev reflection measurements to directly determine the spin polarization, which was found to be approximately $50\%\pm 5\%$ for both compositions. These results are consistent with suggestions that the ferromagnetism observed in certain oxide semiconductors may be carrier mediated.

We study the electronic structure of the junctions between a single graphene layer and carbon nanotubes, using a tight-binding model and the continuum theory based on Dirac fermion fields. The latter provides a unified description of different lattice structures with curvature, which is always localized at six heptagonal carbon rings around each junction. When these are evenly spaced, we find that it is possible to curve the planar lattice into armchair $\left(6n,6n\right)$ as well as zigzag $\left(6n,0\right)$ nanotubes. We show that the junctions fall into two different classes, regarding the low-energy electronic behavior. One of them, constituted by the junctions made of the armchair nanotubes and the zigzag $\left(6n,0\right)$ geometries when $n$ is a multiple of 3, is characterized by the presence of two quasibound states at the Fermi level, which are absent for the rest of the zigzag nanotubes. These states, localized at the junction, are shown to arise from the effective gauge flux induced by the heptagonal carbon rings, which has a direct reflection in the local density of states around the junction. Furthermore, we also analyze the band structure of the arrays of junctions, finding out that they can also be classified into two different groups according to the low-energy behavior. In this regard, the arrays made of armchair and $\left(6n,0\right)$ nanotubes with $n$ equal to a multiple of 3 are characterized by the presence of a series of flat bands, whose number grows with the length of the nanotubes. We show that such flat bands have their origin in the formation of states confined to the nanotubes, with little overlap in the region between the junctions. This is explained in the continuum theory from the possibility of forming standing waves in the mentioned nanotube geometries, as a superposition of modes with opposite momenta and the same quantum numbers under the $C_{6v}$ symmetry of the junction.

We present a study of quantum corrections to the conductivity of thin ferromagnetic gadolinium films. In situ magnetotransport measurements were performed on a series of thin films with thickness $d<150\text{Å}$. For sheet resistances $R_0\le 2840\Omega$ and temperatures $T\le 30\text{K}$, we observe a localizing, linear in temperature contribution to the conductivity in addition to the logarithmic temperature dependence expected from well-known quantum corrections in two dimensions. We show that such a linear $T$ dependence can arise from a spin-wave-mediated Altshuler-Aronov-type correction.

Inelastic neutron measurements of the high-temperature lattice excitations in NaI show that in thermal equilibrium at 555 K an intrinsic mode, localized in three dimensions, occurs at a single frequency near the center of the spectral phonon gap, polarized along [111]. At higher temperatures the intrinsic localized mode gains intensity. Higher energy inelastic neutron and x-ray scattering measurements on a room-temperature NaI crystal indicate that the creation energy of the ground state of the intrinsic localized mode is 299 meV.

X-ray resonant magnetic scattering studies of rare-earth magnetic ordering were performed on perovskite manganites $R{\text{MnO}}_3\left(R=\text{Dy},\text{Gd}\right)$ in an applied magnetic field. The data reveal that the field-induced threefold polarization enhancement for $\mathbf{H}\parallel a\left(H\approx 20\text{kOe}\right)$ observed in ${\text{DyMnO}}_3$ below 6.5 K is due to a re-emergence of the Mn-induced Dy spin order with propagation vector ${\tau }^{\text{Dy}}={\tau }^{\text{Mn}}=0.385{\mathbf{b}}^{\ast }$, which accompanies the suppression of the independent Dy magnetic ordering, ${\tau }^{\text{Dy}}=1/2{\mathbf{b}}^{\ast }$. For ${\text{GdMnO}}_3$, the Mn-induced ordering of Gd spins is used to track the Mn-ordering propagation vector. The data confirm the incommensurate ordering reported previously, with ${\tau }^{\text{Mn}}$ varying from $0.245{\mathbf{b}}^{\ast }$ to $0.16{\mathbf{b}}^{\ast }$ on cooling from $T_N^{\text{Mn}}$ down to a transition temperature $T^{\prime }$. New superstructure reflections which appear below $T^{\prime }$ suggest a propagation vector ${\tau }^{\text{Mn}}=1/4{\mathbf{b}}^{\ast }$ in zero magnetic field, which may coexist with the previously reported $A$-type ordering of Mn. The Gd spins order with the same propagation vector below 7 K. Within the ordered state of Gd at $T=1.8\text{K}$ we find a phase boundary for an applied magnetic field $\mathbf{H}\parallel b$, $H=10\text{kOe}$, which coincides with the previously reported transition between the ground-state paraelectric and the ferroelectric phases of ${\text{GdMnO}}_3$. Our results suggest that the magnetic ordering of Gd in magnetic field may stabilize a cycloidal ordering of Mn that, in turn, produces ferroelectricity.

