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Temperature-dependent atomic structural evolutions of liquid ${\mathrm{Ag}}_{60}{\mathrm{Ga}}_{40}$ and ${\mathrm{Ag}}_{70}{\mathrm{Ga}}_{30}$ alloys have been studied by in situ high-energy x-ray-diffraction (HEXRD) experiments combined with ab initio molecular-dynamics simulations. The experimental data show a reversible structural crossover at about $1050\ensuremath{\sim}1100 (\ifmmode\pm\else\textpm\fi{}50)\phantom{\rule{4pt}{0ex}}\mathrm{K}$ in both liquid ${\mathrm{Ag}}_{60}{\mathrm{Ga}}_{40}$ and ${\mathrm{Ag}}_{70}{\mathrm{Ga}}_{30}$ alloys. Obvious changes of the electrical resistivity, absolute thermoelectric power, and atomic diffusivity around the similar temperature range for both Ag-Ga liquids strongly support the HEXRD results. The origin of the liquid-to-liquid crossover in both Ag-Ga liquids was suggested to link with the rearrangements of Ag and Ga atoms, i.e., Ag and Ga atoms prefer to associate with themselves in the higher temperature range above 1100 K, consistent with the accelerated increase of the strong covalently bonded Ga--Ga dimers in both Ag-Ga liquids. In addition, more studies from the energy aspect are still desirable to understand the rearrangements of Ag and Ga atoms in the higher temperature range in both Ag-Ga liquids.

The $p\text{\ensuremath{-}}T$ conditions of the solid-solid phase transition from the wurtzite to rocksalt structure in GaN are determined both experimentally and by ab initio calculations. Experimental evaluation was based on the x-ray absorption measurements in a laser-heated diamond anvil cell. At 300 K, the transition was observed near 47 GPa. At lower pressures, the wurtzite to rocksalt transition has been induced by high temperature: 1420 K at 42 GPa and about 2100 K at 37 GPa. Thus the slope of the wurtzite-rocksalt borderline could be evaluated as negative and nonlinear. On the part of the theory, the $p\text{\ensuremath{-}}T$ borderline was determined from a comparative analysis of the temperature dependences of the Gibbs potential of the wurtzite and rocksalt structures for different pressure. The Gibbs potentials were calculated within the quasiharmonic approximation and the self-consistent phonon approach. The results obtained with the self-consistent phonon approach show that the inclusion of the anharmonic phonon effects is indispensable to obtain a very good agreement with the experimental data. Possible consequences of the observed anharmonicity for the still unknown melting behavior of GaN are discussed. In particular, it is suggested that the melting temperature of the rocksalt-GaN, at pressure around 37 GPa, is not much higher than 2100 K.

Temperature dependence of the band structure, as well as the spin-polarization on the topologically trivial insulator ${\mathrm{TlBiS}}_{2}$ and nontrivial insulator $\mathrm{Tl}\mathrm{Bi}{\mathrm{Se}}_{2}$, have been investigated by spin- and angle-resolved photoelectron spectroscopy. Despite the recent theoretical prediction [Phys. Rev. Lett. 117, 246401 (2016).], topological phase transition (trivial to nontrivial for ${\mathrm{TlBiS}}_{2}$, nontrivial to trivial for $\mathrm{Tl}\mathrm{Bi}{\mathrm{Se}}_{2}$) does not occur in the observed temperature ranges (from 50 to 400 K for ${\mathrm{TlBiS}}_{2}$, from 50 to 475 K for $\mathrm{Tl}\mathrm{Bi}{\mathrm{Se}}_{2}$). Our results indicate that the discrepancy between the theory and the experiment can be understood by considering the overestimation of lattice parameters and the effect of lattice expansion that is neglected in the previous theory. Although some spin-polarized states are observed in ${\mathrm{TlBiS}}_{2}$, it is probably due to the Rashba-like surface (resonant) states and not the topological surface states. On the other hand, the topological surface states in $\mathrm{Tl}\mathrm{Bi}{\mathrm{Se}}_{2}$ is very robust even at 400 K and the spin-polarized states can be used for the real spintronic devices in the large temperature range.

Lithium carbide, Li2C2, is a fascinating material that combines strong covalent and weak ionic bonding resulting in a wide range of unusual properties. The mechanism of its phase transition from th ...

