We report the results of de-Haas-van-Alphen (dHvA) measurements in ${\text{CeCoIn}}_5$ and ${\text{LaCoIn}}_5$. Cd doping is known to induce an antiferromagnetic order in the heavy-fermion superconductor ${\text{CeCoIn}}_5$, whose effect can be reversed with applied pressure. We find a slight but systematic change of the dHvA frequencies with Cd doping in both compounds, reflecting the chemical potential shift due to the addition of holes. The frequencies and effective masses are close to those found in the nominally pure compounds with similar changes apparent in the Ce and La compounds with Cd substitution. We observe no abrupt change to the electron Fermi surface volume in the high field paramagnetic state for $x\sim x_c$ corresponding to the onset of antiferromagnetic ordering at $H=0$ in $\text{CeCo}{\left({\text{In}}_{1-x}{\text{Cd}}_x\right)}_5$. Our results make it unlikely that a change of Fermi surface from large to small accompanies the change in the ground state meaning that the Fermi surface of ${\text{CeCoIn}}_5$ doped with Cd likely includes a significant contribution from $\text{Ce}f$ electrons even for $x>x_c$.

We calculate the effect of local magnetic moments on the electron-phonon coupling in doped ${\text{BaFe}}_2{\text{As}}_2$ using the density-functional perturbation theory. We show that the magnetism enhances the total electron-phonon coupling by $\sim 50\mathrm{\%}$, up to $\lambda \lesssim 0.35$, still not enough to explain the high critical temperature, but strong enough to have a non-negligible effect on superconductivity, for instance, by frustrating the coupling with spin fluctuations and inducing order-parameter nodes. The enhancement comes not from the phonon softening but mostly from a renormalization of the electron-phonon matrix elements. We also investigate, in the rigid band approximation, the effect of doping, and find that $\lambda$ versus doping does not mirror the behavior of the density of states; while the latter decreases upon electron doping, the former does not, and even increases slightly.

Synchrotron x-ray diffraction investigations of two single crystals of sodium cobaltate ${\text{Na}}_x{\text{CoO}}_2$ from different batches with composition $x=0.525–0.530$ reveal homogeneous incommensurate sodium ordering with propagation vector (0.53 0.53 0) at room temperature. The incommensurate $\left(qq0\right)$ superstructure exists between 220 and 430 K. The value of $q$ varies between $q=0.514$ and 0.529, showing a broad plateau at the latter value between 260 and 360 K. On cooling, an unusual reversible phase segregation into two volume fractions is observed. Below 220 K, one volume fraction shows the well-known commensurate orthorhombic $x=0.50$ superstructure while a second volume fraction with $x=0.55$ exhibits another commensurate superstructure, presumably with a $6a\times 6a\times c$ hexagonal supercell. We argue that the commensurate-to-incommensurate transition is an intrinsic feature of samples with Na concentrations $x=0.5+\delta$ with $\delta \approx 0.03$.

The nature of the phase transition of regularly frustrated vector spin systems in three dimensions is investigated based on a Ginzburg-Landau-type effective Hamiltonian. On the basis of the variational analysis of this model, Onoda and Nagaosa recently suggested the possible occurrence of a chiral phase, where the vector chirality exhibits a long-range order without the long-range order of the spin [S. Onoda and N. Nagaosa, Phys. Rev. Lett. 99, 027206 (2007)]. In the present paper, we elaborate their analysis by considering the possibility of a first-order transition which was not taken into account in their analysis. We find that the first-order transition indeed occurs within the variational approximation, which significantly reduces the stability range of the chiral phase while the chiral phase still persists in a restricted parameter range. Then, we perform an extensive Monte Carlo simulation focusing on such a parameter range. Contrary to the variational result, however, we do not find any evidence of the chiral phase. The range of the chiral phase, if any, is estimated to be less than 0.1% in the temperature width.

