Dependence of relaxor behavior of incompletely filled tetragonal tungsten bronze uniaxial relaxor ferroelectric calcium barium niobate (Ca${}_x$Ba${}_{1-x}$Nb${}_2$O${}_6$, CBN-$x$) on its composition was investigated by varying Ca/Ba ratio (0.18 ≤ $x$ ≤ 0.35) and studying its thermal and elastic properties. Recently, we have reported the relaxor behavior CBN-28 with the evidence of the existence of the Burns temperature $T_{\mathrm{B}}$, and the intermediate characteristic temperature $T^{*}$ [Pandey et al., Phys. Rev. B 84, 174102 (2011)]. In this work, we show that the dynamics of polar nanoregions (and hence the relaxor behavior) strongly varies with the Ca/Ba ratio. Evidence is found for a more pronounced relaxor behavior with increasing $x$. The Curie temperature and the Burns temperature are also very sensitive to the composition, whereas the characteristic temperature $T^{*}$ appears unaffected from the Ca/Ba ratio. The bonding interaction has been explained on the basis of bulk modulus, Poisson's ratio, and deviation from Cauchy relations. Presented results open the perspective to understand the variation of relaxor behavior of CBN-$x$ $(\sim 0.18\le x\le \sim 0.35)$ above Curie temperature.

Multiscale coupling of a quantum mechanical (QM) domain to a coarser-scale material description in a larger surrounding domain should yield forces and energies in the QM domain that are the same as would be achieved in a QM simulation of the entire system. Here, such a coupling is achieved by using constrained density functional theory (DFT) in which the quantum mechanical interaction between the domains is captured via a constraint potential arising from an imposed constraint on the charge density in a boundary region between the two domains. The implementation of the method, including the construction of the constraint charge density and the calculation of the constraint potential, is presented. The method is applied to problems in three different metals (Al, Fe, and Pd) and is validated against periodic DFT calculations. The method reproduces the QM charge density and magnetic moments of bulk materials, produces a reasonable edge dislocation core structure for Fe, and also gives accurate vacancy formation energy for Al and chemisorption energy on a flat Pd surface. Finally, the method is used to study the chemisorption energy of CO on a stepped Pd surface. In general, the method can mitigate fictitious interactions between surface steps and other extended defects, and accommodate long-range deformation fields, and thus improves upon periodic DFT calculations.

It is well known that screw dislocation motion dominates the plastic deformation in body-centered-cubic metals at low temperatures. The nature of the nonplanar structure of screw dislocations gives rise to high lattice friction, which results in strong temperature and strain rate dependence of plastic flow. Thus the nature of the Peierls potential, which is responsible for the high lattice resistance, is an important physical property of the material. However, current empirical potentials give a complicated picture of the Peierls potential. Here, we investigate the nature of the Peierls potential using density functional theory in the bcc transition metals. The results show that the shape of the Peierls potential is sinusoidal for every material investigated. Furthermore, we show that the magnitude of the potential scales strongly with the energy per unit length of the screw dislocation in the material.

First-principles calculations are used to explore the possibility of enhancing the Néel temperature $T_N$ of the magnetoelectric antiferromagnet Cr${}_2$O${}_3$ by substitutional doping. We describe the electronic structure of transition metal (V, Ti, Mn, Fe, Co, and Ni) and anion (N and B) impurities and evaluate their effect on the exchange interaction. We find that, although transition-metal impurities and N are likely to reduce $T_N$, substitution of O by B is likely to increase it. Both N and B impurities introduce impurity states mediating strong hybridization and magnetic interaction between the neighboring Cr ions. For N impurities, this leads to magnetic frustration, but in the case of B substitution, the stability of the ground antiferromagnetic state is enhanced.

Density-functional calculations have been carried out to investigate the origin of the peculiar magnetic properties of Ba${}_4$Ru${}_3$O${}_{10}$. These calculations suggest that, under the influence of Ru-Ru covalent bonding, structural trimers behave in this compound as strong $S=1$ antiferromagnetic dimers ordering below $T_N$ under the influence of weaker intertrimer couplings.

