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Although, as part of a general phenomenon, the piezoelectric response of $\mathrm{Ba}(\mathrm{T}{\mathrm{i}}_{1\ensuremath{-}y}{M}_{y}){\mathrm{O}}_{3}$ ($M=\mathrm{Zr}$, Sn, Hf) increases in the vicinity of the orthorhombic ($Amm2$)-tetragonal ($P4mm$) and orthorhombic ($Amm2$)-rhombohedral ($R3m$) polymorphic phase boundaries, experiments in the last few years have shown that the same phase boundaries show significantly enhanced weak-field piezoproperties in the Ca-modified variants of these ferroelectric alloys, i.e., $(\mathrm{Ba},\mathrm{Ca})(\mathrm{Ti},M){\mathrm{O}}_{3}$. So far there is a lack of clarity with regard to the unique feature(s) which Ca modification brings about that enables this significant enhancement. Here, we examine this issue from a structural standpoint with $M=\mathrm{Sn}$ as a case study. We carried out a comprehensive comparative structural, ferroelectric, and piezoelectric analysis of the $Amm2$ phase in the immediate vicinity of the $P4mm\text{\ensuremath{-}}Amm2$ phase boundaries of (i) Ca-modified $\mathrm{Ba}(\mathrm{Ti},\mathrm{Sn}){\mathrm{O}}_{3}$, as per the nominal formula $(1\ensuremath{-}x)\mathrm{BaT}{\mathrm{i}}_{0.88}\mathrm{S}{\mathrm{n}}_{0.12}{\mathrm{O}}_{3}\text{\ensuremath{-}}(x)\mathrm{B}{\mathrm{a}}_{0.7}\mathrm{C}{\mathrm{a}}_{0.3}\mathrm{Ti}{\mathrm{O}}_{3}$ and (ii) without Ca modification, i.e., $\mathrm{Ba}(\mathrm{T}{\mathrm{i}}_{1\ensuremath{-}y}\mathrm{S}{\mathrm{n}}_{y}){\mathrm{O}}_{3}$. We found that the spontaneous lattice strain of the $Amm2$ phase is noticeably smaller in the Ca-modified counterpart. Interestingly, this happens along with an improved spontaneous polarization by enhancing the covalent character of the Ti-O bond. Our study suggests that the unique role of Ca modification lies in its ability to induce these seemingly contrasting features (reduction in spontaneous lattice strain but increase in polarization).

Low-temperature thermal conductivity measurements were performed on single crystals of $\mathrm{TlNi}{}_{2}\mathrm{Se}{}_{2}$, a nickel chalcogenide heavy-electron superconductor with ${T}_{c}\ensuremath{\simeq}3.7$ K. In zero field, the residual electronic contribution at $T\ensuremath{\rightarrow}0$ K (${\ensuremath{\kappa}}_{0}/T$) was well separated from the total thermal conductivity, which is less than 0.45% of its normal-state value. Such a tiny residual ${\ensuremath{\kappa}}_{0}/T$ is unlikely contributed by the nodal quasiparticles. The nodeless gap structure is supported by the very weak field dependence of ${\ensuremath{\kappa}}_{0}(H)/T$ in low magnetic fields. In the whole field range, ${\ensuremath{\kappa}}_{0}(H)/T$ exhibits an $\mathsf{S}$-shaped curve, as in the case of nickel pnictides $\mathrm{BaNi}{}_{2}\mathrm{As}{}_{2}$ and $\mathrm{SrNi}{}_{2}\mathrm{P}{}_{2}$. This common feature of nickel-based superconductors can be explained by multiple nodeless superconducting gaps.

We have undertaken a study of the propagation of carriers in one-dimensional (1D) aperiodic systems within the tight-binding scheme under the action of electric fields. We have concentrated on the Fibonacci (F) and Thue-Morse (TM) lattices, which are of great practical importance since these structures can be fabricated thanks to recent technological advances. We show that superdiffusion takes place for both lattices in the field-free case. The mean-square displacement (MSD) follows the law 〈${\mathit{n}}^{2}$〉\ensuremath{\propto}${\mathit{t}}^{\mathrm{\ensuremath{\alpha}}}$ with \ensuremath{\alpha}=1.55 for the F lattice and \ensuremath{\alpha}=1.65 for the TM lattice. When the field is included, the particle localizes, i.e., the MSD is bound, even for weak field intensities.

