Physical Review B

An experimental study of the statistical distribution of acoustic emission avalanches in martensitic transitions is presented. The critical exponents characterizing the power-law distributions of avalanches are measured as a function of cycling under soft-driving (stress-driving) and hard-driving (s...

We present synchrotron x-ray measurements in a diamond anvil cell of the molecular structure factor of H₂O and D₂O fluids up to 4.5 GPa and 500 K. We observe large changes in the structure factor and a dramatic increase in the oxygen coordination number over a 2 GPa pressure range. A P-T diagr...

The physics of heat conduction in layered, anisotropic crystals is probed by measurements of the cross-plane elastic constant C₃₃ and thermal conductivity Λ of muscovite mica as a function of hydrostatic pressure. Picosecond interferometry and time-domain thermoreflectance provide high-precisio...

We present a thermal transport phenomenon, a unidirectional selection-rule blockade, and show how it produces unprecedented rectification of bosonic heat flow through molecular or mesoscopic quantum systems. Rectification arises from the quantization of energy levels of the conduction element and se...

The magnetic phase diagram of the Y -type hexaferrite Ba₂Mg₂Fe₁₂O₂₂ has been studied using single-crystal neutron diffraction. The result indicates successive phase transitions where the magnetic modulation wave number changed discontinuously when a magnetic field is applied and the temperature i...

Soft x-ray magnetic dichroism, magnetization, and magnetotransport measurements demonstrate that the competition between different magnetic interactions (exchange coupling, electronic reconstruction, and long-range interactions) in La_0.7Sr_0.3FeO₃(LSFO)/La_0.7Sr_0.3MnO₃(LSMO) perovskite oxide superla...

We study finite temperature properties of a generic spin-orbital model relevant to transition metal compounds, having coupled quantum Heisenberg-spin and Ising-orbital degrees of freedom. The model system undergoes a phase transition, consistent with that of a two-dimensional Ising model, to an orbi...

Transition-metal ferromagnetic films with perpendicular magnetic anisotropy (PMA) have ferromagnetic resonance (FMR) linewidths that are one order of magnitude larger than soft magnetic materials, such as pure iron (Fe) and Permalloy (NiFe) thin films. A broadband FMR setup has been used to investig...

The optical response of spiral magnets is studied theoretically, with special attention to its electromagnon features. We show that these features trace back to the resonant magnetoelectric response resulting from the spiral ordering (irrespective of any concomitant ferroelectricity). This response,...

We study two-dimensional Heisenberg antiferromagnets with additional multispin interactions which can drive the system into a valence-bond-solid state. For standard SU(2) spins, we consider both four- and six-spin interactions. We find continuous quantum phase transitions with the same critical expo...

The magnetic properties of BiCu₂PO₆ have been analyzed by means of magnetic-susceptibility and inelastic neutron-scattering measurements on powder samples by evaluating the spin-exchange interactions on the basis of density-functional calculations and by simulating the inelastic neutron scattering...

The longitudinal resistance R_xx of the SrTiO₃/LaAlO₃ interface with magnetic fields applied perpendicular to the interface has an antisymmetric term [namely, R_xx(H)≠R_xx(−H) ] which increases with decreasing temperature and increasing field. We argue that the origin of this phenomenon is a n...

We report time and frequency domain studies of spin-torque-driven vortex self-oscillations at zero magnetic field. We observe two types of abrupt fluctuations in the frequency and amplitude, with very long random mean lifetimes ( ∼10² to ∼10⁴ oscillation cycles). First, we observe fluctuation...

Recent measurements on extremely underdoped YBa₂Cu₃O_6+y [Phys. Rev. Lett. 99, 237003 (2007)] have allowed the critical temperature T_c , superfluid density ρ_s0≡ρ_s(T⪡T_c) , and dc conductivity σ_dc(T≳T_c) to be determined for a series of electronic dopings for T_c≃3–17 K . The genera...

We report on the investigation of the quasiparticle local density of states and superconducting gap in the iron chalcogenide superconductor Fe_1+δSexTe_x(T_c∼14 K) . The surface of a cleaved crystal revealed an atomic square lattice, superimposed on the inhomogeneous background, with a lattic...

We identify and investigate the parameter regime of small charge solitons in one-dimensional arrays of Josephson junctions. We obtain the dispersion relation of the soliton and show that it unexpectedly flattens in the outer region of the Brillouin zone. We demonstrate Lorentz contraction of the sol...