Ultrafast control of a ferromagnet (FM) via exchange coupling with an antiferromagnet (AFM) is demonstrated in a Co/SmFeO${}_3$ heterostructure. Employing time-resolved photoemission electron microscopy combined with x-ray magnetic circular dichroism, a sub-100-ps change of the Co spins orientation by up to 10${}^{\circ }$ driven by the ultrafast heating of the SmFeO${}_3$ orthoferrite substrate through its spin reorientation phase transition is revealed. Numerical modeling of the ultrafast-laser-induced heat profile in the heterostructure, and the subsequent coupled spins dynamics and equilibration of the spin systems suggest that the localized laser-induced spin reorientation is hindered compared with the static case. Moreover, numerical simulations show that a relatively small Co/SmFeO${}_3$ exchange interaction could be sufficient to induce a complete and fast spin reorientation transition (SRT).

Novel imaging methods show that the mobile dopants in optimum doped La${}_2$CuO${}_{4+y}$ (LCO) get self-organized, instead of randomly distributed, to form an inhomogeneous network of nanoscale metallic puddles with ordered oxygen interstitials interspersed with oxygen-depleted regions. These puddles are expected to be metallic, being far from half filling because of high dopant density, and to sustain superconductivity having a size in the range 5–20 nm. However, the electronic structure of these heavily doped metallic puddles is not known. In fact the rigid-band model fails because of ordering of dopants and supercell calculations are required to obtain the Fermi surface reconstruction. We have performed advanced band calculations for a large supercell La${}_{16}$Cu${}_8$O${}_{32+N}$ where $N=1$ or 2 oxygen interstitials form rows in the spacer La${}_{16}$O${}_{16+N}$ layers intercalated between the CuO${}_2$ layers as determined by scanning nano x-ray diffraction. The additional occupied states made by interstitial oxygen orbitals sit well below the Fermi level ($E_F$) and lead to hole doping as expected. The unexpected results show that in the heavily doped puddles the altered Cu(3$d$)-O(2$p$) band hybridization at $E_F$ induces a multiband electronic structure with the formation of multiple Fermi surface spots: (a) Small gaps appear in the folded Fermi surface, (b) three minibands cross $E_F$ with reduced Fermi energies of 60, 150, and 240 meV, respectively, (c) the density of states and band mass at $E_F$ show substantial increases, and (d) spin-polarized calculations show a moderate increase of antiferromagnetic spin fluctuations. All calculated features are favorable to enhance superconductivity; however, the comparison with experimental methods probing the average electronic structure of cuprates will require the description of the electronics of a network of multigap superconducting puddles.

The interest in twin-boundary (TB) planes as a source of vortex pinning has been recently renewed with the discovery of the new iron-arsenide pnictide superconductors. In the family of compounds Ba(Fe${}_{1-x}$Co${}_x$)${}_2$As${}_2$ a structural transition from a tetragonal to orthorhombic lattice takes place for compounds with $x<x_{\mathrm{cr}}\sim 0.065$. Approaching the critical doping, domain structure shrinks with sizes ultimately becoming comparable to vortex cores. In this work we investigate the changes in anisotropy produced by subtle differences in the Co doping level, in the neighborhood of the structural transition, in good-quality single crystals. Using a scaling approach we are able to determine the angular regions where correlated or uncorrelated disorder prevails. In the tetragonal samples ($x>x_{\mathrm{cr}}$) there is no twinning and we find good agreement with the expected scaling function under uncorrelated disorder, with small anisotropy values similar to those reported in the literature. We show that in the orthorhombic samples ($x<x_{\mathrm{cr}}$), TBs act as correlated disorder in a broad angular range. We propose that the observed angular dependence could be due to an increase in the vortex liquid-glass transition temperature.

We show that electrodynamic dipolar interactions, responsible for long-range fluctuations in matter, play a significant role in the stability of molecular crystals. Density functional theory calculations with van der Waals interactions determined from a semilocal “atom-in-a-molecule” model result in a large overestimation of the dielectric constants and sublimation enthalpies for polyacene crystals from naphthalene to pentacene, whereas an accurate treatment of nonlocal electrodynamic response leads to an agreement with the measured values for both quantities. Our findings suggest that collective response effects play a substantial role not only for optical excitations, but also for cohesive properties of noncovalently bound molecular crystals.

We have observed a ferromagnetic-induced piezoresistance component in a micromechanical cantilever integrated with a GaMnAs piezoresistor. The temperature and the magnetic-field dependence of the piezoresistance around the ferromagnetic transition reveal that the piezoresistance is modulated by ferromagnetic ordering. The sign of the ferromagnetic-induced component of the piezoresistance is opposite to that of the conventional piezoresistance, though of comparable magnitude. This ferromagnetic-induced piezoresistance has a delayed response to mechanically induced strain, with an experimentally determined response time of 230 ± 35 ns.

