Phase transition in sarcosine phosphite single crystals

Single crystals of sarcosine phosphite (SarcH₃PO₃) have been grown. The amino acid sarcosine is an isomer of the protein amino acid alanine. Both amino acids are described by the same chemical formula but have different structures; or, more specifically, in contrast to the alanine molecule, the sarcosine molecule has a symmetric structure. It has been found that the sarcosine phosphite compound undergoes a structural phase transition at a temperature of approximately 200 K. This result has demonstrated that compounds of achiral amino acids are more susceptible to structural phase transitions.     

Dislocation arrangement in deformed very dilute Cu-Cd alloy single crystals

The influence of the precipitation annealing on the CRSS and dislocation arrangement of the Cu-0·03 at. % Cd single crystals is observed experimentally. Dislocation distributions are determined by TEM of sections (111) and (101) in stage I and in the transition region. The CRSS increases due to precipitation annealing. There are not observed any second phase particles, but nevertheless there exist some differences in the dislocation arrangement of the solution annealed single crystals and the precipitation annealed ones.     

Metamagnetic transition in Na 0.85 CoO2 single crystals

We report the magnetization, specific heat, and transport measurements of a high quality Na(0.85)CoO2 single crystal in applied magnetic fields up to 14 T. At high temperatures, the system is in a paramagnetic phase. It undergoes a magnetic phase transition below approximately 20 K. For the field H||c, the measurement data of magnetization, specific heat, and magnetoresistance reveal a metamagnetic transition from an antiferromagnetic state to a quasiferromagnetic state at about 8 T at low temperatures. However, no transition is observed in the magnetization measurements up to 14 T for H perpendicular c. The low temperature magnetic phase diagram of Na(0.85)CoO2 is determined.     

Studies of metal-insulator transition in TiSxSe2-x single crystals

A temperature induced metal-insulator transition has been found in TiS_xSe_2-x single crystals. The M-I transition is found to occur over the temperature range 250° to 300°C for 1.0 ≤ x ≤ 1.7. The present observation of the M-I transition has been compared and contrasted with the earlier reported M-I transition in TiS_1.7 single crystals. Evidence is presented and arguments are put forward in support of the occurrence of the M-I transition in terms of the variation in disorder of the extra Ti atoms in the van der Waals gap. The possible reason for the suppression of the M-I transition in TiS_xSe_2-x for x ≤ 1 is outlined.     

Acoustic emission at phase transition in vanadium sesquioxide single crystals

The metal-semiconductor phase transition in vanadium sesquioxide is investigated by the acoustic emission method. It is shown that the acoustic emission in single crystals of this compound is due to thermoelastic stresses arising in the crystal upon the phase transition. Transformation of the acoustic emission activity and an increase in the phase transition temperature are revealed in the temperature cycling of the sample. Observation of peaks of the acoustic emission activity at a temperature of 5–6 K above the critical temperature indicates that crystal nuclei of the monoclinic phase appear in the high-temperature (trigonal) phase of the crystal.     

Flicker noise in degenerately doped Si single crystals near the metal-insulator transition

In this paper we report some of the important results of experimental investigations of the flicker noise near the metal-insulator (MI) transition in doped silicon single crystals. This is the first comprehensive work to study low-frequency noise in heavily doped Si over an extensive temperature range (2 K<T<500 K). The measurements of conductance fluctuations (flicker noise) were carried out in the frequency range 10⁻²<f<4 × 10¹ Hz in single crystalline Si across the MI transition by doping with phosphorous and boron. The magnitude of noise in heavily doped Si is much larger than that seen in lightly doped Si over the whole temperature range. The extensive temperature range covered allowed us to detect two distinct noise mechanisms. At low temperatures (T<100 K) universal conductance fluctuations (UCF) dominate and the spectral dependence of the noise is determined by dephasing the electron from defects with two-levels (TLS). At higher temperatures (T>200 K) the noise arises from activated defect dynamics. As the MI transition is approached, the 1/f spectral power, typical of the metallic regime, gets modified by the presence of discrete Lorentzians which arise from generation-recombination process which is the characteristic of a semiconductor.     

Metal insulator transition characteristics of macro-size single domain VO2 crystals

The metal insulator transition (MIT) characteristics of macro-size single-domain VO₂ crystal were investigated. At the MIT, the VO₂ crystal exhibited a rectangular shape hysteresis curve, a large change in resistance between the insulating and the metallic phases, in the order of ～10⁵, and a small transition width (i.e. temperature difference before and after MIT) as small as 10^?3°C. These MIT characteristics of the VO₂ crystals are discussed in terms of phase boundary motion and the possibility of controlling the speed of the phase boundary, with change in size of crystal, is suggested.     

Optical study of the spontaneous ferroelectric phase transition in lead scandoniobate single crystals

A study is reported of the optical transmission and small-angle light scattering (SAS) in ordered and disordered stoichiometric single crystals of lead scandoniobate (PSN) with a zero and a dc electric field applied. It is shown that the spontaneous phase transition occurring in both crystals is accompanied by a sharp peak in SAS intensity, which implies the percolation nature of this transition. A field-temperature phase diagram has been constructed for the PSN single crystals studied in the work.     

