Magnetic properties of La0.70Sr0.30MnO2.85 anion-deficient manganite under hydrostatic pressure

The magnetic properties of La_0.70Sr_0.30MnO_2.85 anion-deficient manganite are studied experimentally under hydrostatic pressure. The results show that, in the whole pressure range under investigation (0–1 GPa), the sample is a spin glass with a smeared phase transition to the paramagnetic state. It is found that the spin glass state arises from the frustration of the exchange coupling of the ferromagnetic clusters embedded in the antiferromagnetic matrix. The fraction of the sample volume occupied by the ferromagnetic phase is found to be V _fer ～ 13%. Under hydrostatic pressure, the freezing temperature T _f of the magnetic moments of the ferromagnetic clusters increases at a rate of 4.30 K/GPa and the magnetic ordering temperature T _MO increases at a rate of 12.90 K/GPa. In addition, the ferromagnetic part of the sample increases by ΔV _fer ～ 5%. The enhancement of the ferromagnetic properties of La_0.70Sr_0.30MnO_2.85 anion-deficient manganite under hydrostatic pressure is explained by the redistribution of oxygen vacancies and a decrease in the unit-cell parameters.     

Thermoelectric properties of tin clathrates under hydrostatic pressure

We report the temperature dependence of electrical resistance (R) and thermopower (S) of clathrate Cs₈Sn₄₄ under high pressure up to 1.2 GPa. We observe a reversible gap widening, prominent relaxation effect of R, irreversible increase of |S| under high pressure. We also find that the power factor S²σ (σ: electrical conductivity) reaches a maximum at pressure of 0.3 GPa. Comparison of the experimental results with band structure calculations suggests that the intrinsic vacancy in the clathrate structure of Cs₈Sn₄₄ plays an important role in transport properties under high pressure. Measurements on Cs₈Zn₄Sn₄₂, a clathrate which has defects other than vacancies, are compared with Cs₈Sn₄₄. The results indicate that replacing Sn by Zn has similar effect as the intrinsic vacancy on S.     

Electronic and optical properties of LiMgN, LiMgP and LiMgAs under hydrostatic pressure

First principles calculations, by means of the full-potential linearized augmented plane wave method within the local density approximation, were carried out for the effect of pressure on the electronic and optical properties of the filled tetrahedral compounds LiMgN, LiMgP and LiMgAs. The bandgap pressure coefficient trend in the ternaries is found to be similar to the one encountered in the zinc-blende-like AlX. The first order bandgap pressure coefficient a^Γ-Γ in LiMgN is larger than the corresponding one in AlN, while it is smaller in LiMgP and LiMgAs compared to the one in AlP and AlAs. The predicted values of the dielectric constants for LiMgN, LiMgP and LiMgAs are close to those of the binary compounds AlN, AlP and AlAs.     

Electronic and optical properties of CdTe under hydrostatic pressure effect

We have determined the pressure dependence of the direct bandgap, ionicity, transverse effective charge and refractive index for CdTe. The study is performed using the sp³s ^* semi-empirical tight-binding method with the scaling law and Murnaghan`s equation of state. All these properties were found to have a sublinearity behaviour with the pressure. The results are compared with other works.     

Superconducting and normal state properties of tantalumcarbonitrides under hydrostatic pressure

We have investigated the formation and the normal- and superconducting properties of TaC_xN_y. By analogy to NbCN the superconducting transition temperature of the nitrogen-rich samples increases with pressure and decreases for carbon-rich samples. This is correlated with a different resistivity vs temperature behaviour.     

Electronic and optical properties of distorted rare-earth manganite under hydrostatic pressure

The effect of hydrostatic pressure on the crystal structure, electronic and optical properties of distorted rare-earth manganite TbMnO₃ has been studied on the basis of first-principle calculations. The results reveal that the band gaps reduce quadratically with increasing pressure. The optical properties of TbMnO₃ predict that the peaks in the dielectric function shift to higher photon energy due to the transformation of inner electronic states with increasing pressure. Otherwise, the peaks in the reflectivity spectra and loss function were also found to move to higher photon energy with pressure increasing and the relationships between the positions of these peaks and pressure can be fitted by third order polynomial expressions.     