Using scanning tunneling microscopy and Ginzburg-Landau simulations, we explore vortex configurations in magnetically coupled NbSe₂/permalloy superconductor/ferromagnet bilayer. The permalloy film with stripe domain structure induces periodic local magnetic induction in the superconductor, creatin...

We use the functional renormalization group to analyze the phase diagram of a four-band model for the iron pnictides subject to band interactions with certain A_1g momentum dependence. We determine the parameter regimes where an extended s -wave pairing instability with and without nodes emerges. ...

We demonstrate in this Rapid Communication that in an assembly of stacked metallic U-shaped resonators, pure magnetic and electric responses are, respectively, realized, and the magnetic and electric responses can be switched at the same frequency by changing the polarization of incident light for ...

Metamaterials have paved the way to unprecedented control of the electromagnetic field. The conjunction with space coordinate transformation has led to a “relativity inspired” approach for the control of light propagation. “Invisibility cloak” is the most fascinating proposed device. However...

We present an interferometric scheme producing orbital entanglement in a quantum Hall system upon electron-hole pair emission via tunneling. The proposed setup is an electronic version of the optical interferometer proposed by Cabello [Phys. Rev. Lett. 102, 040401 (2009)] and is feasible with the p...

We have studied the zero-temperature statistics of charge transfer between the two edges of quantum Hall liquids with filling factors ν_0,1=1/(2m_0,1+1) forming Mach-Zehnder interferometer. The known Bethe ansatz solution for symmetric interferometer is used to obtain the cumulant-generating functi...

For a single semiconductor quantum dot embedded in a microcavity, we theoretically and experimentally investigate phonon-assisted transitions between excitons and the cavity mode. Within the framework of the independent boson model we find that such transitions can be very efficient, even for relati...

Experimental evidence of strong coupling between excitons confined in a quantum well and the photonic modes of a two-dimensional dielectric lattice is reported. Both resonant scattering and photoluminescence spectra at low temperature show the anticrossing of the polariton branches, fingerprint of s...

Spin injection efficiency is shown to strongly depend on the interfacial structure between Fe contacts and Al_xGa_1−xAs in spin-based light emitting diodes. Both the magnitude and sign of the injected carriers are dependent on the atomic structure of the contacts and can be controlled through chan...

Traditionally the charge ratchet effect is considered as a consequence of either the spatial symmetry breaking engineered by asymmetric periodic potentials, or time asymmetry of the driving fields. Here we demonstrate that electrically and magnetically driven quantum dissipative systems with spin-or...

In a GaMnAs/AlGaAs resonant tunneling diode (RTD) structure, we observe that both the magnitude and polarity of magnetoresistance are bias dependent when tunneling from a three-dimensional GaMnAs layer through a two-dimensional GaMnAs quantum well. This magnetoresistance behavior results from a shif...

Critical factors that control the vacuum work function of the TiC_xN_1−x ternary system surfaces were determined using detailed density functional theory calculations. Surface chemistry (i.e., orientation, stoichiometry, and defect density) was found to play the most important role in determining ...

Hyperfine interaction (HFI) in carbon nanotube and graphene quantum dots is due to the presence of ¹³C atoms. We theoretically show that in these structures the short-range nature of the HFI gives rise to a coupling between the valley degree of freedom of the electron and the nuclear spin, in addi...

We report an excitation-energy dependence of ultrafast changes of exciton absorptions in isolated semiconducting single-walled carbon nanotubes. For a photoexcitation far below the exciton transition, a blueshift of exciton absorptions originating from the optical Stark effect due to exciton-photon ...

The physics of heat conduction in layered, anisotropic crystals is probed by measurements of the cross-plane elastic constant C₃₃ and thermal conductivity Λ of muscovite mica as a function of hydrostatic pressure. Picosecond interferometry and time-domain thermoreflectance provide high-precisio...

We demonstrate in this Rapid Communication that in an assembly of stacked metallic U-shaped resonators, pure magnetic and electric responses are, respectively, realized, and the magnetic and electric responses can be switched at the same frequency by changing the polarization of incident light for ...

We present a numerical method for the evaluation of dynamical response functions at finite temperatures in one-dimensional strongly correlated systems. The approach is based on the density-matrix renormalization group method, combined with the finite-temperature Lanczos diagonalization. The feasibil...

