Pressure-induced Electronic Transitions Research Articles

Pressure dependent thermoreflectance spectroscopy induced by interband transitions in metallic nano-film

SummaryUtilizing high-pressure to modulate optical properties, such as thermoreflectance (dR/dT), over a wide range has received much attention. Nevertheless, how the pressure exerts on the complex dielectric constant and finally on dR/dT remains elusive. Here, we perform a thoroughly experimental and theoretical investigation on dR/dT of Al nano-film from 0 to 25 GPa. The dR/dT values exhibit a sine-like pressure-dependence, with the zero-crossing appearing at around 6 GPa. These special phenomena are well explained from electron transition viewpoints. The first-principles calculations show that the energy difference of parallel bands is enlarged from 1.45 to 2 eV, thereby increasing the threshold for electron transitions. The lifted threshold changes the optical absorption rates of Al and the density of states of the electrons involving interband transitions; finally, the resulting dR/dT exhibits such a pressure-dependent behavior. Our findings provide a deep insight on pressure-induced electronic transitions and photon-electron interactions in metals.

Pressure Dependent Thermoreflectance Spectroscopy Induced by Interband Transitions in Metallic Nano-Film

Utilizing high-pressure to modulate optical properties, such as thermoreflectance (dR/dT), over a wide range has received much attention. Nevertheless, how the pressure exerts on the complex dielectric constant and finally on dR/dT remains elusive. Here, we perform a thoroughly experimental and theoretical investigation on dR/dT of Al nano-film from 0 to 25 GPa. The dR/dT values exhibit a sine-like pressure-dependence, with the zero-crossing appearing at around 6 GPa. These special phenomena are well explained from electron transition viewpoints. The first-principles calculations show that the energy difference of parallel bands is enlarged from 1.45 to 2 eV, thereby increasing the threshold for electron transitions. The lifted threshold changes optical absorption rates of Al and the density of states of electrons involving interband transitions, finally resulting in that dR/dT exhibits such a pressure-dependent behavior. Our findings provide a deep insight on pressure-induced electronic transitions and photon-electron interactions in metals.

Phase stability and electronic structure of iridium metal at the megabar range

The 5d transition metals have attracted specific interest for high-pressure studies due to their extraordinary stability and intriguing electronic properties. In particular, iridium metal has been proposed to exhibit a recently discovered pressure-induced electronic transition, the so-called core-level crossing transition at the lowest pressure among all the 5d transition metals. Here, we report an experimental structural characterization of iridium by x-ray probes sensitive to both long- and short-range order in matter. Synchrotron-based powder x-ray diffraction results highlight a large stability range (up to 1.4 Mbar) of the low-pressure phase. The compressibility behaviour was characterized by an accurate determination of the pressure-volume equation of state, with a bulk modulus of 339(3) GPa and its derivative of 5.3(1). X-ray absorption spectroscopy, which probes the local structure and the empty density of electronic states above the Fermi level, was also utilized. The remarkable agreement observed between experimental and calculated spectra validates the reliability of theoretical predictions of the pressure dependence of the electronic structure of iridium in the studied interval of compressions.

Higher superconducting transition temperature by breaking the universal pressure relation

By investigating the bulk superconducting state via dc magnetization measurements, we have discovered a common resurgence of the superconducting transition temperatures (Tcs) of the monolayer Bi2Sr2CuO6+δ (Bi2201) and bilayer Bi2Sr2CaCu2O8+δ (Bi2212) to beyond the maximum Tcs (Tc-maxs) predicted by the universal relation between Tc and doping (p) or pressure (P) at higher pressures. The Tc of underdoped Bi2201 initially increases from 9.6 K at ambient to a peak at 23 K at 26 GPa and then drops as expected from the universal Tc-P relation. However, at pressures above 40 GPa, Tc rises rapidly without any sign of saturation up to 30 K at 51 GPa. Similarly, the Tc for the slightly overdoped Bi2212 increases after passing a broad valley between 20 and 36 GPa and reaches 90 K without any sign of saturation at 56 GPa. We have, therefore, attributed this Tc resurgence to a possible pressure-induced electronic transition in the cuprate compounds due to a charge transfer between the Cu 3[Formula: see text] and the O 2p bands projected from a hybrid bonding state, leading to an increase of the density of states at the Fermi level, in agreement with our density functional theory calculations. Similar Tc-P behavior has also been reported in the trilayer Br2Sr2Ca2Cu3O10+δ (Bi2223). These observations suggest that higher Tcs than those previously reported for the layered cuprate high-temperature superconductors can be achieved by breaking away from the universal Tc-P relation through the application of higher pressures.