We investigate the generation and temperature-induced evolution of optically driven spin photocurrents in ${\mathrm{WTe}}_{2}$ and ${\mathrm{MoTe}}_{2}$. By correlating the scattering-plane dependence of the spin photocurrents with the symmetry analysis, we find that a sizable spin photocurrent can be controllably driven along the chain direction by optically exciting the system in the high-symmetry $y\ensuremath{-}z$ plane. Temperature dependence measurements show that pronounced variations in the spin photocurrent emerge at temperatures that coincide with the onset of anomalies in their transport and optical properties. The decreasing trend in the temperature dependence starting below $\ensuremath{\sim}150\phantom{\rule{0.28em}{0ex}}\mathrm{K}$ is attributed to the temperature-induced Lifshitz transition. The sign inversion of the spin photocurrent, observed around 50 K in ${\mathrm{WTe}}_{2}$ and around 120 K in ${\mathrm{MoTe}}_{2}$, may have its origin in an interaction that involves multiple kinds of carriers.

We investigated the bulk-derived electronic structure of the temperature-induced valence transition system ${\mathrm{EuNi}}_{2}$(${\mathrm{Si}}_{1\ensuremath{-}x}{\mathrm{Ge}}_{x}$)${}_{2}$ ($x=0.70$, 0.79, and 0.82) by means of hard x-ray photoemission spectroscopy (HAXPES). The HAXPES spectra clearly show distinct temperature dependencies in the spectral intensities of the ${\mathrm{Eu}}^{2+}$ and ${\mathrm{Eu}}^{3+}\phantom{\rule{4pt}{0ex}}3d$ components. For $x=0.70$, the changes in the ${\mathrm{Eu}}^{2+}$ and ${\mathrm{Eu}}^{3+}\phantom{\rule{4pt}{0ex}}3d$ spectral components with temperature reflect a continuous valence transition, whereas the sudden changes for $x=0.79$ and 0.82 reflect first-order valence transitions. The Eu $3d$ spectral shapes for all $x$ and particularly the drastic changes in the ${\mathrm{Eu}}^{3+}\phantom{\rule{4pt}{0ex}}3d$ feature with temperature are validated by a theoretical calculation based on the single-impurity Anderson model (SIAM). SIAM analysis reveals that the valence transition for each $x$ is controlled by the $c\ensuremath{-}f$ hybridization strength and the charge-transfer energy. Furthermore, the $c\ensuremath{-}f$ hybridization strength governs the valence transition of this system, which is either first order or continuous, consistent with Kondo volume collapse.

We show that a structural energy difference, $\ensuremath{\Delta}E,$ must include explicit symmetry-breaking changes of the electronic structure due to temperature-induced configurational excitations, and why $\ensuremath{\Delta}E$ at $T=0 \mathrm{K}$ is not necessarily relevant to thermodynamic and mechanical modeling. In ${\mathrm{Ni}}_{3}\mathrm{V},$ we calculate a tenfold decrease of $\ensuremath{\Delta}E$ between ${D0}_{22}$ and ${L1}_{2}$ structures from $T=0 \mathrm{K}$ to states of order relevant to experiment. $\ensuremath{\Delta}E$ calculated directly from states with short-range order (8 meV) or with low partial order (7--12 meV) agree with high-T experiment (10 meV).

Photoluminescence of ZnSe, Zn0.84Mn0.16Se alloy, and ZnSe/Zn0.84Mn0.16Se superlattice (SL) have been measured in the temperature range from 10 to 300 K. It is found that the band gap of the ZnSe was smaller than that of the Zn0.84Mn0.16Se alloy at 10 K, but larger than that of the alloy at 300 K. Then the well and barrier layers of the ZnSe/Zn0.84Mn0.16Se SL would be expected to turn over at about 180 K. This type of turn over was observed in the SL sample. The turn over took place at 80 K, somewhat lower than the expected temperature. A calculation including the strain in the ZnSe/Zn0.84Mn0.16Se SL indicates that the heavy-hole bands begin crossing at 75 K, which agrees well with experimental results. [S0163-1829(99)13127-8].

The surface of ${\mathrm{La}}_{1\ensuremath{-}x}{\mathrm{Ca}}_{x}{\mathrm{MnO}}_{3}$ (LCMO) films with $x\phantom{\rule{0ex}{0ex}}=\phantom{\rule{0ex}{0ex}}0.1$, $0.33$, $0.5$, and $0.66$ has been studied; a highly ordered grain pattern induced by strain is observed in films with $x\phantom{\rule{0ex}{0ex}}=\phantom{\rule{0ex}{0ex}}0.5$ and $0.66$. The strain is identified to result from thermal expansion mismatch between substrate and film, and thermal expansion anomaly may exist at a high temperature induced by possible phase transition in LCMO with $x\phantom{\rule{0ex}{0ex}}=\phantom{\rule{0ex}{0ex}}0.5$ and $0.66$. Such phenomenon is arresting on film growth and understanding colossal magnetoresistance materials as well as providing a new way to make ordered structures for application.