A low energy beam of spin polarized ${}^8\text{L}\text{i}$ has been used to investigate nuclear spin relaxation in the multiband superconductor ${\text{NbSe}}_2$. In low magnetic fields there is significant cross relaxation between the ${}^8\text{L}\text{i}$ and the host ${}^{93}\text{N}\text{b}$ spins, which is driven by low frequency fluctuations in the nuclear magnetic dipolar interaction. The rate of cross relaxation is strongly field dependent and thus the $1/T_1$ spin relaxation rate of the ${}^8\text{L}\text{i}$ is a sensitive monitor of the static local magnetic field $B$ just below the surface. This in turn is used to determine the absolute value of the magnetic penetration depth $\lambda$ in the Meissner state. The temperature variations in $1/T_1$ and $\lambda$ are consistent with a wide distribution of superconducting gaps expected for a multiband superconductor.

Water is an omnipresent polar impurity that is expected to be the origin of many electric degradation phenomena observed in organic semiconductors. Here, we describe a microscopic model for polaron formation in the outermost layer of a pentacene crystal due to the polarization of a nearby water layer. The efficient coupling of a classical force field that describes the liquid with a tight-binding model that represents the $\pi$ system of the organic layer permits the calculation of nanosecond length trajectories. The model predicts that the reorientation of water dipoles stabilizes positive charge carriers on average by 0.6 eV and thus leads to a polaron trap state at the liquid interface. Thermal fluctuations of the water molecules provoke two-dimensional diffusive hopping of the charge carrier parallel to the interface with mobilities of up to $0.6{\text{cm}}^2{\text{s}}^{-1}{\text{V}}^{-1}$ and lead to an amorphous broadening of the valence-band tail. As a consequence, water-filled nanocavities act as trapping sites in pentacene transistors. Instead, a complete wetting of the organic film is expected to result in fast thermally activated hopping transport. Polaron trapping is thus not expected to be a limiting factor for transistor-based sensors that operate under water.

We discuss the Monte Carlo method of simulating lattice field theories as a means of studying the low-energy effective theory of graphene. We also report on simulational results obtained using the Metropolis and Hybrid Monte Carlo methods for the chiral condensate, which is the order parameter for the semimetal-insulator transition in graphene, induced by the Coulomb interaction between the massless electronic quasiparticles. The critical coupling and the associated exponents of this transition are determined by means of the logarithmic derivative of the chiral condensate and an equation-of-state analysis. A thorough discussion of finite-size effects is given, along with several tests of our calculational framework. These results strengthen the case for an insulating phase in suspended graphene, and indicate that the semimetal-insulator transition is likely to be of second order, though exhibiting neither classical critical exponents, nor the predicted phenomenon of Miransky scaling.

We show that on superconducting spherical nanoshells, the coexistence of the Meissner state with a variety of vortex patterns drives the phase transition to higher magnetic fields. The spherical geometry leads to a Magnus-Lorentz force pushing the nucleating vortices and antivortices toward the poles, overcoming local pinning centers, preventing vortex-antivortex recombination, and leading to the appearance of a Meissner belt around the sphere equator. In sufficiently small and thin spherical shells paramagnetic vortex states can be stable, enabling spatial separation of freely moving shells with different radii and vorticity in an inhomogeneous external magnetic field.

Ultrathin films of the itinerant ferromagnet ${\text{SrRuO}}_3$ were studied using the transport and magneto-optic polar Kerr effect. We find that below 4 monolayers, the films become insulating and their magnetic character changes as they loose their simple ferromagnetic behavior. We observe a strong reduction in the magnetic moment which for 3 monolayers and below lies in the plane of the film. Exchange-bias behavior is observed below the critical thickness and may point to induced antiferromagnetism in contact with ferromagnetic regions.

The wave-vector $\left(\mathbf{q}\right)$ and doping $\left(x,y\right)$ dependences of the magnetic energy, iron moment, and effective exchange interactions in ${\text{LaFeAsO}}_{1-x}{\text{F}}_x$ and ${\text{Ba}}_{1-2y}{\text{K}}_{2y}{\text{Fe}}_2{\text{As}}_2$ are studied by self-consistent LSDA calculations for co-planar spin spirals. For the undoped compounds $\left(x=0,y=0\right)$, the minimum of the calculated total energy, $E\left(\mathbf{q}\right)$, is for $\mathbf{q}$ corresponding to stripe antiferromagnetic order. Already at low levels of electron doping $\left(x\right)$, this minimum becomes flat in ${\text{LaFeAsO}}_{1-x}{\text{F}}_x$ and for $x\gtrsim 5\%$, it shifts to an incommensurate $\mathbf{q}$. In ${\text{Ba}}_{1-2y}{\text{K}}_{2y}{\text{Fe}}_2{\text{As}}_2$, stripe order remains stable for hole doping up to $y=0.3$. These results are explained in terms of the band structure. The magnetic interactions cannot be accurately described by a simple classical Heisenberg model and the effective exchange interactions fitted to $E\left(\mathbf{q}\right)$ depend strongly on doping. The doping dependence of the $E\left(\mathbf{q}\right)$ curves is compared with that of the noninteracting magnetic susceptibility for which similar trends are found.