We present an interferometric scheme producing orbital entanglement in a quantum Hall system upon electron-hole pair emission via tunneling. The proposed setup is an electronic version of the optical interferometer proposed by Cabello [Phys. Rev. Lett. 102, 040401 (2009)] and is feasible with the p...

We have studied the zero-temperature statistics of charge transfer between the two edges of quantum Hall liquids with filling factors ν_0,1=1/(2m_0,1+1) forming Mach-Zehnder interferometer. The known Bethe ansatz solution for symmetric interferometer is used to obtain the cumulant-generating functi...

Forming a chemically stable low-resistance back contact for CdTe thin-film solar cells is critically important to the cell performance. This paper reports theoretical study of the effects of the back-contact material, Sb₂Te₃ , on the performance of the CdTe solar cells. First-principles calculation...

The terahertz (THz) photoresponse of a two-dimensional electron gas in the quantum Hall regime is investigated. We use a sample structure which is topologically equivalent to a Corbino geometry combined with a cross-gate technique. This quasi-Corbino geometry allows us to directly investigate the TH...

The commonly accepted Stranski-Krastanow model, according to which island formation occurs on top of a wetting layer (WL) of a certain thickness, predicts for the morphological evolution an increasing island aspect ratio with volume. We report on an apparent violation of this thermodynamic understan...

Calculating highly accurate thermochemical properties of condensed matter via wave-function-based approaches (such as, e.g., Hartree-Fock or hybrid functionals) has recently attracted much interest. We here present two strategies providing accurate Hartree-Fock energies for solid LiH in a large Gaus...

In two-dimensional quantum site-percolation square lattice models, the von Neumann entropy is extensively studied numerically. At a certain eigenenergy, the localization-delocalization transition is reflected by the derivative of von Neumann entropy which is maximal at the quantum percolation thresh...

The quantum phase transition (QPT) of the one-dimensional (1D) quantum compass model in a transverse magnetic field is studied in this paper. An exact solution is obtained by using an extended Jordan and Wigner transformation to the pseudospin operators. The fidelity susceptibility, the concurrence,...

Neutron scattering experiments of Cr/V(001) superlattices are discussed, which show that the incommensurate spin-density-wave (SDW) in thick Cr layers becomes suppressed when the V spacer layers are loaded with hydrogen. The hydrogen loading triggers a transition from the incommensurate SDW state to...

Single magnetic nanodots, exchange coupled to an antiferromagnetic (AF) matrix, can produce large exchange bias, while superparamagnetic behavior of the nanodots is suppressed. The exchange bias originates from the formation of a (quasi)spherical domain wall inside the AF matrix when the particle mo...

The magnetic phase diagram of the Y -type hexaferrite Ba₂Mg₂Fe₁₂O₂₂ has been studied using single-crystal neutron diffraction. The result indicates successive phase transitions where the magnetic modulation wave number changed discontinuously when a magnetic field is applied and the temperature i...

We study the Shannon entropy of the probability distribution resulting from the ground-state wave function of a one-dimensional quantum model. This entropy is related to the entanglement entropy of a Rokhsar-Kivelson-type wave function built from the corresponding two-dimensional classical model. In...

We report ambient-pressure magnetization, heat capacity, and thermal-expansion measurements of the ferromagnetic superconductor UGe₂ in high magnetic fields. An analysis of the magnetic heat capacity derived from both magnetization and specific-heat data shows that UGe₂ is well described in the ...

The effects of hydrogenation on Mn-doped GaN are studied with electron-paramagnetic resonance (EPR), local vibrational mode (LVM) spectroscopy, and density-functional theory (DFT) calculations. With EPR, we find two distinct Mn complexes which, in particular, differ in the size and orientation of th...

In order to provide insight into the magnetic properties of semiconductors with ferromagnetic inclusions, we present a thorough quantitative analysis of (Zn,Co)O films with high Co concentration, which give rise to high saturation magnetization values (∼110 kA m⁻¹) at room temperature. Fro...

We present a theory of the influence of band renormalization and excitonic electron-electron interaction effects on the optical conductivity σ(ω) of doped bilayer graphene. Using the Keldysh formalism, we derive a kinetic equation from which we extract numerical and approximate analytic results ...

Critical factors that control the vacuum work function of the TiC_{span style="font-style:italic">x}N_1−x ternary system surfaces were determined using detailed density functional theory calculations. Surface chemistry (i.e., orientation, stoichiometry, and defect density) was found to play the most important role in determining ...

