Low-temperature Configuration Research Articles

Next-Nearest-Neighbor Hopping in the Falicov-Kimball Model

Results of Monte Carlo simulations for the spinless Falicov–Kimball model with the next-nearest-neighbor hopping are presented. We find the critical value of the next-nearest-neighbor hopping integral, below which the low temperature configuration of the localized particles is the same as in the presence of only the nearest-neighbor hopping. Beyond this critical value the localized particles form horizontal or vertical stripes.

Structural and magnetic investigations on a new molecular quantum magnet

We report structural and magnetic investigations on a new hexacopper (II) siloxanolate cluster compound [C 92 H 102 Cu 6 N 8 O 29 Si 10 ]. The system, crystallizing in the triclinic space group P 1 ̄ (No. 2), has two identical macromolecules per unit cell. Each cluster contains six interacting Cu 2+ ions. We find a crossover from a 12-spin paramagnetic state at high temperatures to a 4-spin low-temperature configuration accompanied by the formation of an S =1 cluster ground state. A minimal magnetic model is discussed which may account for these observations.

Theoretical Studies of Solid Nitromethane

A classical potential to simulate the dynamics of a nitromethane crystal as a function of temperature and pressure is described. The intramolecular part of the potential was taken as superposition of bond stretching, bond bending, and torsional angles terms. These terms were parametrized on the basis of the geometric and spectroscopic (vibrational frequencies and eigenvectors) data obtained using ab initio molecular orbital calculations performed at the B3LYP/6-31G* level on an isolated molecule. The intermolecular potential used is of the Buckingham 6-exp form plus charge−charge Coulombic interactions and has been previously developed by us (Sorescu, D. C.; Rice, B. M.; Thompson, D. L. J. Phys. Chem. 1997, B101, 798) to simulate crystals containing nitramine molecules and several other classes of nitro compounds. The analyses performed using constant pressure and temperature molecular dynamics simulations and molecular packing calculations indicate that the proposed potential model is able to reproduce accurately the changes of the structural crystallographic parameters as functions of temperature or pressure for the entire range of values investigated. In addition, the calculated bulk modulus of nitromethane was found in excellent agreement with the corresponding experimental results. Moreover, it was determined that the present potential predicts correctly an experimentally observed 45° change in methyl group orientation in the high-pressure regime relative to the low-temperature configuration. The analysis of the linear expansion coefficients and linear compression data indicate anisotropic behavior for the unit cell edges.

Reactivity of Unsaturated Linear C6Hydrocarbons on Pd(111) and H(D)/Pd(111)

The adsorption and reactivity of 1-hexene, 1,5-hexadiene, 1,3-hexadiene, and 1,3,5-hexatriene on clean Pd(111) and hydrogen (deuterium)-saturated/Pd(111) were investigated using temperature-programmed reaction spectroscopy. The low-temperature adsorption configuration for the linear C6hydrocarbons is proposed to be a weakly π-bonded species. The adsorbed molecules first desorb molecularly with a fraction converting to a more tightly bonded half-hydrogenated state, which either β-hydride eliminates to release the alkene or dehydrogenates completely to adsorbed carbon and hydrogen. Dehydrocyclization to benzene is observed on this surface, whereas it does not occur on Pd(100). Cyclization of 1,3,5-hexatriene to benzene occurs at temperatures as low as 333 K on Pd(111). Although hydrogenation of 1,3-hexadiene adsorbed on clean Pd(111) and of 1,3-hexadiene and 1,5-hexadiene adsorbed on H/Pd(111) to hexene was observed, and hexatriene was hydrogenated to hexadiene and hexene, hydrogenation of the alkenes to hexane was not observed for any of the unsaturated species. H–D exchange into all the adsorbed alkenes was observed. The exchange reaction is proposed to occur through the reversible C–H bond formation in a half-hydrogenated intermediate.

