The in-plane effective mass of quantum well states in thin Pb films on a Bi reconstructed Si(111) surface is studied by angle-resolved photoemission spectroscopy. It is found that this effective mass is a factor of 3 lower than the unusually high values reported for Pb films grown on a Pb reconstructed Si(111) surface. Through a quantitative low-energy electron diffraction analysis the change in effective mass as a function of coverage and for the different interfaces is linked to a change of about 2% in the in-plane lattice constant. To corroborate this correlation, density functional theory calculations are performed on freestanding Pb slabs with different in-plane lattice constants. These calculations show an anomalous dependence of the effective mass on the lattice constant including a change of sign for values close to the lattice constant of Si(111). This unexpected relation is due to a combination of reduced orbital overlap of the $6p_z$ states and altered hybridization between the $6p_z$ and the $6p_{\mathit{xy}}$ derived quantum well states. Furthermore, it is shown by core-level spectroscopy that the Pb films are structurally and temporally stable at temperatures below 100 K.

The interplay of magnetic energies in a Kondo lattice is the underlying physics of a heavy fermion system. Creating an artificial Kondo lattice system by localizing the moments in an ordered metallic array provides a prototype system to tune and study the energetic interplay while avoiding the complications introduced by random alloying of the material. In this paper, we create a Kondo lattice system by fabricating a hexagonally ordered nanostructured array using niobium as the host metal and cobalt as the magnetic constituent. Electrical transport measurements and magnetoresistivity measurements of these artificial lattices show that the competing exchange coupling properties can be easily tuned by controlling the impurity percentage. These artificial Kondo lattice systems enable the exploration of an artificial superconductor which should lead to a deep understanding of the role of magnetism in unconventional superconductors.

In a recent experiment on ${\mathrm{CoNb}}_2{\mathrm{O}}_6$, R. Coldea et al. [Science 327, 177 (2010)] found experimental evidence of the exceptional Lie algebra $E_8$. The emergence of this symmetry was theoretically predicted long ago for the transverse quantum Ising chain in the presence of a weak longitudinal field. We consider an accurate microscopic model of ${\mathrm{CoNb}}_2{\mathrm{O}}_6$ incorporating additional couplings and calculate numerically the dynamical structure function using a recently developed matrix-product-state method. The excitation spectra show bound states characteristic of the weakly broken $E_8$ symmetry. We compare the observed bound-state signatures in this model to those found in the transverse Ising chain in a longitudinal field and to experimental data.

Quantum many-body systems divide into a variety of phases with very different physical properties. The questions of what kinds of phases exist and how to identify them seem hard, especially for strongly interacting systems. Here we make an attempt to answer these questions for gapped interacting quantum spin systems whose ground states are short-range correlated. Based on the local unitary equivalence relation between short-range-correlated states in the same phase, we classify possible quantum phases for one-dimensional (1D) matrix product states, which represent well the class of 1D gapped ground states. We find that in the absence of any symmetry all states are equivalent to trivial product states, which means that there is no topological order in 1D. However, if a certain symmetry is required, many phases exist with different symmetry-protected topological orders. The symmetric local unitary equivalence relation also allows us to obtain some simple results for quantum phases in higher dimensions when some symmetries are present.

We consider the problem of constructing Wannier functions for ${\mathbb{Z}}_2$ topological insulators in two dimensions. It is well known that there is a topological obstruction to the construction of Wannier functions for Chern insulators, but it has been unclear whether this is also true for the ${\mathbb{Z}}_2$case. We consider the Kane-Mele tight-binding model, which exhibits both normal (${\mathbb{Z}}_2$-even) and topological (${\mathbb{Z}}_2$-odd) phases as a function of the model parameters. In the ${\mathbb{Z}}_2$-even phase, the usual projection-based scheme can be used to build the Wannier representation. In the ${\mathbb{Z}}_2$-odd phase, we do find a topological obstruction, but only if one insists on choosing a gauge that respects the time-reversal symmetry, corresponding to Wannier functions that come in time-reversal pairs. If, instead, we are willing to violate this gauge condition, a Wannier representation becomes possible. We present an explicit construction of Wannier functions for the ${\mathbb{Z}}_2$-odd phase of the Kane-Mele model via a modified projection scheme, followed by maximal localization, and confirm that these Wannier functions correctly represent the electric polarization and other electronic properties of the insulator.