Mechanical properties of single- and double-walled carbon nanotubes under hydrostatic pressure

Based on a molecular mechanics coupled with atomistic-based continuum theory, a closed-form formula is presented to examine the elastic properties of single- and double-walled carbon nanotubes subjected to hydrostatic pressure. Following the present model, the effects of the armchair and zigzag CNT structures on the pressure behavior are theoretically investigated. The computational result indicates that the bulk modulus is less sensitive to the chiral structures except for very small tube diameters. Moreover, closed-end nanotubes under hydrostatic pressure exhibit a larger bulking modulus than open ended nanotubes. The cap of the zigzag tubes has a larger effect on the bulk modulus when compared to the armchair tubes, especially in small diameter nanotubes. The predicted strain and the bulk modulus are in good agreement with existing theoretical results. PACS 61.46.+w; 62.20.Dc; 62.20.-x; 62.25.+g     

Photoluminescence of GaSb under hydrostatic pressure

Photoluminescence measurements on GaSb samples were carried out at low temperatures (2 – 5 K) and high pressures (0 – 9 kbar). The energy shift of the direct gap was determined: $\text{d}\text{E}{}^{\mathrm{\Gamma }}\text{/dP= 14.5± 0.3 meV/kbar}$. At pressures above 8 kbar the spectra showed additional structure from the indirect L-conduction band minima. The energy shift of the L-conduction bands were determined: $\text{d}\text{E}\text{L/d}\text{P}\text{= 5.5± 1. meV/kbar.}$ From these data the critical pressure for the inversion of the two conduction bands was calculated: $\text{p}{}^{\mathrm{h}}{}_{\mathrm{c}}\text{= 10.5 ± 1. kbar}$.     

Transport properties of topological nodal-line semimetal candidate CaAs_3 under hydrostatic pressure

We report the transport properties of the CaAs_3 single crystal, which has been predicted to be a candidate for topological nodal-line semimetals. At ambient pressure, CaAs_3 exhibits semiconducting behavior with a small gap, while in some crystals containing tiny defects or impurities, a large "hump" in the resistivity is observed around 230 K. By applying hydrostatic pressure, the samples appear to a tendency towards metallic behavior, but not fully metallized up to 2 GPa.Further high pressure studies are needed to explore the topological characteristics for CaAs_3.     

Photoluminescence properties of Sm-doped BaBr2 under hydrostatic pressure

Photoluminescence spectra of Sm²⁺-doped BaBr₂ have been measured under hydrostatic pressures up to 17 GPa at room temperature. In the low pressure range a red-shift of the broad 5d-4f transition of −145 cm⁻¹/GPa is observed. From 5 to 8 GPa a phase mixture of the initial orthorhombic phase and the high-pressure monoclinic phase gives rise to two 5d-4f bands, which are strongly overlapping. Above 8 GPa the crystal is completely transformed to its high-pressure phase where two different Sm²⁺ sites exist, but only one broad 5d-4f transition is detected. It exhibits a red-shift of −36 cm⁻¹/GPa. In addition, the line shifts of the ⁵D₀→⁷F_J (J=0, 1, 2) transitions are investigated. Linear shifts of −19 cm⁻¹/GPa for J=0, 2 and of −13 cm⁻¹/GPa for J=1 are observed in the pressure range from 0 to 5 GPa.     

Magnetic and structural phase transition of CeAg at hydrostatic pressure

Pressure effects on the Curie temperature and the crystallographic phase transition temperature of CeAg were determined from an electrical resistivity measurement from 4.2 to 300 K at hydrostatic pressures up to 5 kb. Results correspond to the alloying effect in Ce(Ag,In).     

Calculated optical properties of GaX (X=P,As, Sb) under hydrostatic pressure

The hydrostatic pressure dependence of the principal energy gaps and of the optical properties of GaX (X = P, As and Sb) has been calculated using the full potential-linearized augmented plane wave (FP-LAPW) method. The generalized gradient approximation (GGA) for the exchange and correlation potential is applied. Also, we have used the Engel–Vosko GGA formalism, which optimizes the corresponding potential for band-structure and the optical properties calculations. Structural properties such as equilibrium lattice constants, the bulk modulus, and its pressure derivatives were calculated for GaP, GaAs, and GaSb in the zinc-blende structure (ZB). We have found that the results of the structural properties calculations are in agreement with those of ab initio and experimental data. In general, the pressure dependence of the principal energy gaps is compared to other values. The same is for the pressure coefficient. However, for the same structure, the comparison of our results with those of experimental and theoretical calculations shows good agreement. On the other hand, the effect of the applied pressure is clearly seen in the optical properties especially near the energy transition regions.     

