Physical Review B

An anomalous abrupt drop in the electrical conductivity has been observed at the ferroelastic phase transition of a proton-irradiated system of hydrogen-bonded TlH2PO4. As a result of the high-resolution ³¹P NMR chemical-shift measurements, distinct changes in the atomic displacements due to the irradiation were identified in the ferroelastic and paraelastic phases. Besides, ¹H NMR spin-spin relaxation measurements revealed a change due to the irradiation in the proton dynamics at the ferroelastic phase transition, apparently accounting for the much-reduced electrical conductivity in the paraelastic phase of the irradiated system.

Low-temperature (2  KT350  K) heat capacity and room-temperature shear modulus measurements (=1.4  MHz) have been performed on bulk Pd41.25Cu41.25P17.5 in the initial glassy, relaxed glassy, and crystallized states. It has been found that the height of the low-temperature Boson heat capacity peak strongly correlates with the changes in the shear modulus upon high-temperature annealing. It is this behavior that was earlier predicted by the interstitialcy theory, according to which dumbbell interstitialcy defects are responsible for a number of thermodynamic and kinetic properties of crystalline, (supercooled) liquid, and solid glassy states.

Co islands and films are characterized by x-ray magnetic circular dichroism photoemission electron microscopy. The spatial resolution capabilities of the technique together with atomic growth control permit obtaining perfectly flat triangular islands with a given thickness (3 ML), very close to an abrupt spin-reorientation transition. The magnetic domain configurations are found to depend on island size: while small islands can be magnetized in a single-domain state, larger islands show more complex patterns. Furthermore, the magnetization pattern of the larger islands presents a common chirality. By means of dichroic spectromicroscopy at the Co L absorption edges, an experimental estimate of the ratio of the spin and orbital magnetic moment for three monolayer thick films is obtained.

First-principles full-potential linearized augmented plane-wave studies reveal that a surface magnetocrystalline anisotropy (MCA) modification by an external electric field arises from a dipole formation mechanism. The precise calculations demonstrate that the formation of dipoles on Fe(001) surface atoms, which counteract the electric-field-induced charge in the vacuum region, changes the surface states around the Fermi level in the minority-spin d bands, and yields a modification of the surface MCA. These findings greatly advance our understanding of the electric-field-induced MCA modifications in itinerant ferromagnetic surfaces.

In contrast to almost all anisotropic superconductors, the upper critical field of URu2Si2 is larger when the field is oriented along the less conducting direction. We present a study of resistivity and Seebeck coefficient extended down to sub-Kelvin temperature range uncovering a singular case of anisotropy. When the current is injected along the c axis URu2Si2 behaves as a low-density Fermi liquid. When it flows along the a axis, even in presence of a large field, resistivity remains T-linear down to Tc and the Seebeck coefficient undergoes a sign change at very low temperatures. We conclude that the characteristic energy scale is anisotropic and vanishingly small in the basal plane.

Considering the competing spin density wave (SDW) and d-wave superconductivity interactions, we investigate the effects of a nonmagnetic impurity on the vortex-core state of cuprate superconductors within the Bogoliubov–de Gennes formalism. We illustrate that the local SDW order is induced by the impurity on top of the magnetic field induced SDW order. The local density of states of a pinned vortex core exhibit a resonance peak at negative energy, which is drastically different from the double-peak structure observed in an unpinned vortex core. This resonance peak is insensitive to the impurity-scattering strength. Consequently, the impurity resonance peak may be used to identify the nature of vortex-core state of cuprate superconductors.