Magnetic order and crystal structure in the superconductingRNi2B2C materials

Neutron-diffraction measurements have been carried out to investigate the crystal structure, magnetic structures, and magnetic phase transitions in ${\mathrm{RNi}}_{2}$${\mathrm{B}}_{2}$C (R= Y, Ce, Pr, Nd, Tb, Dy, Ho, Er, Tm, and Yb). The materials that order magnetically exhibit a wide variety of both commensurate and incommensurate magnetic structures, which argues strongly that the dominant exchange interactions are of the indirect Ruderman-Kittel-Kasuya-Yosida type. The Nd system exhibits a commensurate antiferromagnetic ordering at 4.8 K, with wave vector \ensuremath{\delta}=(1/2,0,1/2) and moment direction along a (or equivalently with \ensuremath{\delta}=(0,1/2,1/2) and moment direction along b in this tetragonal system). For Dy (${\mathrm{T}}_{\mathrm{N}}$=10.6 K), Pr (${\mathrm{T}}_{\mathrm{N}}$=4.0 K), and the low-temperature phase of Ho, the magnetic structure is also a commensurate antiferromagnet that consists of ferromagnetic sheets of rare-earth moments in the a-b plane, with the sheets coupled antiferromagnetically along the c axis [\ensuremath{\delta}=(0,0,1)]. Pr is not superconducting, while for Dy (${\mathrm{T}}_{\mathrm{c}}$=6 K) and Ho (${\mathrm{T}}_{\mathrm{c}}$=8 K) this magnetic order coexists with superconductivity. For Ho, though, the magnetic state that initially forms at ${\mathrm{T}}_{\mathrm{N}}$\ensuremath{\approx}8.5 K is an incommensurate spiral antiferromagnetic state along the c axis in which the direction of these ferromagnetic sheets are rotated in the a-b plane by \ensuremath{\sim}17\ifmmode^\circ\else\textdegree\fi{} from their low-temperature antiparallel configuration [\ensuremath{\delta}=(0,0,0.91)]. The intensity for this spiral state reaches a maximum near the reentrant superconducting transition (\ensuremath{\sim}5 K); the spiral state then collapses at lower temperature in favor of the commensurate antiferromagnetic state. An incommensurate a-axis modulation, with \ensuremath{\delta}=(0.55,0,0), is also observed above the spiral-antiferromagnetic transition, but it exists over a narrower temperature range than the spiral state, and also collapses near the reentrant superconducting transition. The Er system forms an incommensurate, transversely polarized spin-density wave (SDW) state at ${\mathrm{T}}_{\mathrm{N}}$=6.8 K, with \ensuremath{\delta}=(0.553,0,0) and moment direction along b (or with \ensuremath{\delta} along b and the moment direction along a). The SDW squares up at low T, and coexists with the superconducting state (${\mathrm{T}}_{\mathrm{c}}$=11 K) over the full temperature range where magnetic order is present. Tb, which does not superconduct, orders with a very similar wave vector, but the SDW is longitudinally polarized in this case and again squares up at low T. Tm orders at ${\mathrm{T}}_{\mathrm{N}}$=1.5 K in a transversely polarized SDW state, but with the moments along the c axis and \ensuremath{\delta}=(0.093,0.093,0). This state is coexistent with superconductivity (${\mathrm{T}}_{\mathrm{c}}$=11 K). No significant magnetic moment is found to develop on the Ni site in any of the materials, and there is no magnetic ordering of any kind in the Y, Yb, or Ce materials. Profile refinements have also been carried out on these same samples to investigate the systematics of the crystallography, and the crystal structure is I4/mmm over the full range of compositions and temperatures investigated. The area of the a-b plane and the volume of the unit cell both decrease smoothly with either decreasing lanthanide radius or decreasing temperature, but the strong boron-carbon and nickel-carbon bonding then forces the c axis to expand.

Molecular Dynamics Simulations of the Melting of a Hexane Monolayer: Isotropic versus Anisotropic Force Fields

Molecular dynamics simulations of a hexane monolayer on the basal-plane surface of graphite have been performed to investigate the effect of an anisotropic force field on the melting process. In two sets of simulations, which are carried out for a number of temperatures ranging from the solid to the fluid state of the monolayer, the molecules are described either by a skeletal model, where the interaction sites are represented by a “united atom” model, or by a model where the interaction sites are shifted away from the mass site (the “anisotropic united atom” model). Independent of the model, the low temperature configuration is an orientationally ordered herringbone structure, which on heating undergoes an orientational phase transition to a rectangular-centered structure where the molecules tend to align along one direction. The solid subsequently melts at approximately 175 K. However, the anisotropic force field promotes a perpendicular orientation of the backbone of the hexane molecules and, in contrast to the “united atom” model, molecules exhibit a smaller tilt and a lower percentage of gauche defects at melting. This is compensated by increased librational motion in the plane of the substrate.