We report the ${}^{121/123}$Sb-NMR/nuclear quadrupole resonance (NQR) measurements on the superconductor BaTi${}_2$Sb${}_2$O with a two-dimensional Ti${}_2$O square-net layer formed with Ti${}^{3+}$ (3$d^1$). NQR measurements revealed that the in-plane four-fold symmetry is broken at the Sb site below $T_{\mathrm{A}}\sim 40$ K, without an internal field appearing at the Sb site. These exclude a spin-density wave (SDW)/ charge density wave (CDW) ordering with incommensurate correlations, but can be understood with the commensurate CDW ordering at $T_{\mathrm{A}}$. The spin-lattice relaxation rate $1/T_1$, measured at the four-fold symmetry breaking site, decreases below superconducting (SC) transition temperature $T_{\mathrm{c}}$, indicative of the microscopic coexistence of superconductivity and the CDW/SDW phase below $T_{\mathrm{A}}$. Furthermore, $1/T_1$ of ${}^{121}$Sb-NQR shows a coherence peak just below $T_{\mathrm{c}}$ and decreases exponentially at low temperatures. These results are in sharp contrast with those in cuprate and iron-based superconductors, and strongly suggest that its SC symmetry is classified to an ordinary $s$-wave state.

White-beam x-ray fluorescence holograms were recorded using a 50 watt x-ray tube for a Cu${}_3$Au single crystal. Element sensitivity for this model sample was explicitly demonstrated by measuring distinct holograms using Cu K and Au L x-ray fluorescence. The phase sensitivity of the recorded holograms enabled us to obtain projections of the local atomic structure and image the electron density within the unit cell, relative to either Au or Cu atoms. Since the experimental setup does not require any optics and the angular resolution requirements are loose, the application of more powerful laboratory x-ray sources is straightforward.

We show that the nonequilibrium time evolution after interaction quenches in the one-dimensional, integrable Hubbard model exhibits a dynamical transition in the half-filled case. This transition ceases to exist upon doping. Our study is based on systematically extended equations of motion. Thus it is controlled for small and moderate times; no relaxation effects are neglected. Remarkable similarities to the quench dynamics in the infinite-dimensional Hubbard model are found, suggesting that dynamical transitions are a general feature of quenches in such models.

The electronic structure, optical and x-ray absorption spectra, angle dependence of the cyclotron masses and extremal cross sections of the Fermi surface, phonon spectra, electron-phonon Eliashberg and transport spectral functions, temperature dependence of electrical resistivity of the $M$B${}_2$ ($M=\text{Ti}$ and Zr) diborides were investigated from first principles using the fully relativistic and full potential linear muffin-tin orbital methods. The calculations of the dynamic matrix were carried out within the framework of the linear response theory. A good agreement with experimental data of optical and x-ray absorption spectra, phonon spectra, electron-phonon spectral functions, electrical resistivity, cyclotron masses, and extremal cross sections of the Fermi surface was achieved.

In the theory of quantum spin liquid, gauge fluctuation is an emergent excitation at low energy. The gauge magnetic field is proportional to the scalar spin chirality ${\mathit{S}}_1·{\mathit{S}}_2\times {\mathit{S}}_3$. It is therefore highly desirable to measure the fluctuation spectrum of the scalar spin chirality. We show that in the Kagome lattice with a Dzyaloshinskii-Moriya term, the fluctuation in $S_z$, which is readily measured by neutron scattering, contains a piece which is proportional to the chirality fluctuation.

Binary nanowires (Co${}_{50}$Fe${}_{50}$/Ni${}_{80}$Fe${}_{20}$) that are 260-nm wide, 25-nm thick, and 4-mm long are fabricated using “self-aligned shadow deposition” lithography. Each wire is separated from the next by a gap of 115 nm. Ferromagnetic resonance experiments performed on the nanowires indicate resonant frequencies associated with the Co${}_{50}$Fe${}_{50}$ and the Ni${}_{80}$Fe${}_{20}$ parts of the nanowires. These frequencies are shifted from those measured in nanowires made from just one material or the other due to exchange and dipolar coupling between the two materials. A semianalytic theory is outlined that correctly predicts the resonant frequencies in the binary stripes at various values of static applied field. This calculation is checked against a micromagnetic simulation, and the agreement between both methods and the experiment is good.