Angle-resolved photoemission spectroscopy and first-principles calculations were employed to analyze unusual features in the electronic structure of ultrathin Ag films grown on Ge(111). The Ag sp -derived quantum well states exhibit hexagonal-like constant energy contours with different in-plane or...

The interaction of energetic (up to 200 eV/atom) size-selected Co_n clusters with HOPG is studied both experimentally and theoretically. Etching of the radiation damaged areas introduced by cluster impacts provides a measure of the depth to which the collision cascades are developed and allows a co...

The electronic and structural properties of different charge-imbalanced perovskite oxide NaNbO₃/SrTiO₃ superlattices are investigated with density-functional theory (local density approximation and local spin density approximation +U ) methods. Metallic or insulating behavior of such a superlattic...

Structure formation of the highly polar molecule cytosine on the (111) cleavage plane of calcium fluoride is investigated in ultrahigh vacuum using noncontact atomic force microscopy at room temperature. Molecules form well-defined trimer structures, covering the surface as homogeneously distributed...

We present an efficient generalization of the k-space interpolation scheme for electronic structure presented by E. L. Shirley, Phys. Rev. B 54, 16464 (1996). The method permits the construction of a compact k-dependent Hamiltonian using a numerically optimal basis derived from a coarse-grained set of effective single-particle electronic structure calculations (based on density functional theory in this work). We provide some generalizations of the initial approach which reduce the number of required initial electronic structure calculations, enabling accurate interpolation over the entire Brillouin zone based on calculations at the zone-center only for large systems. We also generalize the representation of non-local Hamiltonians, leading to a more efficient implementation which permits the use of both norm-conserving and ultrasoft pseudopotentials in the input calculations. Numerically interpolated electronic eigenvalues with accuracy that is within 0.01 eV can be produced at very little computational cost. Furthermore, accurate eigenfunctions - expressed in the optimal basis - provide easy access to useful matrix elements for simulating spectroscopy and we provide details for computing optical transition amplitudes. The approach is also applicable to other theoretical frameworks such as the Dyson equation for quasiparticle excitations or the Bethe-Salpeter equation for optical responses.

Based on the Hubbard model of strongly correlated systems, a reduction in the bandwidth of the electrons can yield a substantial change in the properties of the material. One method to modify the bandwidth is geometrically confined doping, i.e. the introduction of a (thin) dopant layer in a material. In this paper, the magnetic properties of LaVO3/SrVO3 superlattices, in which the geometrically confined doping is produced by a one monolayer thick SrVO3 film, are presented. In contrast to the solid solution La1-xSrxVO3, such superlattices have a finite magnetization up to room temperature. Furthermore, the total magnetization of the superlattice depends on the thickness of the LaVO3 layer, indicating an indirect coupling of the magnetization that emerges at adjacent dopant layers. Our results show that geometrically confined doping, like it can be achieved in superlattices, reveals a way to induce otherwise unaccessible phases, possibly even with a large temperature scale.

First-principles molecular dynamics simulations based on a new exchange-correlation functional show that self-diffusion in the refractory metal molybdenum is associated with strongly temperature dependent activation energies for vacancy formation and migration. While static calculations of self-diffusion rates based on transition-state theory deviate systematically from experiments, with up to two orders of magnitude, the current results are accurate to within a mean deviation of 4 over the experimental range in temperature.

One of the unusual features of the Mn+1AXn phases (where M is a transition metal, A is a group A element, X is carbon or nitrogen and n = 1,2,3...) is that for a given M-A-X system, only certain values of n are found to occur and there is no systematic behaviour between the different systems. Density functional theory (DFT) was used to verify the stability of the different phases by comparing their total energy to that the appropriate competing phases. Five systems (Ti-Al-C, Ti-Si-C, Ti-Al-N, Ti-Si-N, Cr-Al-C) were studied for n=1-4. Complete agreement with observed occurrences of these phases was found. Very small energy differences suggest that it may be possible to fabricate Ti2SiC, Ti2SiN and Ti3AlN2 as metastable phases. None of the M5AX4 phases were predicted to occur and in all cases the \alpha - phases were found to be more energetically favourable than the \beta -phases.

