We present theoretical and experimental studies of the amplitude of the terahertz (THz) light emitted by biased superlattices excited by fast optical pulses as a function of the pump photon energy. The comparison of the measured transient signal and of its dependence upon the excitation photon energy with a model calculation allows a clear separation between the contributions of the dissociated carriers and bound pairs to the emitted THz field.

When epitaxial graphene layers are formed on SiC(0001), the first carbon layer (known as the “buffer layer”), while relatively easy to synthesize, does not have the desirable electrical properties of graphene. The conductivity is poor due to a disruption of the graphene $\pi$ bands by covalent bonding to the SiC substrate. Here we show that it is possible to restore the graphene $\pi$ bands by inserting a thin oxide layer between the buffer layer and SiC substrate using a low temperature, complementary metal-oxide semiconductor-compatible process that does not damage the graphene layer.

We present measurements of the electronic compressibility, $K$, of bilayer graphene in both zero and finite magnetic fields up to 14 T, and as a function of both the carrier density and electric field perpendicular to the graphene sheet. The low-energy hyperbolic band structure of bilayer graphene is clearly revealed in the data, as well as a sizable asymmetry between the conduction and valence bands. A sharp increase in $K^{-1}$ near zero density is observed for increasing electric field strength, signaling the controlled opening of a gap between these bands. At high magnetic fields, broad Landau level (LL) oscillations are observed, directly revealing the doubled degeneracy of the lowest LL and allowing for a determination of the disorder broadening of the levels.

In this paper we give the full microscopic derivation of the model Hamiltonian for the three-dimensional topological insulators in the ${\mathrm{Bi}}_2{\mathrm{Se}}_3$ family of materials (${\mathrm{Bi}}_2{\mathrm{Se}}_3$, ${\mathrm{Bi}}_2{\mathrm{Te}}_3$ and ${\mathrm{Sb}}_2{\mathrm{Te}}_3$). We first give a physical picture to understand the electronic structure by analyzing atomic orbitals and applying symmetry principles. Subsequently, we give the full microscopic derivation of the model Hamiltonian introduced by Zhang et al. [Nat. Phys. 5, 438 (2009)] based both on symmetry principles and the $\mathbf{k}\cdot \mathbf{p}$ perturbation theory. Two different types of $k^3$ terms, which break the in-plane full rotation symmetry down to threefold rotation symmetry, are taken into account. An effective Hamiltonian is derived for the topological surface states. Both bulk and surface models are investigated in the presence of an external magnetic field, and the associated Landau level structure is presented. For a more quantitative fitting to the first principle calculations, we also present a model Hamiltonian including eight energy bands.

We analyze a number of proposed explanations for spectroscopic anomalies observed in atomic hydrogen defects embedded in a solid molecular hydrogen matrix. In particular, we critically evaluate the possibility that these anomalies are related to Bose-Einstein condensation (both global and local). For each proposed mechanism we discuss which aspects of the experiment can be explained and make predictions for future experiments.

We report the observation of hysteretic synchronization of point-contact spin-torque nano-oscillators by a microwave magnetic field. The hysteresis is asymmetric with respect to the frequency detuning of the driving signal and appears in the region of a strong dependence of the oscillation frequency on the bias current. Theoretical analysis shows that hysteretic synchronization occurs when the width of the synchronization range, enhanced by the oscillator’s nonlinearity, becomes comparable to the dissipation rate, while the observed asymmetry is a consequence of the nonlinear dependence of frequency on the bias current. Hysteretic synchronization is a general property of strongly nonlinear oscillators, and can therefore be expected to affect the dynamics of a variety of physical systems.