We show how efficient loop updates, originally developed for Monte Carlo simulations of quantum spin systems at finite temperature, can be combined with a ground-state projector scheme and variational calculations in the valence-bond basis. The methods are formulated in a combined space of spin $z$ components and valence bonds. Compared to schemes formulated purely in the valence-bond basis, the computational effort is reduced from up to $O\left(N^2\right)$ to $O\left(N\right)$ for variational calculations, where $N$ is the system size, and from $O\left(m^2\right)$ to $O\left(m\right)$ for projector simulations, where $m⪢N$ is the projection power. These improvements enable access to ground states of significantly larger lattices than previously. We demonstrate the efficiency of the approach by calculating the sublattice magnetization $M_s$ of the two-dimensional Heisenberg model to high precision, using systems with up to $256\times 256$ spins. Extrapolating the results to the thermodynamic limit gives $M_s=0.30743\left(1\right)$. We also discuss optimized variational amplitude-product states, which were used as trial states in the projector simulations, and compare results of projecting different types of trial states.

We investigate the contribution of the low-energy electronic excitations toward the Raman spectrum of bilayer graphene for the incoming photon energy $\Omega \gtrsim 1\text{eV}$. Starting with the four-band tight-binding model, we derive an effective scattering amplitude that can be incorporated into the commonly used two-band approximation. Due to the influence of the high-energy bands, this effective scattering amplitude is different from the contact interaction amplitude obtained within the two-band model alone. We then calculate the spectral density of the inelastic light scattering accompanied by the excitation of electron-hole pairs in bilayer graphene. In the absence of a magnetic field, due to the parabolic dispersion of the low-energy bands in a bilayer crystal, this contribution is constant and in doped structures has a threshold at twice the Fermi energy. In an external magnetic field, the dominant Raman-active modes are the $n^{-}\to n^{+}$ inter-Landau-level transitions with crossed polarization of in/out photons. We estimate the quantum efficiency of a single $n^{-}\to n^{+}$ transition in the magnetic field of 10 T as $I_{n^{-}\to n^{+}}\sim {10}^{-12}$.

The structural misfit compound ${\left(\text{PbSe}\right)}_{1.16}{\left({\text{TiSe}}_2\right)}_2$ is reported. It is a superconductor with a $T_{\text{c}}$ of 2.3 K. ${\left(\text{PbSe}\right)}_{1.16}{\left({\text{TiSe}}_2\right)}_2$ derives from a parent compound, ${\text{TiSe}}_2$, which shows a charge-density wave transition and no superconductivity. The crystal structure, characterized by high-resolution electron microscopy and powder x-ray diffraction, consists of two layers of 1T-${\text{TiSe}}_2$ alternating with a double layer of (100) PbSe. Transport measurements suggest that the superconductivity is induced by charge transfer from the PbSe layers to the ${\text{TiSe}}_2$ layers.

It has been noted a long time ago that a term of the form $\theta \left(e^2/2\pi h\right)\mathbf{B}\cdot \mathbf{E}$ may be added to the standard Maxwell Lagrangian without modifying the familiar laws of electricity and magnetism. $\theta$ is known to particle physicists as the “axion” field and whether or not it has a nonzero expectation value in vacuum remains a fundamental open question of the standard model. A key manifestation of the axion term is the Witten effect: a unit magnetic monopole placed inside a medium with $\theta \ne 0$ is predicted to bind a (generally fractional) electric charge $-e\left(\theta /2\pi +n\right)$ with $n$ integer. Here we conduct a test of the Witten effect based on the recently established fact that the axion term with $\theta =\pi$ emerges naturally in the description of the electromagnetic response of a class of crystalline solids called topological insulators—materials distinguished by strong spin-orbit coupling and nontrivial band structures. Using a simple physical model for a topological insulator we demonstrate the existence of a fractional charge bound to a monopole by an explicit numerical calculation. We also propose a scheme for generating an “artificial” magnetic monopole in a topological insulator film that may be used to facilitate an experimental test of Witten’s prediction.

We experimentally demonstrate a general optical pump-probe technique to observe the spin Hall effect of light (SHEL) in an absorbing medium. In essence, a locally confined pump-induced modification of a material’s absorptivity can effectively be used as an induced aperture allowing one to detect the transversely displaced circular polarization components of an incident beam through differential transmission techniques. We consider linear absorption mechanisms such as free-carrier absorption and Pauli blocking as well as nonlinear absorption processes such as two-photon absorption. For absorption mechanisms that do not depend on light polarization, the SHEL of the probe beam is obtained directly, while polarization-dependent properties give an effective SHEL displacement that depends on the action of the SHEL on both pump and probe beams. Using 150 fs pump, 820 nm pump and probe pulses we observe SHEL effects in silicon via free-carrier absorption. SHEL effects are also observed via Pauli blocking at 820 nm and two-photon absorption at 1550 nm in GaAs using $\sim 150\text{fs}$ pump and probe pulses.