A mixed-stack organic charge-transfer crystal of 2-chloro-5-methyl- $p$-phenylenediamine-2,5-dimethyl dicyanoquinonediimine (ClMePD-DMeDCNQI), which undergoes a temperature-induced neutral-ionic transition (TINIT), is reported. From infrared spectroscopy, the molecular ionicity (\ensuremath{\rho}) of ClMePD-DMeDCNQI was found to change continuously from \ensuremath{\rho}=0.3 at 300 K to \ensuremath{\rho}=0.6 at 100 K. The neutral-ionic interface was crossed without a discontinuity in \ensuremath{\rho} at around 200 K, accompanied by stack dimerization. These features were quite different from the discontinuous change in \ensuremath{\rho} observed for the TINIT of tetrathiafulvalene-$p$-chloranil (TTF-CA) and tetramethylbenzidine-tetracyanoquinodimethane (TMB-TCNQ). The transition characteristics in ClMePD-DMeDCNQI are discussed by drawing a comparison with those seen in TTF-CA and TMB-TCNQ.

We have studied the sol-gel transition, the viscoelastic and the structural properties of networks constituted of semiflexible actin filaments cross-linked by \ensuremath{\alpha}-actinin. Cross-linking was regulated in a reversible way by varying the temperature through the association-dissociation equilibrium of the actin--\ensuremath{\alpha}-actinin system. Viscoelastic parameters [shear storage modulus G\ensuremath{'}(\ensuremath{\omega}), phase shift tan(\ensuremath{\Phi})(\ensuremath{\omega}), creep compliance J(t)] were measured as a function of temperature and actin-to-cross-linker ratio by a magnetically driven rotating disc rheometer. G\ensuremath{'}(\ensuremath{\omega}) and tan(\ensuremath{\Phi})(\ensuremath{\omega}) were studied at a frequency \ensuremath{\omega} corresponding to the elastic plateau regime of the G\ensuremath{'}(\ensuremath{\omega}) versus \ensuremath{\omega} spectrum of the purely entangled solution. The microstructure of the networks was viewed by negative staining electron microscopy (EM). The phase shift tan(\ensuremath{\Phi}) (or equivalently the viscosity \ensuremath{\eta}) diverges and reaches a maximum when approaching the apparent gel point from lower and higher temperatures, and the maximum defines the gel point (temperature ${\mathit{T}}_{\mathit{g}}$). The elastic plateau modulus ${\mathit{G}}_{\mathit{N}}^{\ensuremath{'}}$ diverges at temperatures beyond this gel point T${\mathit{T}}_{\mathit{g}}$ but increases only very slightly at Tg${\mathit{T}}_{\mathit{g}}$. The cross-linking transition (corresponding to a sol-gel transition at zero frequency) is interpreted in terms of a percolation model and the divergence of ${\mathit{G}}_{\mathit{N}}^{\ensuremath{'}}$ at T${\mathit{T}}_{\mathit{g}}$ is analyzed by a power law of the form ${\mathit{G}}_{\mathit{N}}^{\ensuremath{'}}$\ensuremath{\sim}[p(T)-${\mathit{p}}_{\mathit{c}}$${]}^{\ensuremath{\gamma}}$ where p(T) is the temperature dependent fraction of crosslinks formed. A power of \ensuremath{\gamma}=1.5--1.8 is found. Negative staining EM shows (1) that the gel is essentially homogeneous above the cross-linking transition (Tg${\mathit{T}}_{\mathit{g}}$), (2) that microscopic segregation takes place at T\ensuremath{\leqslant}${\mathit{T}}_{\mathit{g}}$ leading to local formation of clusters (a state termed microgel), and (3) that at low actin--\ensuremath{\alpha}-actinin ratios (${\mathit{r}}_{\mathit{A}\mathrm{\ensuremath{\alpha}}}$\ensuremath{\le}10) and low temperatures (T\ensuremath{\le}10 \ifmmode^\circ\else\textdegree\fi{}C) macroscopic segregation into bundles of cross-linked actin filaments and a diluted solution of actin filaments is observed. The three regimes of network structure are represented by an equivalent phase diagram. \textcopyright{} 1996 The American Physical Society.