Within the framework of time-dependent density functional theory combined with the Korringa-Kohn-Rostoker Green function formalism, we present a real-space methodology to investigate dynamical magnetic excitations from first principles. We set forth a scheme which enables one to deduce the correct effective Coulomb potential needed to preserve the spin-invariance signature in the dynamical susceptibilities, that is, the Goldstone mode. We use our approach to explore the spin dynamics of $3d$ adatoms and different dimers deposited on a Cu(001) with emphasis on their decay to particle-hole pairs.

We investigate the magnetotransport properties of n-type (B,Ga,In)As alloys and an n-type GaAs reference sample under hydrostatic pressure up to 16 kbar in the temperature range from 1.6 to 300 K. The free carrier concentration and the mobility of the reference sample are almost independent of the applied hydrostatic pressure. In contrast, the free carrier concentration and the mobility in ${\mathrm{B}}_{0.027}$${\mathrm{Ga}}_{0.913}$${\mathrm{In}}_{0.06}$As alloys drop by orders of magnitude over the accessible pressure range. The observations can be explained by assuming that a boron-related density of localized states exists in the vicinity of the conduction band edge of the alloy. The analysis of the pressure-induced changes of the free carrier concentration yields an image of the boron-related density of localized states with characteristic features that are found to be in good agreement with theoretical calculations using a linear combination of isolated states model. Our results provide strong evidence that boron-related states act as isovalent traps in (B,Ga,In)As alloys.

We measure simultaneously the in-plane electron $g$ factor and spin-relaxation rate in a series of undoped inversion-asymmetric (001)-oriented GaAs/AlGaAs quantum wells by spin-quantum beat spectroscopy. In combination the two quantities reveal the absolute values of both the Rashba and the Dresselhaus coefficients and prove that the Rashba coefficient can be negligibly small despite huge conduction-band potential gradients which break the inversion symmetry. The negligible Rashba coefficient is a consequence of the “isomorphism” of conduction- and valence-band potentials in quantum systems where the asymmetry is solely produced by alloy variations.

We study excitonic effects in two-dimensional massless Dirac fermions with Coulomb interactions by solving the ladder approximation to the Bethe-Salpeter equation. It is found that the general four-leg vertex has a power-law behavior with the exponent going from real to complex as the coupling constant is increased. This change of behavior is manifested in the antisymmetric response, which displays power-law behavior at small wave vectors reminiscent of a critical state, and a change in this power law from real to complex that is accompanied by poles in the response function for finite-size systems, suggesting a phase transition for strong enough interactions. The density-density response is also calculated, for which no critical behavior is found. We demonstrate that exciton correlations enhance the cusp in the irreducible polarizability at $2k_F$, leading to a strong increase in the amplitude of Friedel oscillations around a charged impurity.

We report on the remarkably robust ferroelectric state in the multiferroic compound ${\mathrm{Mn}}_{1-x}{\mathrm{Zn}}_x{\mathrm{WO}}_4$. Substitution of the magnetic ${\mathrm{Mn}}^{2+}$ with nonmagnetic Zn${}^{2+}$ reduces the magnetic exchange and provides control of the various magnetic and multiferroic states of ${\mathrm{MnWO}}_4$. Only $5\%$ of Zn substitution results in complete suppression of the frustrated collinear (paraelectric) low-temperature phase. The helical magnetic and ferroelectric phase develops as the ground state. The multiferroic state is stable up to a high level of substitution of more than $50\%$. The magnetic, thermodynamic, and dielectric properties, as well as the ferroelectric polarization of single crystals of ${\mathrm{Mn}}_{1-x}{\mathrm{Zn}}_x{\mathrm{WO}}_4$, are studied for different substitutions up to $x=0.5$. The magnetic phases have been identified in single-crystal neutron-scattering experiments. The ferroelectric polarization scales with the neutron intensity of the incommensurate peak of the helical phase.