Cross-sectional scanning tunneling microscopy and spectroscopy (XSTM/S) were used to map out the band alignment across the complex oxide interface of ${\text{La}}_{2/3}{\text{Ca}}_{1/3}{\text{MnO}}_3/\text{Nb}$-doped ${\text{SrTiO}}_3$. By a controlled cross-sectional fracturing procedure, unit-cell high steps persist near the interface between the thin film and the substrate in the noncleavable perovskite materials. The abrupt changes in the mechanical and electronic properties were visualized directly by XSTM/S. Using changes in the density of states as probe by STM, the electronic band alignment across the heterointerface was mapped out providing an approach to directly measure the electronic properties at complex oxide interfaces.

The contact area dependence of the interfacial friction experienced during the translation of antimony nanoparticles deposited on a graphite substrate is studied under different conditions using the tip of an atomic force microscope as a manipulation tool. In vacuum a dual behavior of the friction-area curves is found, characterized by the observation that some particles exhibit friction below the detection limit while other similarly sized particles showed constant shear stress values. Detailed investigations prove the reproducibility of this effect, revealing that neither the particle’s morphology nor their alignment relative to the substrate lattice influence the findings. In contrast, we observe that a temporary exposure to ambient air can lead to a drastic increase in the particle’s friction.

The electronic band structure of as-grown and doped graphene grown on the carbon face of SiC is studied by high-resolution angle-resolved photoemission spectroscopy, where we observe both rotations between adjacent layers and $AB$ stacking. The band structure of quasifreestanding $AB$ bilayers is directly compared with bilayer graphene grown on the Si face of SiC to study the impact of the substrate on the electronic properties of epitaxial graphene. Our results show that the C-face films are nearly free standing from an electronic point of view due to the rotations between graphene layers.

Recently we have shown that the reduction in the photoluminescence linewidth of many deep luminescence centers in highly enriched ${}^{28}\text{S}\text{i}$ results in well-resolved isotopic fingerprints. This allows for a better characterization of a defect center, as not only the involvement of a specific element but also the number of atoms of that element within the complex can be determined. Surprisingly, we have found that many well-known luminescence centers have a different composition than originally supposed. In addition, we have found a large number of four- and five-atom luminescence centers involving the elements Cu, Au, and Li. Here we introduce series of four- and five-atom deep luminescence centers involving a single Pt atom together with Cu and Li, similar to what has been seen previously for Au-containing luminescence centers.

The temperature dependence of the lattice parameter of ${\text{Ge}}_{1-y}{\text{Sn}}_y$ alloys deposited on Si substrates has been determined from an analysis of their x-ray reciprocal-space maps. It is found that over the range $0<y<0.03$ the alloy thermal expansivity increases by up to 20% as a function of $y$. This implies a strong deviation from a linear interpolation between the end compounds since the thermal expansivities of pure Ge and $\alpha \text{-Sn}$ are nearly the same. Alternative interpolation formulas based on a Debye model and a mixed Debye-Einstein model of the phonon structure are tested and it is found that they also fail to explain the observed increase in thermal expansivity.