A combination of atomistic simulation techniques has been employed to predict ordered structures for a series of A4B3O12 -phase compounds, where A is a 3+ cation ranging in size from Sc³⁺ to Ho³⁺ and B is a 4+ cation ranging from Ti⁴⁺ to Zr⁴⁺. Experimentally, a fully ordered cation structure has yet to be resolved for any of these compounds. Monte Carlo energy-minimization calculations using short-range pair potentials identified three low-energy arrangements of A³⁺ and B⁴⁺ cations. The details of these three structures were analyzed with the layer motif method. To quantitatively determine the -phase structure of each composition, the three configurations were reevaluated with density-functional theory. We also used special quasirandom structures to compare the ordered low-energy configurations to cation disorder. For all compositions considered, we find that at least one of the three ordered structures is lower in energy than the disordered structure, suggesting the thermodynamic stability of an ordered phase. Of the three ordered structures identified by this approach, one has not been identified previously in the literature for any composition. In addition, we discuss the stability of -phase compounds with respect to other “ABO4−x” fluorite-derivative compositions and predict the structure of compositions for which none has been reported.

We developed a global structure optimization method, genetic algorithm, for a fast and efficient prediction of grain-boundary structures. Using this method we predicted the most stable structures and a number of low-energy metastable structures for Si[001] symmetric tilted grain boundaries with various tilted angles. We show that most of the grain-boundary structures can be described by the structural unit model with the units being the dislocation cores and perfect-crystal fragments. The energies of the grain-boundary structures obtained from the genetic algorithm optimization are evaluated by tight-binding calculations using the environment-dependent Si tight-binding potential developed previously and found to be in very good agreement with the first-principles calculation results.

We consider a quantum model of a nanomechanical flexing beam resonator interacting with a bath comprising a few damped tunneling two-level systems. In contrast with a resonator interacting bilinearly with an ohmic free oscillator bath (modeling clamping loss, for example), the mechanical resonator damping is amplitude dependent, while the decoherence of quantum superpositions of mechanical position states depends only weakly on their spatial separation.

The dislocation core structure of self-interstitial atom (SIA) clusters in bcc iron and fcc copper is determined using the hybrid ab initio continuum method of Banerjee et al. [Philos. Mag. 87, 4131 (2007)]. To reduce reliance on empirical potentials and to facilitate predictions of the effects of local chemistry and stress on the structure of defects, we present here a hybrid extension of the Peierls-Nabarro continuum model, with lattice resistance to slip determined separately from ab initio calculations. A method is developed to reconstruct atomic arrangements and geometry of SIA clusters from the hybrid model. The results are shown to compare well with molecular-dynamics simulations. In iron, the core structure does not show dependence on the size of the self-interstitial cluster, and is nearly identical to that of a straight edge dislocation. However, the core structure of SIA clusters in Cu is shown to depend strongly on the cluster size. Small SIA clusters are found to have nondissociated compact dislocation cores, with a strong merging of Shockley partial dislocations and a relatively narrow stacking fault (SF) region. The compact nature of the SIA core in copper is attributed to the strong dependence of the self-energy on the cluster size. As the number of atoms in the SIA cluster increases, Shockley partial dislocations separate and the SF region widens, rendering the SIA core structure to that of an edge dislocation. The separation distance between the two partials widens as the cluster size increases, and tends to the value of a straight edge dislocation for cluster sizes above 400 atoms. The local stress is found to have a significant effect on the atomic arrangements within SIA clusters in copper and the width of the stacking faults. An applied external shear can delocalize the core of an SIA cluster in copper, with positive shear defined to be on the (111) plane along the [2] direction. For an SIA cluster containing 1600 atoms, a positive 1 GPa shear stress delocalizes the cluster and expands the SF to 30b, while a negative shear stress of 2 GPa contracts the core to less than 5b, where b is the Burgers vector magnitude.

Much of the discussion in the literature of the low-frequency part of the density of states of amorphous solids was dominated for years by comparing measured or simulated density of states to the classical Debye model. Since this model is hardly appropriate for the materials at hand, this created some amount of confusion regarding the existence and universality of the so-called “boson peak” which results from such comparisons. We propose that one should pay attention to the different roles played by different aspects of disorder, the first being disorder in the interaction strengths, the second positional disorder, and the third coordination disorder. These have different effects on the low-frequency part of the density of states. We examine the density of states of a number of tractable models in one and two dimensions and reach a clearer picture of the softening and redistribution of frequencies in such materials. We discuss the effects of disorder on the elastic moduli and the relation of the latter to frequency softening, reaching the final conclusion that the boson peak is not universal at all.