It was predicted by Wigner in 1934 that an electron gas will undergo a transition to a crystallized state when its density is very low. Whereas significant progress has been made toward the detection of electronic Wigner states, their clear and direct experimental verification still remains a challenge. Here we address signatures of Wigner molecule formation in the transport properties of InSb nanowire quantum-dot systems, where a few electrons may form localized states depending on the size of the dot (i.e., the electron density). Using a configuration interaction approach combined with an appropriate transport formalism, we are able to predict the transport properties of these systems, in excellent agreement with experimental data. We identify specific signatures of Wigner state formation, such as the strong suppression of the antiferromagnetic coupling, and are able to detect the onset of Wigner localization, both experimentally and theoretically, by studying different dot sizes.

In the linear-response approximation, coherently driven microcavity polaritons in the pump-only configuration are expected to satisfy the Landau criterion for superfluidity at either strong enough pump powers or small flow velocities. Here, we solve nonperturbatively the time-dependent Gross-Pitaevskii equation describing the resonantly driven polariton system. We show that, even in the limit of asymptotically large densities, where in linear-response approximation the system satisfies the Landau criterion, the fluid always experiences a residual drag force when flowing through the defect. We explain the result in terms of the polariton lifetime being finite, finding that the equilibrium limit of zero drag can only be recovered in the case of perfect microcavities. In general, both the drag force exerted by the defect on the fluid, as well as the height of Cherenkov radiation, and the percentage of particles scattered by the defect, show a smooth crossover rather than a sharp thresholdlike behavior typical of superfluids which obey the Landau criterion.

We present electronic-structure calculations, electrical resistivity data, and the first specific-heat measurements in the normal and superconducting states of quasi-one-dimensional $M_2{\text{Mo}}_6{\text{Se}}_6$ $\left(M=\text{Tl},\text{In},\text{Rb}\right)$. ${\text{Rb}}_2{\text{Mo}}_6{\text{Se}}_6$ undergoes a metal-insulator transition at $\sim 170\text{K}$: electronic-structure calculations indicate that this is likely to be driven by the formation of a dynamical charge-density wave. However, ${\text{Tl}}_2{\text{Mo}}_6{\text{Se}}_6$ and ${\text{In}}_2{\text{Mo}}_6{\text{Se}}_6$ remain metallic down to low temperature, with superconducting transitions at $T_c=4.2\text{K}$ and 2.85 K, respectively. The absence of any metal-insulator transition in these materials is due to a larger in-plane bandwidth, leading to increased interchain hopping which suppresses the density wave instability. Electronic heat-capacity data for the superconducting compounds reveal an exceptionally low density of states $D_{E_F}=0.055$ states ${\text{eV}}^{-1}{\text{atom}}^{-1}$, with BCS fits showing $2\Delta /k_B{T}_c\ge 5$ for ${\text{Tl}}_2{\text{Mo}}_6{\text{Se}}_6$ and 3.5 for ${\text{In}}_2{\text{Mo}}_6{\text{Se}}_6$. Modeling the lattice specific heat with a set of Einstein modes, we obtain the approximate phonon density of states $F\left(\omega \right)$. Deconvolving the resistivity for the two superconductors then yields their electron-phonon transport coupling function ${\alpha }_{tr}^{2}F\left(\omega \right)$. In ${\text{Tl}}_2{\text{Mo}}_6{\text{Se}}_6$ and ${\text{In}}_2{\text{Mo}}_6{\text{Se}}_6$, $F\left(\omega \right)$ is dominated by an optical “guest ion” mode at $\sim 5\text{meV}$ and a set of acoustic modes from $\sim 10$ to 30 meV. ${\text{Rb}}_2{\text{Mo}}_6{\text{Se}}_6$ exhibits a similar spectrum; however, the optical phonon has a lower intensity and is shifted to $\sim 8\text{meV}$. Electrons in ${\text{Tl}}_2{\text{Mo}}_6{\text{Se}}_6$ couple strongly to both sets of modes, whereas ${\text{In}}_2{\text{Mo}}_6{\text{Se}}_6$ only displays significant coupling in the 10–18 meV range. Although pairing is clearly not mediated by the guest ion phonon, we believe it has a beneficial effect on superconductivity in ${\text{Tl}}_2{\text{Mo}}_6{\text{Se}}_6$, given its extraordinarily large coupling strength and higher $T_c$ compared to ${\text{In}}_2{\text{Mo}}_6{\text{Se}}_6$.