We experimentally studied the effects of a moderated electromigration field on the dynamics of the step bunching process on the Si(111) surface at 1130 ◦C (regime II) and 1270 ◦C (regime III). The surfaces with step bunch morphologies were created by annealing vicinal Si(111) at fixed temperatures while the applied electric field E was adjusted for every experiment. Scaling relations, ym ∼ hE , between the slope of a step bunch ym, step bunch height h, and electromigration field E were experimentally probed. Scaling exponents α ≈ 2/3 and q ≈ 1/3 were extracted from the step bunch morphologies created by annealing Si(111) in the regime III (1270 ◦C), which are in good agreement with the predictions of the generalized BCF theory. Scaling exponents α ≈ 3/5 and q ≈ 1/3 were extracted from the morphologies created by annealing in regime II (1130 ◦C). This result was compared to the scaling relations derived within the frame of the transparent step model, which correctly predicts the formation of the step bunching instability by step-up adatom electromigration. The scaling relation obtained by experiment was found to differ from the model predictions. We measured values of critical electric field (Ecr), i.e., minimum electric field required for the step bunching to take place. A relatively weak field of E > 0.5 V/cm was found to be sufficient to initiate the step bunching process in regime II. This contrasts with regime III, where Ecr = 1.0 and 2.0 V/cm were measured for Si miscut from the (111) plane by 1.1◦ and 2.5◦, respectively. The increased values of Ecr were attributed to the enhanced step-step repulsion in regime III. The theoretically predicted formation of compressed step density waves was observed upon annealing in both regimes with E < Ecr.

It was recently observed [van der Beek et al., Phys. Rev. B 54, R792 (1996)] that irradiated layered high-T c superconductors subject to perpendicular magnetic field manifest two similar scaling behaviors corresponding totwo different critical temperatures. We show that this phenomenon is explained within the framework of the Ginzburg-Landau theory, In the weak field region, the scaling function differs from that of a clean sample mainly because of the renormalization of the critical temperature, due to defects. In the strong field region, defects are effectively suppressed and the scaling function, as well as the critical temperature, are the same as in a clean superconductor. The theory presented here agrees quite satisfactory with experiment.

A many-body theory of a two-dimensional electron gas in a magnetic field is presented. The field dependence of the Fermi momentum shows a two-dimensional peculiarity. It consists of the spin-dependent paramagnetic part and the orbital diamagnetic part. The former increases more strongly with ${r}_{s}$ than the latter. The paramagnetic and diamagnetic susceptibilities are found as functions of ${r}_{s}$. The field-dependent ground-state energy is evaluated. Its paramagnetic part decreases steadily with ${r}_{s}$, while the diamagnetic part shows a maximum. No such maximum has been found for the three-dimensional case in the same approximation which is valid for high density and low magnetic fields. From the susceptibilities, an effective $g$ factor and an effective mass are derived. They are given approximately by $\frac{{g}^{*}}{g}=1+0.87\ifmmode\times\else\texttimes\fi{}{10}^{6}{n}^{\ensuremath{-}\frac{1}{2}}$ or more accurately by $\frac{{g}^{*2}}{{g}^{2}}=1+1.74\ifmmode\times\else\texttimes\fi{}{10}^{6}{n}^{\ensuremath{-}\frac{1}{2}}\ensuremath{-}0.708\ifmmode\times\else\texttimes\fi{}{10}^{12}{n}^{\ensuremath{-}1}$ and by $\frac{{m}^{*}}{m}=1+3.21\ifmmode\times\else\texttimes\fi{}{10}^{4}{n}^{\ensuremath{-}\frac{1}{2}}$, where $n$ is the number density.

Weak-field magnetoresistance measurements on $p$-type SnTe exhibit a peculiar symmetry which is temperature and carrier-concentration dependent. A Fermi-surface model is described which can account for these results and reconcile them with several other kinds of experimental measurements on SnTe. The Fermi surface consists of four prolate $〈111〉$ valleys with three $〈100〉$-oriented knobs protruding from each end of each valley. The proposed model is also used to bring out some previously unreported similarities between the band structures of the ${\mathrm{Pb}}_{1\ensuremath{-}x}{\mathrm{Sn}}_{x}\mathrm{Te}$ and ${\mathrm{Bi}}_{1\ensuremath{-}x}{\mathrm{Sb}}_{x}$ systems, and to examine the broader question of the connection between weak-field magnetoresistance behavior and band-structure characteristics.

We have measured the weak-magnetic-field electron g factor of the two-dimensional electron gas found in a modulation-doped heterojunction formed from (In,Ga)As and InP. The g factor was determined by observing the spin-splitting zeros in the Shubnikov--de Haas oscillations as a function of total magnetic field in the temperature interval from 2 to 10 K. Extrapolating to zero field gives \ensuremath{\Vert}${g}^{\mathrm{*}}$\ensuremath{\Vert}=4.5\ifmmode\pm\else\textpm\fi{}0.25, independent of temperature in the low-temperature limit. Estimates of the band-edge g factor in bulk (In,Ga)As lead to an estimate of 0.5 for the exchange enhancement of the g factor of this two-dimensional electron gas.

The granular magnetization model of Finnemore et al. (1987) has been tested on a well-characterized sample of Y1Ba2Cu3Ox. The temperature dependence of the magnetic susceptibility is in excellent agreement with the model. In addition, the grain size deduced from the magnetization data (1-2 microns) is in reasonable agreement with direct scanning-electron-microscopy measurements of the microstructure.