We report superconducting properties of AgSnSe${}_2$, which is a conventional type II superconductor in the very dirty limit due to intrinsically strong electron scatterings. While this material is an isotropic three-dimensional (3D) superconductor with a not-so-short coherence length where strong vortex fluctuations are not expected, we found that the magnetic-field-induced resistive transition at fixed temperatures becomes increasingly broader toward zero temperature and, surprisingly, that this broadened transition is taking place largely above the upper critical field determined thermodynamically from the specific heat. This result points to the existence of an anomalous metallic state possibly caused by quantum phase fluctuations in a strongly disordered 3D superconductor.

We analyze a two-dimensional Pomeranchuk-nematic instability, trigger by the Landau parameter $F_2<0$, in the presence of a small magnetic field. Using Landau Fermi-liquid theory in the isotropic phase, we analyze the collective modes near the quantum critical point $F_2=-1,{\omega }_c=0$ (where ${\omega }_c$ is the cyclotron frequency). We focus on the effects of parity symmetry breaking on the Fermi-surface deformation. We show that the linear response approximation of the Landau-Silin equation is not sufficient to study the critical regime and it is necessary to compute corrections at least of order ${\omega }_c^2$. Identifying the slowest oscillation mode in the disordered phase, we compute the phase diagram for the isotropic/nematic phase transition in terms of $F_2$ and ${\omega }_c$.

We study the interplay of $3d$ and $4f$ electrons in the iron pnictides CeFe${}_{1-x}$Co${}_x$AsO and GdFe${}_{1-y}$Co${}_y$AsO, which correspond to two very different cases of $4f$-magnetic moment. Both CeFeAsO and GdFeAsO undergo a spin-density-wave (SDW) transition associated with Fe $3d$ electrons at high temperatures, which is rapidly suppressed by Fe/Co substitution. Superconductivity appears in a narrow doping range: $0.05<x<0.2$ for CeFe${}_{1-x}$Co${}_x$AsO and $0.05<y<0.25$ for GdFe${}_{1-y}$Co${}_y$AsO, showing a maximum transition temperature $T_{\mathrm{sc}}$ of about 13.5 K for Ce and 19 K for Gd. In both compounds, the $4f$ electrons form an antiferromagnetic (AFM) order at low temperatures over the entire doping range and Co $3d$ electrons are ferromagnetically ordered on the Co-rich side; the Curie temperature reaches $T_{\mathrm{C}}^{\mathrm{Co}}\approx$ 75 K at $x=1$ and $y=1$. In the Ce compounds, the Néel temperature $T_{\mathrm{N}}^{\mathrm{Ce}}$ increases upon suppressing the SDW transition of Fe and then remains nearly unchanged with further increasing Co concentration up to $x\simeq 0.8$ ($T_{\mathrm{N}}^{\mathrm{Ce}}\approx$ 4 K). Furthermore, evidence of Co-induced polarization on Ce moments is observed on the Co-rich side. In the Gd compounds, the two magnetic species of Gd and Co are coupled antiferromagnetically to give rise to ferrimagnetic behavior in the magnetic susceptibility on the Co-rich side. For $0.7\le y<1.0$, the system undergoes a possible magnetic reorientation below the Néel temperature of Gd ($T_{\mathrm{N}}^{\mathrm{Gd}}$). Our results suggest that the effects of both electron hybridizations and magnetic exchange coupling between the $3d$-$4f$ electrons give rise to a rich phase diagram in the rare-earth iron pnictides.

Single- and two-particle excitation spectra of the one-dimensional, half-filled Holstein-Hubbard model are calculated using the continuous-time quantum Monte Carlo method. In the metallic phase, the results are consistent with a Luther-Emery liquid that has gapped spin and single-particle excitations but a gapless charge mode. However, given the initially exponential dependence of the spin gap on the backscattering matrix element, the numerical excitation spectra appear gapless in the weak-coupling regime, and therefore resemble those of a Luttinger liquid. The Mott phase has the expected charge gap and gapless spin excitations. The Peierls state shows a charge, spin, and single-particle gap, a soft phonon mode, backfolded shadow bands, and soliton excitations. Arguments and numerical evidence for the existence of a nonzero spin gap throughout the metallic phase are provided in terms of equal-time spin and charge correlation functions.