We study the magnetic susceptibility of mixed-valent La${}_{2\ensuremath{-}x}$Sr${}_{x}$CoO${}_{4}$ single crystals in the doping range of $0.5\ensuremath{\leqslant}x\ensuremath{\leqslant}0.8$ for temperatures up to 1000 K. The magnetism below room temperature is described by paramagnetic Co${}^{2+}$ in the high-spin state and by Co${}^{3+}$ in the nonmagnetic low-spin state. At high temperatures, an increase in susceptibility is seen, which we attribute to a temperature-induced spin-state transition of Co${}^{3+}$. The susceptibility is analyzed by comparison to full-multiplet calculations for the thermal population of the high- and intermediate-spin states of Co${}^{3+}$.

We introduce a novel mechanism of temperature dependence of friction at the nanoscale which is determined by a decrease of the slip length with temperature T. We find that the effect of temperature on the slip length may result in a rich temperature dependence of friction, including a peak and/or plateau in F as a function of T, and a sharp increase or decrease of F with T. This mechanism is of primary importance in the multiple-slip regimes of motion when the tip slips over a number of lattice spacings. The influence of normal load and driving velocity on the temperature dependence of friction is discussed. We predict that the presence of surface defects or adsorbate may strongly influence the temperature dependence of friction.

We report a detailed spectroscopic investigation of temperature-induced valence and structural instability of the mixed-stack organic charge-transfer (CT) crystal $4,{4}^{\ensuremath{'}}$-dimethyltetrathiafulvalene-chloranil (DMTTF-CA). DMTTF-CA is a derivative of tetrathiafulvalene-chloranil (TTF-CA), the first CT crystal exhibiting the neutral-ionic transition by lowering temperature. We confirm that DMTTF-CA undergoes a continuous variation of the ionicity on going from room temperature down to $\ensuremath{\sim}20\phantom{\rule{0.3em}{0ex}}\mathrm{K}$, but remains on the neutral side throughout. The stack dimerization and cell doubling, occurring at $65\phantom{\rule{0.3em}{0ex}}\mathrm{K}$, appear to be the driving forces of the transition and of the valence instability. In a small temperature interval just below the phase transition, we detect the coexistence of molecular species with slightly different ionicities. The Peierls mode(s) precursors of the stack dimerization are identified.

We analyse phase averaging in one-dimensional interacting mesoscopic systems with several barriers and show that for incommensurate positions an independent average over several phases can be induced by finite temperature. For three strong barriers with conductances G_i and mutual distances larger than the thermal length, we obtain G ~ sqrt{G_1 G_2 G_3} for the total conductance G. For an interacting wire, this implies power laws in G(T) with novel exponents, which we propose as an experimental fingerprint to distinguish temperature induced phase averaging from dephasing., Comment: 6 pages, 5 figures; added one figure; slightly extended

Recent electron paramagnetic resonance (EPR) and electron-nuclear double-resonance research at 9.5 and 34 GHz have demonstrated that two types of $[{\mathrm{RhCl}}_{6}{]}^{4\ensuremath{-}}$ complexes may coexist in AgCl crystals, depending on the crystal growth method and sample pretreatments. Based on these results, the temperature-induced changes to the EPR spectrum of ${\mathrm{Rh}}^{2+}$-doped AgCl single crystals are convincingly explained by the difference in the dynamical behavior of the two complexes.

${\mathrm{G}\mathrm{d}}_{5}{\mathrm{S}\mathrm{n}}_{4}$ exhibits a giant magnetocaloric effect comparable to that reported in the ${\mathrm{G}\mathrm{d}}_{5}(\mathrm{S}\mathrm{i},\mathrm{G}\mathrm{e}{)}_{4}$ system. The giant magnetocaloric effect is associated with a first-order change that occurs at $\ensuremath{\sim}82\text{ }\text{ }\mathrm{K}$ in zero field, and can be reversed by the application of an external field of a few Tesla. $^{119}\mathrm{S}\mathrm{n}$ M\"ossbauer spectroscopy shows that this material is magnetically inhomogeneous over a wide range of temperatures and fields.

A Raman-scattering investigation of stishovite, a high-pressure crystalline polymorph of SiO[sub 2], shows that at around 650 [degree]C it becomes amorphous. The amorphous material produced during the initial stages of amorphization is not identical to ordinary silica since it exhibits features of the high-pressure amorphous polymorph. Above 800 [degree]C there is little remaining evidence of stishovite but the amorphous material has transformed to a state indistinguishable from that of silica. Annealing effects in densified silica are also presented and compared with the stishovite results.

We have shown that a very strong nonlinearity in the temperature dependence of the luminescence quantum yield in Cr-doped laser crystals of the Colquiriite family can lead to a temperature induced intrinsic optical bistability in these materials. The bistability and the hysteresis loop in the dependence of Cr luminescence intensity and transmission of the crystal on cw pumping power has been theoretically predicted and experimentally observed. The experimental results are in a good qualitative and quantitative agreement with the theoretical prediction.