DLTS investigation of GaP under hydrostatic pressure

Abstract

Deep Level Transient Spectroscopy (DLTS) was applied to nitrogen related deep electron trap 0.4 eV in green emitting diodes of GaP under hydrostatic pressure. The pressure coefficient of the level energy is determinated as equal -31 meV/kbar with respect to the valence band edge.     

Electrophysical properties of composite films based on multiwalled carbon nanotubes under hydrostatic pressure

The effect hydrostatic pressure of up to 9 GPa has on the electrical behavior of polymer composites based on multiwalled carbon nanotubes (MWCNT) is investigated at room temperature. The ratio R _9GPa/R ₀ ≈ 7 for samples with resistance in the kilo-ohm range is higher than for samples in the ohm range. Pressure-induced MWCNT features at P ≈ 1.5 GPa are associated with the deformation of the circular form of nanotube walls, in agreement with theoretical predictions. Measurements of direct and reverse resistance and current voltage characteristics under pressure demonstrate the reversibility of electrophysical properties, which could find practical application in creating pressure sensors.     

Electrical properties of TlGaTe2 single crystals under hydrostatic pressure

The effect of hydrostatic pressure (up to 0.82 GPa) on the electric properties of chain TlGaTe₂ single crystals has been investigated in the temperature range 77-296 K. It has been shown that pressure leads to a considerable increase of conductivity (σ_⊥) across the chains of TlGaTe₂ single crystals. Parameters of localized states in the band gap of TlGaTe₂ single crystal according to the low-temperature electrical measurements were obtained at various pressures.     

Photoluminescence on CdSe and CdTe under hydrostatic pressure

Photoluminescence measurements on CdSe and CdTe samples have been made under hydrostatic pressure up to the phase transition. Transition pressure values are reported. Nonlinear pressure dependence of the direct gap is observed for both materials. Pressure coefficients are in good agreement with values calculated from the dielectric theory of the chemical Bond.     

Tunable InSb detectors under uniaxial and hydrostatic pressure

We report on n-InSb photoconductive detectors, which are either mounted on different substrates (metals and synthetic materials) or embedded in a special glue. If these detectors are cooled to 4.2 K, the different coefficients of expansion provide either a uniaxial or a hydrostatic pressure. This pressure causes a shift of the resonances (for the mounted detectors up to 5.5%, for the embedded detector 5.8% at 2.3 T) and a freeze out of electrons, which results in a decrease of the cyclotron resonance (CR) signal in comparison to the impurity cyclotron resonance (ICR) signal for the embedded detector.     

Electroreflectance in GeSi alloys under hydrostatic pressure

From the electroreflectance spectra measured under hydrostatic pressure to 7 kbar we have determined the pressure coefficients for germanium (dE₁/dP = 7.8 ± 0.4⁻⁶eV/bar, dP = 1.4 ± 0.8 10⁻⁶eV/bar), for Si (dE′_o/dP = 1 ± 1 10⁻⁶eV/bar, dE₁dP = 6.2 ± 0.4 10⁻⁶eV/bar) and for GeSi alloys in the entire composition region. For the composition 80–100% of Si which is widely discussed in the literature, we could distinguish two maxima with substantially different pressure coefficients. The absolute experimental values of dE/dP agree rather well with theoretical values which, together with composition shift of electroreflectance peaks, enable us to connect the peak E₁ predominantly with λ and L and E′_o with Г and Δ transitions in the entire composition region.     

Raman scattering in GaTe under hydrostatic pressure

Raman-active lattice vibrational modes of GaTe have been investigated at 300 K in the range 15–300 cm^?1 in equilibrium conditions under different hydrostatic pressures up to 11.25 kbar. The spectra of the Bridgman grown crystals were excited with the 1.06 μm line of the continuously operated YAG:Nd³⁺ laser. The mode-Grüneisen parameters $\text{Γ}{}_{\mathrm{j}}\text{= (}\text{1}\text{β}\text{)(}\text{1}\text{ν}{}_{\mathrm{j}}\text{) =}\text{dν}{}_{\mathrm{j}}\text{d}{}_{\mathrm{p}}$ were determined for all fourteen Raman bands observed. It is shown that there is no low frequency rigid-layer modes or Davydov pairs in Raman spectra of GaTe crystals.