We report on the local electronic structure of Fe impurities in MgO thin films. Using soft x-ray absorption spectroscopy, we verified that the Fe impurities are all in the $2+$ valence state. The fine details in the line shape of the $\text{Fe}{L}_{2,3}$ edges provide direct evidence for the presence of a dynamical Jahn-Teller distortion. We are able to determine the magnitude of the effective $D_{4h}$ crystal field energies. We also observed a strong temperature dependence in the spectra which we can attribute to the thermal population of low-lying excited states that are present due to the spin-orbit coupling in the $\text{Fe}3d$. Using this ${\text{Fe}}^{2+}$ impurity system as an example, we show that an accurate measurement of the orbital moment in ${\text{Fe}}_3{\text{O}}_4$ will provide a direct estimate for the effective local low-symmetry crystal fields on the ${\text{Fe}}^{2+}$ sites, important for the theoretical modeling of the formation of orbital ordering.

Most quantum point contacts (QPCs) fabricated in high-mobility two-dimensional electron gases show a zero-bias conductance peak near pinchoff but the origin of this peak remains a mystery. Previous experiments have primarily focused on the zero-bias peak at moderate conductance, in the range $1–2e^2/h$. Here, measurements are presented of zero-bias peaks that persist down to ${10}^{-4}{e}^2/h$. Magnetic field and temperature dependencies of the zero-bias peak in the low-conductance limit are qualitatively different from the analogous phenomenology at higher conductance with implications for existing theoretical models of transport in low-density QPCs.

We theoretically study a Luttinger liquid (LL) driven out of equilibrium by injection of high-energy electrons. The electrons are injected into the LL locally at a fixed energy and their spectral properties are detected at another spatial point some distance away by evaluating the average tunneling current from the LL into a resonant level with tunable energy. For energies slightly below the injection energy, the dependence of the detected current on the difference between injection, and detection energies is described by a power law whose exponent depends continuously on the Luttinger parameter. In contrast, for tunneling into a chiral LL edge of a fractional quantum-Hall state from the Laughlin sequence, we find that the detected current grows linearly with energy difference, despite a decreasing density of states.

We report on a state characterized by a zero differential resistance observed in very high Landau levels of a high-mobility two-dimensional electron system. Emerging from a minimum of Hall field-induced resistance oscillations at low temperatures, this state exists over a continuous range of magnetic fields extending well below the onset of the Shubnikov–de Haas effect. The minimum current required to support this state is largely independent on the magnetic field while the maximum current increases with the magnetic field tracing the onset of inter-Landau level scattering.

We report a thorough theoretical study of the low temperature phase diagram of ${\mathrm{Cs}}_2\mathrm{Cu}{\mathrm{Cl}}_4$, a spatially anisotropic spin $S=1∕2$ triangular lattice antiferromagnet, in a magnetic field. Our results, obtained in a quasi-one-dimensional limit in which the system is regarded as a set of weakly coupled Heisenberg chains, are in excellent agreement with experiment. The analysis reveals some surprising physics. First, we find that when the magnetic field is oriented within the triangular layer, spins are actually most strongly correlated within planes perpendicular to the triangular layers. This is despite the fact that the interlayer exchange coupling in ${\mathrm{Cs}}_2\mathrm{Cu}{\mathrm{Cl}}_4$ is about an order of magnitude smaller than the weakest (diagonal) exchange in the triangular planes themselves. Second, the phase diagram in such orientations is exquisitely sensitive to tiny interactions, heretofore neglected, of order a few percent or less of the largest exchange couplings. These interactions, which we describe in detail, induce entirely new phases, and a novel commensurate-incommensurate transition, the signatures of which are identified in NMR experiments. We discuss the differences between the behavior of ${\mathrm{Cs}}_2\mathrm{Cu}{\mathrm{Cl}}_4$ and an ideal two-dimensional triangular model, and in particular, the occurrence of magnetization plateaux in the latter. These and other related results are presented here along with a thorough exposition of the theoretical methods, and a discussion of broader experimental consequences to ${\mathrm{Cs}}_2\mathrm{Cu}{\mathrm{Cl}}_4$ and other materials.