With the aim to examine the variation in the electronic properties of CoTiSb due to heat treatment, a comparative study of the as-cast and annealed samples using ⁵⁹Co nuclear magnetic resonance (NMR) spectroscopy was performed. All NMR observations clearly indicate a significant change in the local electronic characteristics for the annealed sample. The spin-lattice relaxation rate measurements further provide an estimate of Co-d Fermi-level density of states, Nd(EF), indicating a substantial reduction in Nd(EF) for the specimen with heat treatment. This finding gives a microscopic interpretation for the larger electrical resistivity and Seebeck coefficient in the annealed half-Heusler alloys, as the samples with higher electrical resistivity and Seebeck coefficient usually are associated with lower carrier densities in the vicinity of the Fermi level.

Motivated by the iron pnictides, we examine the spin excitations in an itinerant antiferromagnet where a spin-density wave (SDW) originates from an excitonic instability of nested electronlike and holelike Fermi pockets. Using the random-phase approximation, we derive the Dyson equation for the transverse susceptibility in the excitonic SDW state. The Dyson equation is solved for two different two-band models, describing an antiferromagnetic insulator and metal, respectively. We determine the collective spin-wave dispersions and also consider the single-particle continua. The results for the excitonic models are compared with each other and also contrasted with the well-known SDW state of the Hubbard model. Despite the qualitatively different SDW states in the two excitonic models, their magnetic response shows many similarities. We conclude with a discussion of the relevance of the excitonic SDW scenario to the iron pnictides.

Dynamical excitations in bulk liquid ⁴He are investigated by using a manifestly microscopic theory of excitations that includes multiple-phonon scattering. The wave function of the dynamic system is represented in terms of one- and two-body excitation amplitudes. Equations of motion for the linear response of boson liquids to a scalar external field are then derived from a stationarity principle. For a consistent treatment of long- and short-wavelength properties of the excitation amplitudes we derive and solve three sets of generic “hypernetted chain” equations determining the basic ingredients of the theory. From those ingredients, we calculate a dynamic structure function for ⁴He at saturation density. It is shown that the complete solution of the hypernetted chain equations leads, partly by the cancellation of errors, to an insignificantly improved theoretical prediction for the dynamic structure function compared with approximations introduced by Jackson, Feenberg, and Campbell. The implications of this result and the need for including higher-order multiparticle fluctuations are discussed.

We have studied nonlinear superconducting resonators: /2 coplanar-waveguide (CPW) resonators with Josephson junctions (JJs) placed in the middle and /4 CPW resonators terminated by JJs, which can be used for the qubit readout as “bifurcation amplifiers.” The nonlinearity of the resonators arises from the Josephson junctions, and because of the nonlinearity, the resonators with appropriate parameters are expected to show a hysteretic response to the frequency sweep, or “bifurcation,” when they are driven with a sufficiently large power. We designed and fabricated resonators whose resonant frequencies were around 10 GHz. We characterized the resonators at low temperatures, T<0.05  K, and confirmed that they indeed exhibited hysteresis. The sizes of the hysteresis, however, are sometimes considerably smaller than the predictions based on the loaded quality factor in the weak drive regime. When the discrepancy appears, it is mostly explained by taking into account the internal loss, which often increases in our resonators with increasing drive power in the relevant power range. As a possible origin of the power-dependent loss, the quasiparticle channel of conductance of the JJs is discussed.