Nature of the magnetic order in superconducting and nonsuperconducting HoNi2-xCoxB2C.

Neutron-diffraction measurements have been carried out to investigate the magnetic properties of superconducting (${\mathit{T}}_{\mathit{c}}$\ensuremath{\sim}8 K) ${\mathrm{HoNi}}_{2}$${\mathrm{B}}_{2}$C and nonsuperconducting (${\mathit{T}}_{\mathit{c}}$3 K) ${\mathrm{HoNi}}_{1.985}$${\mathrm{Co}}_{0.015}$${\mathrm{B}}_{2}$C. Both systems become magnetically long-range ordered below \ensuremath{\sim}8 K, with three types of magnetic order being present. The low-temperature structure is a commensurate antiferromagnetic state that consists of ${\mathrm{Ho}}^{3+}$ moments aligned ferromagnetically in the a-b plane, with the sheets coupled antiferromagnetically along the c axis. The magnetic state that initially forms on cooling, however, is dominated by an incommensurate spiral antiferromagnetic state along the c axis, with wave vector ${\mathit{q}}_{\mathit{c}}$\ensuremath{\approxeq}0.054 A${\mathrm{\r{}}}^{\mathrm{\ensuremath{-}}1}$, in which the relative alignment of each ferromagnetic sheet is rotated in the a-b plane by \ensuremath{\sim}17\ifmmode^\circ\else\textdegree\fi{} from the low-temperature antiparallel configuration. The intensity for this spiral state reaches a maximum near \ensuremath{\sim}5 K; the spiral state then collapses at lower temperature in favor of the commensurate antiferromagnetic state. A smaller amplitude a-axis modulation, with ${\mathit{q}}_{\mathit{a}}$\ensuremath{\approxeq}0.73 A${\mathrm{\r{}}}^{\mathrm{\ensuremath{-}}1}$, is also observed above the spiral antiferromagnetic transition, but over a narrower temperature range than the spiral state. The identical sequence of phase transitions is observed for both the superconducting and nonsuperconducting samples, demonstrating that the reentrant superconductivity and the coexistence of long-range antiferromagnetic order and superconductivity at low temperatures are both controlled by the nature of the magnetic structures present. In the temperature regime where the three magnetic structures are observed simultaneously, the sample dependence of the intensities strongly suggests that they occur in spatially separate regions. \textcopyright{} 1996 The American Physical Society.

The Flow Stress of High-Purity Refractory Body-Centred Cubic Metals and its Modification by Atomic Defects

The strong temperature and strain-rate dependence of the flow stress of high-purity refractory body-centred cubic metals has been shown to be an intrinsic property and is usually ascribed to a high Peierls barrier of a o /2 screw dislocations. These barriers are overcome by the formation of kink pairs on the screw dislocations. The paper reports on recent, very complete flow-stress data on ultra-high purity Mo crystals obtained by two different experimental techniques and covering the temperature range 4 K to 460 K. The results are in accord with earlier work of Brunner and Diehl on α-Fe, who showed that below the so-called knee temperature, T K , three regimes in the temperature variation of the flow-stress should be distinguished. Two of them are fully accounted for by the same glide mechanism, namely elementary glide steps on {211} planes. The so-called upper bend separating these two regimes is an inherent feature of the theory of kink-pair formation and does not indicate a change in the glide mechanism. There is, however, strong evidence that the so-called lower bend, separating the range of {211} elementary glide steps from the low-temperature flow-stress regime, is due to a change in the glide mechanism. It is argued that at the lower bend the screw-dislocation cores undergo a first-order phase transition from a low-temperature configuration that allows glide of a given screw dislocation on any of its three {110} glide planes to a high-temperature configuration that can glide only on one definite {211} plane. Between T K and the lower-bend temperature, T, bcc metals may show the unique phenomena of alloy and irradiation softening. With regard to the latter phenomenon, Brunner and Diehl distinguish between primary and secondary softening. It is shown that alloy softening and the secondary irradiation softening of bcc metals may be explained by an overheating of the phase transition in the dislocation core. Although the underlying mechanism is not yet fully understood, there is fairly strong evidence that the primary irradiation softening is due to the interaction between screw dislocation cores and crowdion self-interstitials.