We develop a microscopic model to calculate point-contact spectra between a metallic tip and a superconducting heavy-fermion system. We apply our tunneling model to the heavy fermion ${\text{CeCoIn}}_5$, both in the normal and superconducting states. In point contact and scanning tunneling spectroscopy many heavy-fermion materials, such as ${\text{CeCoIn}}_5$, exhibit an asymmetric differential conductance, $dI/dV$, combined with a strongly suppressed Andreev reflection signal in the superconducting state. We argue that both features may be explained in terms of a multichannel tunneling model in the presence of localized states near the interface. We find that it is not sufficient to tunnel into two itinerant bands of light and heavy electrons to explain the Fano line shape of the differential conductance. Localized states in the bulk or near the interface are an essential component for quantum interference to occur when an electron tunnels from the metallic tip of the point contact into the heavy-fermion system.

The interplay between quantum Hall ordering and spontaneously broken “internal” symmetries in two-dimensional electron systems with spin or pseudospin degrees of freedom gives rise to a variety of interesting phenomena, including novel phases, phase transitions, and topological excitations. Here we develop a theory of broken-symmetry quantum Hall states, applicable to a class of multivalley systems, where the symmetry at issue is a point-group element that combines a spatial rotation with a permutation of valley indices. The anisotropy of the dispersion relation, generally present in such systems, favors states where all electrons reside in one of the valleys. In a clean system, the valley “pseudospin” ordering occurs via a finite-temperature transition accompanied by a nematic pattern of spatial symmetry breaking. In weakly disordered systems, domains of pseudospin polarization are formed, which prevents macroscopic valley and nematic ordering; however, the resulting state still asymptotically exhibits the quantum Hall effect. We discuss the transport properties in the ordered and disordered regimes, and the relation of our results to recent experiments in AlAs.

Cationic iron-oxide clusters of several sizes and stoichiometries have been synthesized and studied isolated in the gas phase. Vibrational spectra of the clusters have been measured using resonant IR-induced dissociation of ${\text{Fe}}_n\text{O}_{m+2}{}^{+}\to {\text{Fe}}_n\text{O}_m{}^{+}+O_2$ in the $250–1250{\text{cm}}^{-1}$ range. Density-functional theory was used to investigate the geometry and spin configuration of the representative ${\text{Fe}}_4\text{O}_6{}^{0/+}$ cluster. Its lowest-energy state was found to be an almost tetrahedral cagelike structure with a ferrimagnetic arrangement of spins, resulting in total cluster spin of $S=5$ for the neutral cluster. These results were confirmed for ${\text{Fe}}_4\text{O}_6{}^{+}$ by the comparison of the calculated infrared spectrum to the experimentally obtained one.

We show that the competition between magnetism and superconductivity can be used to determine the pairing state in the iron arsenides. To this end we demonstrate that the itinerant antiferromagnetic (AFM) phase and the unconventional $s^{+-}$ sign-changing superconducting (SC) state are near the borderline of microscopic coexistence and macroscopic phase separation, explaining the experimentally observed competition of both ordered states. In contrast, conventional $s^{++}$ pairing is not able to coexist with magnetism. Expanding the microscopic free energy of the system with competing orders around the multicritical point, we find that static magnetism plays the role of an intrinsic interband Josephson coupling, making the phase diagram sensitive to the symmetry of the Cooper-pair wave function. We relate this result to the quasiparticle excitation spectrum and to the emergent SO(5) symmetry of systems with particle-hole symmetry. Our results rely on the assumption that the same electrons that form the ordered moment contribute to the superconducting condensate and that the system is close to particle-hole symmetry. We also compare the suppression of SC in different regions of the FeAs phase diagram, showing that while in the underdoped side it is due to the competition with AFM, in the overdoped side it is related to the disappearance of pockets from the Fermi surface.

The surface electronic structure of the half-metallic alloy NiMnSb is investigated in detail, revealing the presence of Dirac cones in the minority-spin states. Surface-localized carriers of suitable energy will consequently behave as massless Dirac fermions, a remarkable situation for a half-metallic material and potentially of great significance in future spintronic device applications. Unlike related features observed previously in graphene and at the surfaces of topological insulators, the Dirac cones described here are of elliptical cross section and are tilted with respect to the energy axis. Such unique properties are permitted due to the lower symmetry in the present case.