Anisotropy in the length change of gallium single crystals at their superconducting transition

The magnetostriction of gallium single crystals has been measured for the three principal axes between 0.3K and 1.1 K. It is found that the length change at the transition from the superconducting to the normal state is given approximately by (Δl/l)_a = − 3.10⁻⁹, (Δl/l)_b = + 4.4 · 10⁻⁹ and (Δl/l)_c = + 1.8 · 10⁻⁹, for the a, b and c axes at T = 0 K. A short discussion of the results with respect to microscopic theory is presented.     

Phase transition and triplet exciton mobility in anthracene—tetracyanobenzene charge-transfer single crystals

Triplet-Excitons in a mixed 1 : 1 anthracene—tetracyanobenzene single crystal are established by ESR-spectrocopy. These excitons (lifetime 0.5 msec) are characterized by a hopping frequency of v = 1.3 × 10⁶ K at 300K. The temperature dependence of the ESR line shape indicates a phase transition of the crystal at T_t = 205K. Above and below the phase transition the exciton mobility is nearly temperature independent.     

Reentrant spin-glass transition in a dilute magnet

We perform a large-scale Monte Carlo simulation of a dilute classical Heisenberg model with ferromagnetic nearest neighbor and antiferromagnetic next-nearest neighbor interactions. We found that the model reproduces a reentrant spin-glass transition. That is, as the temperature is decreased, the magnetization increases rapidly below a certain temperature, reaches a maximum value, and then disappears at some lower temperature. The low temperature phase was suggested to be a spin-glass phase that is characterized by ferromagnetic clusters.     

Commensurate-incommensurate transition and disordering in dilute graphite-bromine

Results of a x-ray scattering study of the inplane ordering in dilute graphite-bromine in the temperature range from 300 to 400K are presented. Evidence is obtained for a commensurate-incommensurate transition which is found to behave differently from those observed in the case of adsorbed monolayers. The bromine layers undergo continuous order-disorder transition from the incommensurate phase at 374K. Implications of this result and the large value of the measured exponent (β=0.32) are discussed.     

Bose-Einstein transition in a dilute interacting gas

We study the effects of repulsive interactions on the critical density for the Bose-Einstein transition in a homogeneous dilute gas of bosons. First, we point out that the simple mean field approximation produces no change in the critical density, or critical temperature, and discuss the inadequacies of various contradictory results in the literature. Then, both within the frameworks of Ursell operators and of Green's functions, we derive self-consistent equations that include correlations in the system and predict the change of the critical density. We argue that the dominant contribution to this change can be obtained within classical field theory and show that the lowest order correction introduced by interactions is linear in the scattering length, a, with a positive coefficient. Finally, we calculate this coefficient within various approximations, and compare with various recent numerical estimates. Received 15 July 2001     

Effect of electric field on the percolation phase transition in lead scandotantalate single crystals

The effect of a dc electric field on the phase transition process in lead scandotantalate single crystals differing in the degree of ion ordering has been studied by small-angle light scattering (SAS). The spontaneous phase transition occurring in these crystals is shown to be accompanied by a sharp SAS intensity peak indicating the percolation nature of this transition. A phase diagram in the field-temperature coordinates has been constructed for all the PST crystals studied in the work. The electric field and the temperature variation rate have been found to affect the SAS intensity.     

The paramagnetic-to-ferromagnetic transition in B2-structured Fe-Al single crystals: Experiments and calculations

It is well established that single crystals of B2-structured Fe-Al change from paramagnetic to ferromagnetic upon plastic deformation. This strain-induced ferromagnetism arises mostly from Fe atoms which have three or more like nearest neighbours (NNs) in antiphase-boundary (APB) tubes. Such Fe atoms carry magnetic moments according to their local environment. In this study, the saturation magnetizations, M S , of cold-rolled Fe-34 at.% Al, Fe-40 at.% Al and Fe-43 at.% Al single crystals were measured in a vibrating-sample magnetometer from 77 K to room temperature and transmission electron microscopy was used to confirm the presence of APB tubes. After magnetic measurements, rolled single crystals were heated in a differential scanning calorimeter at 20 K min m1 to 973 K and the enthalpy associated with the removal of the APB tubes measured. From the enthalpy measurement, the APB tube density and the number of Fe NNs were estimated. Hence, the M S values at 0 K were calculated. These were in good agreement with the experimentally determined values.     

An acoustic anomaly at the phase transition in GeTeSnTe alloy single crystals

The ultrasonic wave velocities and attenuation have been measured in single crystals of 20 mole.% GeTeSnTe alloys between 90 and 373°K. The ultrasound velocities and thus the elastic constants show a step-like behaviour, which is accompanied by a peak in the attenuation, at the cubic to rhombohedral transformation temperature (240°K). In the vicinity of the transition a strong interaction occurs between the acoustic phonon modes and the soft TO mode.