We provide evidence for the temperature-induced metallization (TIM) of Si(100) surface. A TIM takes place at about 600 K without any change in surface periodicity. Contrary to previous suggestions, photoemission spectroscopy (PES) reveals that the TIM is not related to the (instantaneous) symmetrization of asymmetric dimers. We suggest that Si adatoms produced at elevated temperatures play a role of donor, giving rise to the TIM.

We examine the depinning transitions and the temperature versus driving force phase diagram for magnetically interacting pancake vortices in layered superconductors. For strong disorder the initial depinning is plastic followed by a sharp hysteretic transition to a 3D ordered state for increasing driving force. Our results are in good agreement with theoretical predictions for driven anisotropic charge density wave systems. We also show that a temperature induced peak effect in the critical current occurs due to the onset of plasticity between the layers., Comment: 4 pages, 4 postscript figures

The structural phase transition (PT) of the Si(113) surface upon varying the temperature has been investigated using synchrotron radiation photoemission spectroscopy (SRPES). It was observed that the Si(113) surface reconstructs to a $3\ifmmode\times\else\texttimes\fi{}2$ phase at room temperature (RT). The $3\ifmmode\times\else\texttimes\fi{}2$ surface was found to transform to $3\ifmmode\times\else\texttimes\fi{}1$ at about 800 K. Our PES results show that the local structure of the $3\ifmmode\times\else\texttimes\fi{}2$ surface at RT is the same as that of the $3\ifmmode\times\else\texttimes\fi{}1$ surface at 800 K. The PT upon raising the temperature can be understood via the thermal fluctuation of two types of tetramer on the $3\ifmmode\times\else\texttimes\fi{}2$ surface. We thus propose that the temperature-induced PT is an order-disorder transition.

Sur la base des mesures magneto-optiques sur un super-reseau HgTe-CdTe, la bande de valence doit etre de l'ordre de 40 meV. On montre qu'au lieu de representer la resonance cyclotron par les electrons a l'interieur du super-reseau, les minima dans leurs spectres de magnetotransmission sont beaucoup plus facilement interpretes en termes des electrons quasi-bidimensionnels residant au voisinage de l'interface du super-reseau avec le substrat. Lorsque la non parabolicite est prise en compte, les donnees sont compatibles avec une grande discontinuite de la bande de valence dans ce systeme, contrairement au resultat obtenu anterieurement

The phase transition of ${\mathrm{CaBr}}_{2}$ at about 780 K can be classified as a second-order proper ferroelastic one of the optical type that is induced by an order parameter of ${\mathit{B}}_{1\mathit{g}}$ symmetry. We wish to point out that x-ray- and Raman-spectroscopic results on this transition are in full accordance one with another and with the statements above, whereas the assignments of Raman modes by Raptis, McGreevy, and Seguier [Phys. Rev. B 39, 7996 (1989)] are in part contradictory.

High-energy ion scattering measurements on a flat W(100) surface have been carried out in the temperature range 80--600 K. The results indicate conclusively that the surface atoms are displaced laterally at all temperatures. The lateral displacement is estimated as approx. =0.14 A. The low-temperature (..sqrt..2 x ..sqrt..2 )R45/sup 0/ to high-temperature (1 x 1) phase transition is therefore an order-disorder transition.

Phonon frequencies of longitudinal- and transverse-acoustic modes have been determined in the mixed-valence compound ${\mathrm{CeSn}}_{3}$ at 295 and 10 K. For comparison, temperature-induced phonon-frequency shifts were also measured in the integral-valence isostructural compound ${\mathrm{LaSn}}_{3}$. No evidence has been found that the mixed valence significantly alters the lattice dynamics of ${\mathrm{CeSn}}_{3}$ as a function of temperature. The difference between the thermal expansion coefficients of ${\mathrm{CeSn}}_{3}$ and ${\mathrm{LaSn}}_{3}$ below room temperature is in agreement with differences in the respective mode Gr\"uneisen parameters of the two substances.

The finite-temperature Hartree-Fock-Bogoliubov equations are solved for /sup 148/Sm with the pairing-plus-quadrupole interaction. Although the static shape is spherical at zero temperature, raising the temperature induces a second-order phase transition to a prolate deformed shape. The onset of deformation is related to the thermal destruction of the pairing gap.

The finite-temperature Hartree-Fock-Bogoliubov cranking equation is solved for /sup 166/Er. Increasing the temperature at fixed angular momentum produces a phase transition from a prolate shape rotating collectively to an oblate shape ''rotating'' noncollectively around a symmetry axis. The critical temperature for this shape transition is spin dependent.