We consider a lattice of bosonic atoms, whose number N may be smaller than the number of lattice sites M. We study the Hartree-Fock wave function built up from localized wave functions w(r) of single atoms, with nearest-neighboring overlap. The zero-momentum particle number is expressed in terms of permanents of matrices. In one dimension, it is analytically calculated to be N(M−N+1)/M, with =|w(r)d|²/[(1+2a)l], where a is the nearest-neighboring overlap and l is the lattice constant. is on the order of 1. The result indicates that the condensate fraction is proportional to and of the same order of magnitude as that of the vacancy concentration, hence there is off-diagonal long-range order or Bose-Einstein condensation of atoms when the number of vacancies M−N is a finite fraction of the number of the lattice sites M.

We present theoretical results, based on zero-temperature density-functional theory, for the formation and properties of a negative impurity ion in bulk liquid ⁴He. We first consider Ca which, due to its very low electron affinity, does not easily form a negative ion in vacuum. We show that a neutral Ca atom in bulk liquid ⁴He can easily capture a nearby electron bubble leading to the exothermic formation of a Ca⁻ ion trapped inside a spherical cavity of ~15  Å radius. The Ca negative ion in bulk ⁴He turns out to be a metastable state, the lowest-energy configuration being represented instead by a weakly bound Ca⁻ ion floating over the nearly unperturbed free surface of liquid ⁴He. We have computed the threshold negative pressure at which the trapped Ca⁻ ion bubble explodes and we discuss our results in light of recent experimental measurements. We have also considered the possible ion formation in the case of a Ne atom, i.e., an atomic impurity that does not form a negative ion in vacuum. Despite the long-range attraction between the electron bubble and the Ne atom due to polarization forces, in the minimum-energy configuration the electron bubble and the Ne atom rather than merge together remain spatially separated in bulk liquid ⁴He, forming a weakly bound state that has no analog in vacuum.

The checkerboard pattern in the differential conductance maps on underdoped cuprates appears when the scanning tunneling microscopy is placed above the O sites in the outermost CuO2 plane. In this position the interference between tunneling paths through the apical ions above the neighboring Cu sites leads to an asymmetric weighting of final states in the two antinodal regions of k space. The form of the asymmetry in the differential conductance spectra in the checkerboard pattern favors asymmetry in the localization length rather than a nematic displacement as the underlying origin.

We assessed the infrared-absorption spectra and ¹³C-NMR measurements in a layered organic salt, -(BEDT-TTF)(TCNQ), which exhibits antiferromagnetic transitions at 20 and 3 K. The former originates from the spin in the bis-(ethylenedithio)-tetrathiafulvalene (BEDT-TTF) layers, while the latter originates from the localized spin in the tetracyanoquinodimethane (TCNQ) layers. Using infrared-absorption spectroscopy, we estimated the degree of charge transfer, , between BEDT-TTF and TCNQ as 0.5. Using ¹³C-NMR spectroscopy, we observed an exchange field at the BEDT-TTF site, which is produced by the localized spins of TCNQ dimers. Using the obtained value of and the molecular arrangement of -(BEDT-TTF)(TCNQ), which is similar to that of the highest Tc organic superconductor, -(BEDT-TTF)2ICl2, we concluded that the absence of the pressure-induced superconductivity in -(BEDT-TTF)(TCNQ) results from the presence of this exchange field. The exchange interaction, J, and the exchange field, Hex, were estimated as −12  K and −19  T/µB on the TCNQ dimer unit, respectively. These findings suggest that superconductivity may arise in -(BEDT-TTF)(TCNQ) by the application of an external field of 19 T under high pressure.

The anisotropy in the microwave conductivity of the ortho-II YBa2Cu3O6.50 is studied within the kinetic-energy driven superconducting mechanism. The ortho-II YBa2Cu3O6.50 is characterized by a periodic alternative of filled and empty -axis CuO chains. By considering the CuO chain-induced extended anisotropy impurity scattering, the main features of the anisotropy in the microwave conductivity of the ortho-II YBa2Cu3O6.50 are reproduced based on the nodal approximation of the quasiparticle excitations and scattering processes, including the intensity and line shape of the energy and temperature dependences of the â-axis and -axis microwave conductivities. Our results also confirm that the -axis CuO chain-induced impurity is the main source of the anisotropy.