We have applied density functional theory based ab initio molecular dynamics (AIMD) to examine Li4BN3H10 at temperatures both above and below the experimental melting point. We examine the structure of the liquid, diffusivity, vibrational spectra and compare to both experimental data and analogous properties from solid state calculations. We find the following: 1) The liquid state, like the solid state, is primarily a mixture of Li+, BH4- and NH2- with ionic interactions between the BH4- and NH2- anions and the Li+ cations. 2) We observe the reaction of two amide anions exchanging hydrogen to form ammonia and an imide anion: 2NH2- NH3 + NH2- 3) The liquid demonstrates wide bond angle distributions in the BH4- and NH2- units and thus these anionic units are not simply rigid complexes. 4) The Li+ sub-lattice disorders before the anionic sub-lattices and the liquid exhibits very fast Li+ diffusion. We calculate the activation energy and pre-exponential factor for Li+ diffusivity in the liquid to be ~ 20 kJ/mol and 15x10-4 cm2/s, respectively. 5) Finally, we find that the liquid contains the same generic types of vibrational modes as the solid, however the lower frequency anionic vibration and rotation modes become more prominent with increasing temperature.

In the present article we investigate the influence of the contact region on the distribution of the chemical potential in integer quantum Hall samples, as well as the longitudinal and Hall resistance as a function of the magnetic field. First we use a standard quantum Hall sample geometry and analyse the influence of the length of the leads where current enters/leaves the sample and the ratio of the contact width to the width of these leads. Furthermore we investigate potential barriers in the current injecting leads and the measurement arms in order to simulate non-ideal contacts. Second we simulate nonlocal quantum Hall samples with applied gating voltage at the metallic contacts. For such samples it has been found experimentally that both the longitudinal and Hall resistance as a function of the magnetic field can change significantly. Using the nonequilibrium network model we are able to reproduce most qualitative features of the experiments.

We demonstrate that the presence of charges around a semiconductor quantum dot (QD) strongly affects its optical properties and produces non-resonant coupling to the modes of a microcavity. We show that, besides (multi)exciton lines, a QD generates a spectrally broad emission which efficiently couples to cavity modes. Its temporal dynamics shows that it is related to the Coulomb interaction between the QD (multi)excitons and carriers in the adjacent wetting layer. This mechanism is suppressed by the application of an electric field, making the QD closer to an ideal two-level system.

A novel approach to calculation of reciprocal space maps of x-ray diffraction from partially relaxed multilayered epitaxial structures is reported. The theory takes into account the additional harmonics of wave field caused by the difference in the lateral projections of reciprocal vectors in the sample layers. The reciprocal space maps shown can be simulated on the basis of the dynamical diffraction theory and matrix method for boundary conditions, which are applicable to arbitrary experimental geometry. The developed theory explains the experimental results from several typical epitaxial structures in partially relaxed state.

A unified approach to decoherence and relaxation of energy resolved single electron excitations in Integer Quantum Hall edge channels is presented. Within the bosonization framework, relaxation and decoherence induced by interactions and capacitive coupling to an external linear circuit are computed. An explicit connexion with high frequency transport properties of a two terminal device formed by the edge channel on one side and the linear circuit on the other side is established.

The spin-split states subject to Rashba spin-orbit coupling in two-dimensional systems has long been accepted as pointing inplane and perpendicular to the corresponding wave vectors. This is in general true for free electron model, but exceptions do exist elsewhere. Within the tight-binding model, we unveil the unusual upstanding behavior of those Rashba spins around [`K] and [`K] points in honeycomb lattices. Our calculation (i) explains the recent experiment of the Tl/Si(111)-(1times;1) surface alloy [Phys. Rev. Lett. 102, 096805 (2009)], where abrupt upstanding spin states near [`K] are observed, and (ii) predicts an electrically reversible out-of-plane surface spin polarization.

We present a theory of valence holes as Luttinger spinor based qubits in p-doped self-assembled quantum dots within the 4-band k#183;p formalism. The two qubit levels are identified with the two chiralities of the doubly degenerate ground state. We show that single qubit operations can be implemented with static magnetic field applied along the z axis (growth direction) for [^(s)]z operation and with magnetic field in the quantum dot plane, x direction, for [^(s)]x operation. The coupling of two dots and hence the double qubit operations are shown to be sensitive to the orientation of the two quantum dots. For vertical qubit arrays, there exists an optimal qubit separation suitable for the voltage control of qubit-qubit interactions.