We report inelastic neutron scattering measurements that provide a distinct dynamical “fingerprint” for the multiferroic ground state of 3.5% Ga-doped ${\text{CuFeO}}_2$. The complex ground state is stabilized by the displacement of the oxygen atoms, which contribute to the multiferroic coupling predicted in the “spin-driven” model. By comparing the observed and calculated spectrum of spin excitations, we conclude that the magnetic ground state is a distorted screw-type spin configuration with a distribution of turn angles.

Motivated by the recent numerical evidence [Z. Meng, T. Lang, S. Wessel, F. Assaad, and A. Muramatsu, Nature (London) 464, 847 (2010)] of a short-range resonating valence bond state in the honeycomb lattice Hubbard model, we consider Schwinger boson mean field theories of possible spin liquid states on honeycomb lattice. From general stability considerations the possible spin liquids will have gapped spinons coupled to $Z_2$ gauge field. We apply the projective symmetry group method to classify possible $Z_2$ spin liquid states within this formalism on honeycomb lattice. It is found that there are only two relevant $Z_2$ states, differed by the value of gauge flux, zero or $\pi$, in the elementary hexagon. The zero-flux state is a promising candidate for the observed spin liquid and continuous phase transition into commensurate Néel order. We also derive the critical field theory for this transition, which is the well-studied O(4) invariant theory [A. V. Chubukov, T. Senthil, and S. Sachdev, Phys. Rev. Lett. 72, 2089 (1994); A. V. Chubukov, S. Sachdev, and T. Senthil, Nucl. Phys. B 426, 601 (1994); S. V. Isakov, T. Senthil, and Y. B. Kim, Phys. Rev. B 72, 174417 (2005)], and has an irrelevant coupling between Higgs and boson fields with cubic power of spatial derivatives as required by lattice symmetry. This is in sharp contrast to the conventional theory [S. Sachdev and N. Read, Int. J. Mod. Phys. B 5, 219 (1991)], where such transition generically leads to incommensurate magnetic order. In this scenario the $Z_2$ spin liquid could be close to a tricritical point. Soft boson modes will exist at seven different wave vectors. This will show up as low-frequency dynamical spin susceptibility peaks not only at the $\Gamma$ point (the Néel order wave vector) but also at Brillouin-zone-edge center $M$ points and twelve other points. Some simple properties of the $\pi$-flux state are studied as well. Symmetry allowed further neighbor mean field ansatz are derived in appendices which can be used in future theoretical works along this direction.

We have examined the impact of charged impurity scattering on charge carrier transport in bilayer graphene (BLG) by deposition of potassium in ultrahigh vacuum at low temperature. Charged impurity scattering gives a conductivity which is supralinear in carrier density with a magnitude similar to single-layer graphene for the measured range of carrier densities of $2–4\times {10}^{12}{\text{cm}}^{-2}$. Upon addition of charged impurities of concentration $n_{\text{imp}}$, the minimum conductivity ${\sigma }_{\text{min}}$ decreases proportional to $n_{\text{imp}}^{-1/2}$ while the electron and hole puddle carrier density increases proportional to $n_{\text{imp}}^{1/2}$. These results for the intentional deposition of potassium on BLG are consistent with theoretical predictions for charged impurity scattering assuming a gapless hyperbolic dispersion relation. However, our results also suggest that charged impurity scattering alone cannot explain the observed transport properties of pristine BLG on ${\text{SiO}}_2$ before potassium doping.

Superconductivity-induced changes in phonon lineshapes in niobium have been reinvestigated by high-resolution inelastic neutron scattering. We show that the changes go beyond a simple change in lifetime and frequency when the phonon frequency is close to the superconducting energy gap $2\Delta$. The observed lineshapes in elemental niobium are qualitatively similar to those found previously in borocarbide superconductors and agree very well with those predicted by the theory of Allen et al. [Phys. Rev. B 56, 5552 (1997)]. Our results indicate that the peculiar phonon lineshapes in the superconducting state predicted by the theory of Allen et al. [Phys. Rev. B 56, 5552 (1997)] are a general phenomenon and not restricted to a particular class of compounds.