In this paper we link the physics of topological nonlinear $\sigma$ models with that of Chern-Simons insulators. We show that corresponding to every $2n$-dimensional Chern-Simons insulator there is a $\left(n-1\right)$-dimensional topological nonlinear $\sigma$ model with the Wess-Zumino-Witten term. Breaking internal symmetry in these nonlinear $\sigma$ models leads to nonlinear $\sigma$ models with the $\theta$ term. [This is analogous to the dimension reduction leading from $2n$-dimensional Chern-Simons insulators to $\left(2n-1\right)$ and $\left(2n-2\right)$-dimensional topological insulators protected by discrete symmetries.] The correspondence described in this paper allows one to derive the topological term in a theory involving fermions and order parameters (we shall referred to them as “fermion-$\sigma$ models”) when the conventional gradient-expansion method fails. We also discuss the quantum number of solitons in topological nonlinear $\sigma$ model and the electromagnetic action of the $\left(2n-1\right)$-dimensional topological insulators. Throughout the paper we use a simple model to illustrate how things work.

Working in the crystal-momentum representation, we calculate the optical conductivity of noncentrosymmetric insulating crystals at first order in the wave vector of light. The time-even part of this tensor describes natural optical activity and the time-odd part describes nonreciprocal effects such as gyrotropic birefringence. The time-odd part can be uniquely decomposed into magnetoelectriclike and purely quadrupolar contributions. The magnetoelectriclike component reduces in the static limit to the traceless part of the frozen-ion static magnetoelectric polarizability while at finite frequencies it acquires some quadrupolar character in order to remain translationally invariant. The expression for the orbital contribution to the conductivity at transparent frequencies is validated by comparing numerical tight-binding calculations for finite and periodic samples.

We report on magnetotransport measurements of a two-dimensional electron gas confined in a ${\text{Cd}}_{0.997}{\text{Mn}}_{0.003}\text{Te}$ quantum well structure under conditions of vanishing Zeeman energy. The electron Zeeman energy has been tuned via the $s\text{−}d$ exchange interaction in order to probe different quantum Hall states associated with metallic and insulating phases. We have observed that reducing Zeeman energy to zero does not necessary imply the disappearing of quantum Hall states, i.e., a closing of the spin gap. The spin-gap value under vanishing Zeeman-energy conditions is shown to be dependent on the filling factor. Numerical simulations support a qualitative description of the experimental data presented in terms of a crossing or an avoided crossing of spin split Landau levels with same orbital quantum number $N$.

We perform an analytical renormalization group (RG) study to address the role of Coulomb repulsion, the competition between extended $s$-wave superconducting order $\left(s\pm \right)$, and the spin-density wave (SDW) order and the angular dependence of the superconducting gap in multipocket models of iron-based superconductors. Previous analytic RG studies considered a toy model of one hole and one electron pocket. We consider more realistic models of two electron pockets and either two or three hole pockets, and also incorporate angular dependence of the interactions. In a toy two-pocket model, SDW order always wins over $s\pm$ order at perfect nesting; $s\pm$ order only appears when doping is finite and RG flow extends long enough to overcome intrapocket Coulomb repulsion. For multipocket models, there are two new effects. First, in most cases there exists an attractive component of the interaction in $s\pm$ channel no matter how strong intrapocket repulsion is, such that the system necessary becomes a superconductor once it overcomes the competition from the SDW state. Second, in four-pocket case (but not in five-pocket case), $s\pm$ order wins over SDW order even for perfect nesting, if RG flow extends long enough, suggesting that SDW order is not a necessary precondition for the $s\pm$ order. Our analytic results are in full agreement with recent numerical functional RG studies by Thomale et al. [arXiv:1002.3599 (unpublished)]