We investigate theoretically two-photon processes in a microcavity containing one quantum dot in the strong coupling regime. The cavity mode can be tuned to resonantly drive the two-photon transition between the ground and the biexciton states, while the exciton states are far-off resonance due to the biexciton binding energy. We study the steady state of the quantum dot and cavity field in presence of a continuous incoherent pumping. We identify the regime where the system acts as two-photon emitter and discuss the feasibility and performance of realistic single quantum dot devices for two-photon lasing.

We develop simple density-matrix models to describe the role of coherence in resonant-tunneling (RT) transport of quantum-cascade lasers (QCLs). Specifically, we investigate the effects of coherent coupling between the lasing levels with other levels on the transport properties and gain spectra. In the first part of the paper, we use a three-level density-matrix model to obtain useful analytical expressions for current transport through the injector barrier in a QCL. An expression for the slope discontinuity in the current-voltage characteristics at the lasing threshold is derived. This value is shown to be a direct measure of the population inversion at threshold, and contradicts the previously held belief of it being indicative of ratio of the laser level lifetimes. In the second part of the paper, we use density matrices to compute the gain spectrum for a resonant-phonon terahertz QCL design. The large anticrossing of the doublet of lower radiative levels is reflected in a broad gain linewidth due to a coherent RT assisted depopulation process. At certain bias conditions, the gain spectrum exhibits double peaks which is supported by experimental observations.

We have used X-ray magnetic circular dichroism (XMCD) and ab-initio electronic structure calculation techniques to investigate the magnetic properties of high quality epitaxial (110) and (100) CrO2 thin films. A relatively larger XMCD was observed on the Cr L2,3 edge of (110) oriented CrO2 films compared to (100) oriented CrO2 films at room temperature. Analysis of our data with conventional sum rules for 3d elements show a nearly 50% higher spin moment on (110) films compared to (100) orientation, consistent with bulk magnetometry measurements. The orbital moment is found to be similar for both orientations. Robust magnetism is attributed to increased collinearity of Cr spins in strain-free (110) films as compared to strained (100) films. Zero temperature density functional calculations show opposing trends in nearest neighbor (NN) and next nearest neighbor (NNN) exchange interactions between relaxed and strained CrO2.

With the implementation of a relativistic Korringa-Kohn-Rostoker Green's function and band structure method, we analyze the spin expectation value of the electron states on the Fermi surface of non-magnetic as well as magnetic metals. It is shown that for relatively light elements like Cu the spin states are well defined. A separation of all electron states to upnbsp;and downnbsp;spin polarized states can be done even in the case of quite heavy, but monovalent elements like Au. In contrast, for heavy polyvalent metals like Pt, the expectation value of the spin operator can be close to zero in large regions of the Fermi surface. In this case the non-relativistic language of well defined spin-upnbsp;and spin-downnbsp;states is not valid anymore. For magnetic materials, the relativistic Fermi surfaces change their topology with respect to the non-relativistic majority and minority sheets because of spin-orbit driven avoided crossings of the bands. As a result, regions with vanishing spin polarization appear.

We report measurements of the thermoelectric power (TEP) for a series of Pb1-xTlxTe crystals with x = 0.0 to 1.3%. Although the TEP is very large for x = 0.0, using a single band analysis based on older work for dilute magnetic alloys we do find evidence for a Kondo contribution of 11 - 18 mV/K. This analysis suggests that TK is @ 50 - 70 K, a factor 10 higher than previously thought.

FeVO4 has been studied by heat capacity, magnetic susceptibility, electric polarization and single crystal neutron diffraction experiments. The triclinic crystal structure is made of S-shaped clusters of six Fe3+ ions, linked by VO43- groups. Two long-range magnetic ordering transitions occur at TN1=22K and TN2=15K. Both magnetic structures are incommensurate and below TN2, nbsp;becomes weakly ferroelectric coincidentally with the loss of the collinearity of the magnetic structure in a very similar fashion than in the classical TbMnO3 multiferroic material. However we argue that the symmetry considerations and the mechanisms invoked to explain these properties in TbMnO3 do not straightforwardly apply to . Firstly, the magnetic structures, even the collinear structure, are all acentric so that ferroelectricity in nbsp;is not correlated to the fact magnetic ordering is breaking inversion symmetry. Regarding the mechanism, nbsp;has quenched orbital moments that questions the exact role of the spin orbit interactions.