Pressure-Induced Electronic Transition in Black Phosphorus.

In a semimetal, both electrons and holes contribute to the density of states at the Fermi level. The small band overlaps and multiband effects engender novel electronic properties. We show that a moderate hydrostatic pressure effectively suppresses the band gap in the elemental semiconductor black phosphorus. An electronic topological transition takes place at approximately 1.2GPa, above which black phosphorus evolves into a semimetal state that is characterized by a colossal positive magnetoresistance and a nonlinear field dependence of Hall resistivity. The Shubnikov-de Haas oscillations detected in magnetic field reveal the complex Fermi surface topology of the semimetallic phase. In particular, we find a nontrivial Berry phase in one Fermi surface that emerges in the semimetal state, as evidence of a Dirac-like dispersion. The observed semimetallic behavior greatly enriches the material property of black phosphorus and sets the stage for the exploration of novel electronic states in this material.

High-pressure single-crystal elasticity study of CO2 across phase I-III transition

Sound velocities and elastic moduli of solid single-crystal CO2 were measured at pressures up to 11.7(3) GPa by Brillouin spectroscopy. The aggregate adiabatic bulk modulus (KS), shear modulus (G), and their pressure derivatives for CO2 Phase I are KS0 = 3.4(6) GPa, G0 = 1.8(2) GPa, (dKS/dP)0 = 7.8(3), (dG/dP)0 = 2.5(1), (d2KS/dP2)0 = −0.23(3) GPa−1, and (d2G/dP2)0 = −0.10(1) GPa−1. A small increase of elastic properties was observed between 9.8(1) and 10.5(3) GPa, in agreement with the CO2 I-III transition pressure determined from previous x-ray diffraction experiments. Above the transition pressure PT, we observed a mixture dominated by CO2-I, with minor CO2-III. The CO2-I + III mixture shows slightly increased sound velocities compared to pure CO2-I. Elastic anisotropy calculated from the single-crystal elasticity tensor exhibits a decrease with pressure beginning at 7.9(1) GPa, which is lower than PT. Our results coincide with recent X-ray Raman observations, suggesting that a pressure-induced electronic transition is related to local structural and optical changes.

The Mott insulators at extreme conditions; structural consequences of pressure-induced electronic transitions

Electronic/magnetic transitions and their struc- tural consequences in Fe-based Mott insulators in a re- gime of very high static density are the main issue of this short review paper. The paper focuses on the above-men- tioned topics based primarily on our previous and on- going experimental HP studies employing: (i) diamond anvil cells, (ii) synchrotron X-ray diffraction, (iii) 57 Fe Mossbauer spectroscopy, (iv) electrical resistance and (v) X-ray absorption spectroscopy. It is shown that applying pressure to such strongly correlated systems leads to a number of changes; including quenching of the orbital moment, quenching of Jahn-Teller distortion, spin cross- over, inter-valence charge transfer, insulator-metal tran- sition, moment collapse and volume collapse. These changes may occur simultaneously or sequentially over a range of pressures. Any of these may be accompanied by or be a consequence of a structural phase transition; namely, a change in crystal symmetry. Analyzing this rich variety of phenomena we show the main scenarios which such strongly correlated systems may undergo on the way to a correlation breakdown (Mott transition). To illustrate these scenarios we present recent results for MFeO3 (M ¼ Fe, Ga, Lu, Eu, Pr) and CaFe2O4 ferric oxi- des; FeCl2 and FeI2 ferrous halides, and FeCr2S4 sulfide. Fe3O4 is given as an example case for the impact of Mossbauer Spectroscopy on High Pressure Crystallogra- phy studies.