Exciton emission in BaFBr and BaFCl crystals

The emission of self-trapped excitons produced by vacuum ultraviolet excitation as well as X-ray excitation or photostimulation in BaFCl and BaFBr crystals was studied. The exciton emission bands at 4.2 and 5.15 eV in BaFBr and at 3.3 eV in BaFCl were observed upon irradiation into the Cl or Br exciton absorption bands. The 5.15 eV emission band in BaFBr thermally transforms to the 4.2 eV band with increasing temperature within the range 50-80 K. The 3.3 eV emission in BaFCl increases in the range 20-50 K in BaFCl, while the lifetime of the emission decreases. These facts indicate rather clearly the potential barrier between two different exciton configurations. The low-temperature exciton configuration was tentatively assigned to the Vk+e type and the other to F+H type.

Raman scattering of KI:Ag+: Exploration of a nearly unstable defect-lattice configuration

Abstract The Raman spectra of Ag+ in KI exhibit between 6K and 25K two resonant modes and quasi-elastic scattering (QES), each with a different temperature behavior. The low-temperature Raman and infrared (ir) spectral features can be described within a harmonic force-constant model, which does not however account for the dramatic temperature dependence of the experimental data. They are compatible with a population transfer from a low-temperature on-center configuration to a second elastic configuration, in which the Ag+ ion is displaced from the regular lattice site. The QES, which dominates the Raman spectra above 20K, may reflect different types of rotational diffusion or the presence of a third elastic configuration.

Low-frequency elastic shear constant and low-temperature configuration of K1-xLixTaO3

Elastic shear constant data are presented for K1-xLixTaO3 at very low frequencies near the transition temperature to a glass phase. A comparison made with dielectric-susceptibility data reveals the character of the relaxation modes. The low-temperature state is described in terms of short dipolar but long quadrupolar correlation.

Hydrogen relaxation in yttrium

The internal-friction spectra of the solid solutions α-YH(D) x with x = 0.05 to 0.2 have been measured between liquid nitrogen and room temperature. They exhibit an hydrogen-related isotope-dependent peak at T p ( H) = 219 K and T p ( D) = 223 K and a break in the dynamic-modulus dependence at the same temperature. The results are discussed in the framework of a Zener-type process of stress-induced rearrangement between H and vacant tetrahedral sites in the ordered low-temperature configuration as established by neutron scattering. The relaxation characteristics of α-YH(D) x are compared with those measured earlier on α-ScH(D) x and α-LuH(D) x and the observed trends related to various lattice properties of the host metal.

Theoretical and experimental study of a quantized lattice configuration in a nearly unstable defect system.

Theoretical lattice dynamics analysis and new optical experiments on KI:${\mathrm{Ag}}^{+}$ reveal a nearly unstable low-temperature defect-host configuration, whose population decreases dramatically with temperature, but whose dynamics are consistent with a temperature-independent harmonic model involving collective motion. This behavior is in marked contrast to the temperature-dependent evolution of the dynamics of ``soft-mode'' systems.

Thallium‐belated isoelectronic bound excitons in silicon. A bistable defect at low temperatures

AbstractDetailed investigations are reported on the luminescence spectra of thallium related isoelectronic excitons in silicon, which yield a complete picture of this most unusual luminescence system. The luminescent defect exists in two different configurations, which are able to transform into each other in the temperature range from 10 to 20 K. The stress and Zeeman experiments reveal that the low temperature configuration has trigonal symmetry, whereas the high temperature form of the center is aligned along the [100] directions. Time resolved measurements of the luminescence lines lead to a detailed model for the mechanism of reorientation. In the defect ground state the trigonal symmetry is lowest in energy. The capture of a hole or an exciton changes the relative energies in such a way that now the [100] configuration is energetically favoured.