We performed ab initio density functional theory calculations to investigate the process of hydrogenation of a bilayer of graphene. 50% hydrogen coverage is possible in case that the hydrogen atoms are allowed to adsorb on both sides of the bilayer. In this case interlayer chemical bonding occurs which stabilizes the structure. At maximum coverage, a bilayer of graphane is formed which has properties that are similar to those of a single layer of graphane.

Based on the tight-binding model, we investigate band structures of graphene nanohole (GNH) superlattices as a function of NH size and density. One common origin of band gaps for GNH superlattices with NHs of either armchair or zigzag edges is the quantum confinement effect due to the periodic potential introduced by the NHs, which turns the semi-metallic sheet into a direct gap semiconductor. Additional band gaps also open for GNH superlattices with NHs of zigzag edges in a ferromagnetic ground state, arising from the staggered sublattice potential on the zigzag edges due to edge magnetization. Our calculations reveal a generic scaling relation that both types of band gaps increase linearly with the product of NH size and density.

In this article we explain the origin of the ring structure that sometimes forms a sharp border surrounding field emission electron microscopy patterns from carbon nanotubes (CNTs) at high current. The rings turn out to be due to the self-focusing of thermal field electrons emitted from the near-cap shank of a CNT that reaches high temperature through Joule heating. To prove this we have simulated the electrostatic fields, the dependence of electron emission on field, temperature and position on the CNT, and the emitted electron trajectories for different CNT/electrode geometries. The rings are formed when the manifold of emitted electron trajectories folds over onto itself due to self-focusing by the back support plane. Sufficient thermal electron currents can only be emitted because CNTs can maintain a continuous high temperature self-heating state, a state for now unique to carbon nanotubes. We do not need to evoke space charge effects and a program based on Green's functions is to calculate the field emission current for all values of field and temperature. The original observation of this phenomenon goes back to at least the pulsed high current experiments on W emitters in the 1950's1,2 and thus this work resolves a long-standing riddle in field emission.

We describe two different nuclear quadrupole resonance (NQR) based techniques, designed to measure the local asymmetry of the internal electric field gradient h, and the tilt angle a of the main NQR principal axis from the crystallographic axis. These techniques use the dependence of the NQR signal on the duration of the radio frequency (RF) pulse, and on the direction of the RF field H1 with respect to the crystal axis. The techniques are applied to oriented powder of YBa2Cu 3Oy(YBCO) fully enriched with 63Cu. Measurements were performed at different frequencies, corresponding to different in-plane copper sites with respect to the dopant. Combining the results from both techniques, we conclude that oxygen deficiency in the chain layer lead to a rotation of the NQR main principal axis at the nearby Cu on the CuO2 planes by a @ 205. This occurs with no change to h. The axis rotation associated with oxygen deficiency means that there must be electric field inhomogeneities in the CuO2 planes only in the vicinity of the missing oxygen.

The high-energy kink or the waterfall effect seen in the photoemission spectra of cuprates is suggestive of the coupling of quasiparticles to a high energy bosonic mode with implications for the mechanism of superconductivity. Recent experiments however indicate that this effect may be an artifact produced entirely by matrix element effects, i.e. by the way the photoemitted electron couples to incident photons in the emission process. In order to address this issue directly, we have carried out realistic computations of the photo-intensity in Bi2Sr2CaCu2O8 (Bi2212) where the effects of the matrix element are included together with those of the corrections to the self-energy resulting from electronic excitations. Our results demonstrate that while the photoemission matrix element plays an important role in shaping the spectra, the waterfall effect is a clear signature of the presence of strong coupling of quasiparticles to electronic excitations.

We establish the phase diagram of the disordered three-dimensional Bose-Hubbard model at unity filling which has been controversial for many years. The theorem of inclusions, proven in Ref.nbsp;, states that the Bose glass phase always intervenes between the Mott insulating and superfluid phases. Here, we note that assumptions on which the theorem is based exclude phase transitions between gapped (Mott insulator) and gapless phases (Bose glass). The apparent paradox is resolved through a unique mechanism: such transitions have to be of the Griffiths type when the vanishing of the gap at the critical point is due to a zero concentration of rare regions where extreme fluctuations of disorder mimic a regular gapless system. An exactly solvable random transverse field Ising model in one dimension is used to illustrate the point. A highly non-trivial overall shape of the phase diagram is revealed with the worm algorithm. The phase diagram features a long superfluid finger at strong disorder and on-site interaction. Moreover, bosonic superfluidity is extremely robust against disorder in a broad range of interaction parameters; it persists in random potentials nearly 50 (!) times larger than the particle half-bandwidth. Finally, we comment on the feasibility of obtaining this phase diagram in cold-atom experiments, which work with trapped systems at finite temperature.