We report quantum oscillations in the underdoped high-temperature superconductor ${\text{YBa}}_2{\text{Cu}}_3{\text{O}}_{6+x}$ over a wide range in magnetic field $28\le {\mu }_{0}H\le 85\text{T}$ corresponding to $\approx 12$ oscillations, enabling the Fermi surface topology to be mapped to high resolution. As earlier reported by Sebastian et al. [Nature (London) 454, 200 (2008)], we find a Fermi surface comprising multiple pockets, as revealed by the additional distinct quantum oscillation frequencies and harmonics reported in this work. We find the originally reported broad low-frequency Fourier peak at $\approx 535\text{T}$ to be clearly resolved into three separate peaks at $\approx 460$, $\approx 532$, and $\approx 602\text{T}$, in reasonable agreement with the reported frequencies of Audouard et al. [Phys. Rev. Lett. 103, 157003 (2009)]. However, our increased resolution and angle-resolved measurements identify these frequencies to originate from two similarly sized pockets with greatly contrasting degrees of interlayer corrugation. The spectrally dominant frequency originates from a pocket (denoted $\alpha$) that is almost ideally two-dimensional in form (exhibiting negligible interlayer corrugation). In contrast, the newly resolved weaker adjacent spectral features originate from a deeply corrugated pocket (denoted $\gamma$). On comparison with band structure, the $d$-wave symmetry of the interlayer dispersion locates the minimally corrugated $\alpha$ pocket at the “nodal” point ${\mathbf{k}}_{\text{nodal}}=\left(\pi /2,\pi /2\right)$, and the significantly corrugated $\gamma$ pocket at the “antinodal” point ${\mathbf{k}}_{\text{antinodal}}=\left(\pi ,0\right)$ within the Brillouin zone. The differently corrugated pockets at different locations indicate creation by translational symmetry breaking—a spin-density wave has been suggested from the suppression of Zeeman splitting for the spectrally dominant pocket. In a broken-translational symmetry scenario, symmetry points to the nodal $\left(\alpha \right)$ pocket corresponding to holes, with the weaker antinodal $\left(\gamma \right)$ pocket corresponding to electrons—likely responsible for the negative Hall coefficient reported by LeBoeuf et al. [Nature (London) 450, 533 (2007)]. Given the similarity in $\alpha$ and $\gamma$ pocket volumes, their opposite carrier type and the previous report of a diverging effective mass in Sebastian et al. [Proc. Nat. Am. Soc. 107, 6175 (2010)], we discuss the possibility of a secondary Fermi surface instability at low dopings of the excitonic insulator type, associated with the metal-insulator quantum critical point. Its potential involvement in the enhancement of superconducting transition temperatures is also discussed.

We present a comprehensive ellipsometric study of an untwinned, nearly stoichiometric ${\text{LaMnO}}_3$ crystal in the spectral range 1.2–6.0 eV at temperatures $20\le T\le 300\text{K}$. The complex dielectric response along $b$ and $c$ axes of the $Pbnm$ orthorhombic unit cell, ${\tilde{\epsilon }}^b\left(\nu \right)$ and ${\tilde{\epsilon }}^c\left(\nu \right)$, is highly anisotropic over the spectral range covered in the experiment. The difference between ${\tilde{\epsilon }}^b\left(\nu \right)$ and ${\tilde{\epsilon }}^c\left(\nu \right)$ increases with decreasing temperature, and the gradual evolution observed in the paramagnetic state is strongly enhanced by the onset of $A$-type antiferromagnetic long-range order at $T_N=139.6\text{K}$. In this study we focus on the analysis of excitations observed at high energy $\left(\sim 4–5\text{eV}\right)$ and show that the observed temperature changes of their spectral weight are opposite to those found for the lowest-energy gap excitation at $\sim 2\text{eV}$. We used a classical dispersion analysis to quantitatively determine the temperature-dependent optical spectral-weights shifts between low- and high-energy optical bands. Based on the observation of a pronounced spectral-weight transfer between both features upon magnetic ordering, they are assigned to high-spin and low-spin intersite $d^4{d}^4\rightleftharpoons d^3{d}^5$ transitions by Mn electrons. The anisotropy of the lowest-energy optical band and the spectral-weight shifts induced by antiferromagnetic spin correlations are quantitatively described by an effective spin-orbital superexchange model. An analysis of the multiplet structure of the intersite transitions by $\text{Mn}{e}_g$ electrons allowed us to estimate the effective intra-atomic Coulomb interaction, the Hund exchange coupling, and the Jahn-Teller splitting energy between $e_g$ orbitals in ${\text{LaMnO}}_3$, as well as to extract experimental information concerning the type of orbital order in ${\text{LaMnO}}_3$. This study identifies the lowest-energy optical transition at $\sim 2\text{eV}$ as an intersite $d\text{−}d$ transition whose energy is substantially reduced compared to that obtained from the bare intra-atomic Coulomb interaction.