We investigate the high-frequency dynamics of dysprosium and cobalt gratings fabricated at the surface of a $\text{GaAs}/{\text{Al}}_{0.33}{\text{Ga}}_{0.67}\text{As}$ heterojunction. We detect the collective and localized spin-wave modes of the grating by measuring the photovoltage and the photoresistance induced in the two-dimensional electron gas (2DEG). The magnetic excitations couple to the 2DEG through their stray magnetic field. We perform a spectroscopy of dipolar-exchange spin waves as a function of microwave power, temperature, the tilt angle of the applied magnetic field, and by varying the structural and material parameters to change the strength of dipolar interactions. The data reveal two types of spin waves. Dipolar magnetization waves propagate across the grating through the magnetostatic interaction between the stripes. We derive an analytical expression of their dispersion curve and obtain a good fit of the ferromagnetic resonance broadening from first principles. The second type is dipolar edge spin waves which manifest through a series of sharp resonances at lower magnetic field. These waves are confined near the pole surfaces and interact very little with neighboring stripes. We calculate the eigenfrequencies of the quantized modes and obtain a qualitative explanation of the low-field resonances. The fit yields a value of the exchange stiffness constant of dysprosium, $A=1.5\times {10}^{-12}\text{J}{\text{m}}^{-1}$. Our experiments show that photovoltage measurements in hybrid semiconductor-ferromagnetic structures provide a sensitive and noninvasive tool for probing the spin waves of small magnets (10–500 nm).

We report the characterization of the layered rare-earth manganese oxyselenide ${\text{La}}_2{\text{O}}_3{\text{Mn}}_2{\text{Se}}_2$. The susceptibility data show a broad maximum near 350 K, indicating the existence of two-dimensional (2D) short-range ordering in this compound. A sharp feature associated with a short-range to long-range antiferromagnetic phase transition is seen at 163 K. A very small heat-capacity anomaly is detected around 163 K, indicating that most of the magnetic entropy is lost during the 2D ordering process. Both crystal and magnetic structures were studied by neutron powder diffraction at 300, 200, 150, 100, and 6 K. The structure was refined in space group $I4/mmm$ with $a=4.13939\left(3\right)\text{Å}$ and $c=18.8511\left(2\right)\text{Å}$ at ambient temperature. No structural distortion was detected. The resulted magnetic structure is G-type with a propagation vector of $\mathbf{k}=\left(0,0,0\right)$ and an ordered magnetic moment of $4.147\left(28\right){\mu }_B/\text{Mn}$ along $c$ is found at 6 K. Warren peak shape analysis of the neutron-diffraction data near $22°$ is employed to characterize the increase in correlation length in the 2D magnetic state on approaching the three-dimensional ordering transition.

We present measurements of $ab$-plane resistivity ${\rho }_{\text{ab}}\left(T\right)$ and superfluid density ($n_{\text{s}}\propto {\lambda }^{-2}$, $\lambda =\text{magnetic}$ penetration depth) in ${\text{La}}_{2-x}{\text{Sr}}_x{\text{CuO}}_4$ (LSCO) films. As Sr concentration $x$ exceeds about 0.22, the superconducting transition sharpens dramatically, becoming as narrow as 200 mK near the super-to-normal-metal quantum critical point. At the same time, ${\lambda }^{-2}\left(0\right)$ and transition temperature $T_{\text{c}}$ decrease, and upward curvature develops in ${\lambda }^{-2}\left(T\right)$. Given the sharp transitions, we interpret these results in the context of a homogeneous $d$-wave superconducting state, not a phase-separated state, with elastic scattering that is enhanced relative to underdoped LSCO due to weaker electron correlations.