The computer simulations of the process of single pulse readout from a two state quantum system, describing the phase qubit, is performed in the frame of one-dimensional Schroedinger equation. It has been demonstrated that the readout error can be minimized by choosing the optimal pulse duration and the depth of a potential well, leading to the fidelity of 0.94 for 2ns sinusoidal pulses.

The mixture of the scalar bosonic and the spinless or polarized fermionic cold atoms in the one-dimensional optical lattice is studied. The system is modeled by the Bose-Fermi-Hubbard Hamiltonian, which shows different behavior from that of the Bose-Hubbard or the Fermi-Hubbard models. Because the SU(1|1)-supersymmetric Bethe ansatz solution gives an excellent approximation to this kind of mixed cold atomic systems, the ground state properties of the system such as the densities of state in the momentum space are obtained based on the Bethe ansatz. If the number of bosons is equal to that of fermions and the filling factor is one, it is found that there exists a critical on-site interaction Uc. If U < Uc, the ground state of the system is in the superfluid phase, while if U > Uc, the ground state is in the insulating phase. The superfluid-insulator transition occurs at Uc. From the analysis of the superfluid density, the value of the critical point is determined as Uc=2.79256, which is larger than the Uc=0 for the Fermi-Hubbard model and smaller than the Uc=3.28 for the Bose-Hubbard model. The elementary excitations and effective Hamiltonian in the strong coupling limit are also discussed.

We study the Shannon entropy of the probability distribution resulting from the ground-state wave function of a one-dimensional quantum model. This entropy is related to the entanglement entropy of a Rokhsar-Kivelson-type wave function built from the corresponding two-dimensional classical model. In...

The physics of heat conduction in layered, anisotropic crystals is probed by measurements of the cross-plane elastic constant C₃₃ and thermal conductivity Λ of muscovite mica as a function of hydrostatic pressure. Picosecond interferometry and time-domain thermoreflectance provide high-precisio...

We classify all possible 36 gap-opening instabilities in graphenelike structures in two dimensions, i.e., masses of Dirac Hamiltonian when the spin, valley, and superconducting channels are included. These 36 order parameters break up into 56 possible quintuplets of masses that add in quadrature and...

The symmetry ordering of the valence bands in ZnO is derived from high-resolution magneto-optical measurements of bound excitons. We report on the experimental observation of a hole state related fine splitting for bound excitons in the Voigt configuration. This splitting is related to a nonzero Lan...

An angle-swept high-frequency electron paramagnetic resonance (HFEPR) technique is described that facilitates efficient in situ alignment of single-crystal samples containing low-symmetry magnetic species such as single-molecule magnets (SMMs). This cavity-based technique involves recording HFEPR sp...

We calculate, within a self-consistent Hartree-Fock approximation, the local density of states for different electron crystals in graphene subject to a strong magnetic field. We investigate both the Wigner crystal and bubble crystals with M_e electrons per lattice site. The total density of states ...

This paper presents an efficient algorithm for computing the transition probability in auxiliary field quantum Monte Carlo simulations of strongly correlated electron systems using a Hubbard model. This algorithm is based on a low rank updating of the underlying linear algebra problem, and results i...

We report on the investigation of the quasiparticle local density of states and superconducting gap in the iron chalcogenide superconductor Fe_1+δSe_1−xTe_x(T_c∼14 K) . The surface of a cleaved crystal revealed an atomic square lattice, superimposed on the inhomogeneous background, with a lattic...

We study two-dimensional Heisenberg antiferromagnets with additional multispin interactions which can drive the system into a valence-bond-solid state. For standard SU(2) spins, we consider both four- and six-spin interactions. We find continuous quantum phase transitions with the same critical expo...

In a GaMnAs/AlGaAs resonant tunneling diode (RTD) structure, we observe that both the magnitude and polarity of magnetoresistance are bias dependent when tunneling from a three-dimensional GaMnAs layer through a two-dimensional GaMnAs quantum well. This magnetoresistance behavior results from a shif...