Magnetic Order in Spatially Disordered Systems

Magnetic order in metallic and nonmetallic binary alloys is discussed. A recent model, in which the breakdown of long-range magnetic order in metallic alloys is due to the cooperative change of the moment magnitude with concentration, is illustrated with. examples from nickel alloy systems. Percolation at the critical concentration for long-range order plays a part in these alloy systems, as it does for nonmetallic alloys and metallic alloys in which the magnetic moment magnitude is not so environment dependent. Discussion of the magnetic state in the non long-range ordered composition range leads to a description of spin glasses. The results of recent experiments on the low temperature configuration, the AC susceptibility near the freezing temperature, and the dynamics at all temperatures are presented with the implications for the nature of the spin glass transition.

Internal fields and ferroelectric ordering in potassium ferrocyanide trihydrate

On the basis of the existing experimental data it is assumed that the mechanism responsible for the ferroelectric transition of potassium ferrocyanide trihydrate is a rotational reorientation of the water molecules. Accordingly, the electrostatic energy of the water molecules (approximated by three point charges) is calculated as a function of their orientation. The low temperature configuration is obtained by minimising the electrostatic energy. A possible model is then proposed for the ferroelectric to paraelectric transition, consisting in the switching of the water molecules between two energetically equivalent configurations. The results are in good agreement with the experimental data.

Magnetic phase transitions of CeSb. I. Zero applied magnetic field

The magnetic ordering of the anomalous antiferromagnet CeSb, which has a NaCl crystal structure, was determined in zero applied magnetic field by means of neutron diffraction investigations of single crystals and powder. Below the Neel temperature TN of (16.1+or-0.1)K, there exist six partially disordered magnetic phases of antiphase domain type ((100) superstructures) with (100) orientation of the magnetic moments. At 4.4K, the ordered magnetic moment equals (2.10+or-0.04) mu B, which corresponds to the free-ion value of 2.14 mu mB for Ce3+. The temperature dependence of the ordered moment shows a first-order phase transition at TN. At approximately TN/2 there is a first-order phase transition to a FCC type IA low-temperature configuration. The unusual magnetic properties of CeSb, which result from anisotropic exchange and crystalline electric field effects, resemble those of certain actinide NaCl-type compounds.

Thermal contact problems in low temperature thermocouple thermometry

The temperature difference and thermal contact resistance between a thermocouple and the body to which it is attached have been calculated for a realistic low temperature configuration. For good thermal contact the temperature error at the junction will be of order exp ( −1 α ) less than the average thermocouple-to-body temperature difference, where I is the bond length and α a characteristic length of order 2 mm. The thermocouple-to-body thermal bond resistance is proportional to α and is of order 10 4 K W −1. When thermocouples are used to measure the thermal conductivity of poor conductors, a large error may result because of the shunting action of the wires. This has been calculated for a typical configuration, the fractional error being approximately 3W x W w where W x is the thermal resistance of the body and W W the thermal resistance of the wire between the points of contact.

Two-dimensional bonded lattice fluids. II. Orientable molecule model

For pt. I see ibid. vol. 3, 427-41 (1970). A bonded fluid model on a plane triangular lattice is studied. Each molecule has three bonding directions at angles of 120 degrees to each other and two possible orientations in each of which its bonding directions point to three of the six nearest-neighbour sites. If the molecules of a nearest-neighbour pair have bonding directions pointing towards each other then a bond is formed and the pair has interaction energy -( epsilon +w), while an unbonded nearest-neighbour pair has interaction energy - epsilon ( epsilon >0, w>0). For epsilon /w<1/3 regions of open structure short- range order became important at low temperatures and pressures, with each molecule in such a region bonded to three others and one third of the sites vacant. At higher pressures the predominant low-temperature configuration is close-packed with all sites occupied. Calculations are performed, using a first-order approximation based on a triangle of sites, for epsilon /w=0 and epsilon /w=1/4. Critical points are deduced for separation into liquid and vapour phases, both without long-range order. The behaviour of the density as a function of pressure and temperature in the model resembles that found in fluid water, especially for epsilon /w=1/4. There is a supercritical region where curves of density against temperature at constant pressure show turning points, though at very high pressures the density decreases monotonically. For epsilon /w=1/4 these turning points are also found below the critical pressure in the liquid phase.