MnSi is a cubic compound with small magnetic anisotropy, which stabilizes a helimagnetic spin spiral that reduces to a ferromagnetic and antiferromagnetic state in the long- and short-wavelength limits, respectively. We report a comprehensive inelastic neutron-scattering study of the collective magnetic excitations in the helimagnetic state of MnSi. In our study we observe a rich variety of seemingly anomalous excitation spectra, as measured in well over 20 different locations in reciprocal space. Using a model based on only three parameters, namely, the measured pitch of the helix, the measured ferromagnetic spin wave stiffness and the amplitude of the signal, as the only free variable, we can simultaneously account for all of the measured spectra in excellent quantitative agreement with experiment. Our study identifies the formation of intense, strongly coupled bands of helimagnons as a universal characteristic of systems with weak chiral interactions.

We present results of inelastic neutron scattering experiments performed for the compound $\left[{\text{Mo}}_{12}{\text{O}}_{28}{\left({\mu }_2\text{-OH}\right)}_9{\left({\mu }_3\text{-OH}\right)}_3{\left\{\text{Ni}{\left({\text{H}}_2\text{O}\right)}_3\right\}}_4\right]\cdot 13{\text{H}}_2\text{O}$, which is a molecular magnet with antiferromagnetically coupled ${\text{Ni}}^{2+}$ ions forming nearly ideal tetrahedra in a diamagnetic molybdate matrix. The neutron spectroscopic data are analyzed together with high-field magnetization data (taken from the literature), which exhibit four steps at non-equidistant field intervals. The experimental data can be excellently described by antiferromagnetic Heisenberg-type exchange interactions as well as an axial single-ion anisotropy within a distorted tetrahedron of ${\text{Ni}}^{2+}$ ions characterized by x-ray single-crystal diffraction. Our analysis contrasts to recently proposed models, which are based on the existence of extremely large biquadratic (and three-ion) exchange interactions and/or on a strong field dependence of the Heisenberg coupling parameters.

Since the Lorentz force is perpendicular to the magnetic field, it should not affect the motion of a charge along the field. This argument seems to imply absence of longitudinal magnetoresistance (LMR) which is, however, observed in many materials and reproduced by standard semiclassical transport theory applied to particular metals. We derive a necessary and sufficient condition on the shape of the Fermi surface for nonzero LMR. Although an anisotropic spectrum is a prerequisite for LMR, not all types of anisotropy can give rise to the effect: a spectrum should not be separable in any sense. More precisely, the combination $k_{\rho }{v}_{\varphi }/v_{\rho }$, where $k_{\rho }$ is the radial component of the momentum in a cylindrical system with the $z$ axis along the magnetic field and $v_{\rho }\left(v_{\varphi }\right)$ is the radial (tangential) component of the velocity, should depend on the momentum along the field. For some lattice types, this condition is satisfied already at the level of nearest-neighbor hopping; for others, the required non-separabality occurs only if next-to-nearest-neighbor hopping is taken into account.

Utilizing three-terminal tunnel emission of ballistic electrons and holes in a planar tunnel transistor with a Mott-barrier collector, we have developed a method to self-consistently determine the energy gap of a semiconductor and band discontinuities at a semiconductor heterojunction without using a priori material parameters. Measurements are performed on lattice-matched $\text{GaAs}/{\text{Al}}_x{\text{Ga}}_{1-x}\text{As}$ (100) single-barrier double heterostructures with ${\text{Al}}_x{\text{Ga}}_{1-x}\text{As}$ as the model ternary III–V compounds. Electronic band gaps of the AlGaAs alloys and band offsets at the GaAs/AlGaAs (100) interfaces are measured with a resolution of several meV at 4.2 K. The direct-gap $\Gamma$ band offset ratio for the GaAs/AlGaAs (100) interface is found to be 59:41 $\left(\pm 3\mathrm{\%}\right)$. Reexamination of our previous experiment [W. Yi et al., Appl. Phys. Lett. 95, 112102 (2009)] revealed that, in the indirect-gap regime, ballistic electrons from direct tunnel emissions probe the X valley in the conduction band, while those from Auger-like scattering processes in the metal base film probe the higher-lying L valley. Such selective electron collection may be explained by their different momentum distributions and parallel momentum conservation at the quasiepitaxial Al/GaAs (100) interface. We argue that the present method is in principle applicable to arbitrary type-I semiconductor heterostructures.