Structural consequences of pressure-induced electronic transitions in iron compounds

Structural consequences of pressure-induced electronic transitions in iron compounds

Effect of pressure on the superconducting Tc of MgB 2

Measurements of the superconducting transition temperature T c have been performed by the inductive method on MgB 2 at pressures up to 28 GPa. T c decreases with applied pressure, exhibiting a cusp at about 9 GPa. We interpret the appearance of this anomaly in the T c( P) dependence as a result of the pressure-induced electronic transition. Recent band-structure calculations for MgB 2 give some support for such an explanation.

High-pressure structural study of UM2Al3 (M = Pd,Ni): evidence for a pressure-induced electronic transition inUPd2Al3

High-pressure x-ray diffraction studies on the magnetically orderingheavy-fermion superconductors UM2Al3 (M = Pd, Ni) havebeen performed. For UNi2Al3 only a smooth reduction of thepressure dependence of the unit-cell volume was observed. In thecase of UPd2Al3 a structural phase transition takes placearound p≈25 GPa from the high-symmetry hexagonalPrNi2Al3-type low-pressure structure into a low-symmetryorthorhombic high-pressure phase. The new phase is characterized byspace group Pmmm and closely related to the PrNi2Al3structure at ambient pressure. Its compressibility is about twicethat of the low-pressure phase. The remarkable high-pressurebehaviour is discussed with respect to an electronic transitionrelated to the structural high-pressure phase transition.

Pressure-induced electronic transition in Ce Al2

The room-temperature resistance of CeAl2 passes through a faint maximum around 60 kBar and falls off at higher pressure. Strong anomalies were previously reported for the temperature-dependence of the resistivity under pressure up to 15 kBar by Nicolas-Francillon et al. It has been found that these anomalies gradually disappear with even higher pressures. They are entirely suppressed at pressures exceeding ≃120 kBar resulting in a R-T characteristic being very similar to that of the nonmagnetic compound LaAl2. There is a striking similarity to the R vs. T behavior of pure cerium in the β or γ phase on the one hand and in the a phase on the other hand. We conclude that CeAl2 undergoes a continuous electronic transition with pressure just as Ce above the critical point.

Abstract: Pressure-induced electronic transitions in the mixed compounds Sm1−xGdxS

Abstract: Pressure-induced electronic transitions in the mixed compounds Sm1−<i>x</i>Gd<i>x</i>S

Hugoniot equation of state of the lanthanides

Shock wave studies of the lanthanide series show the existence of a very high pressure phase transition in all members of the series. The data show that this transformation is of necessity to a more incompressible phase and has been identified with melting. Thermodynamic considerations allow calculation of the solid-liquid phase boundary from these data; the results indicate that all the rare earths melt anomalously at sufficiently high pressures. This can be understood in the context of a “two-fluid” theory, in which the composition of the liquid along the phase boundary changes continuously with pressure due to the degree of pressure-induced electronic transition present in the liquid. Hence, at sufficiently high pressure, the density of the liquid becomes greater than the density of the contiguous solid and dP dT becomes negative.

Optical Versus Thermal Transitions in Solids at High Pressure

Pressure-induced electronic transitions have been observed in a wide variety of materials. In particular, Mössbauer resonance studies under high pressure have revealed changes of oxidation state and spin state of iron as a function of pressure. These processes involve the thermal transfer of an electron to a new ground state of the system. The difference in energy between states is frequently measured by optical absorption. In this paper, we relate the energy of the optical absorption peak and its half-width to the difference in thermal energy between the two states as a function of pressure. We show that the optical peak will broaden with pressure only if there is a difference between the force constants of the ground and excited states. These relationships permit the prediction from optical absorption data of the pressure at which a new ground state will be obtained. We demonstrate that the predictions are qualitatively correct for the reduction of Fe(III) in a series of ferric hydroxamates, and for the low-spin to high-spin transition of Fe(II) in ferrous phenanthroline complexes.

Band structure and pressure-induced electronic transitions in calcium

The band structure of calcium has been determined at normal and reduced lattice spacings by a self-consisten APW method using a ‘soft-core’ approximation and Gaspar-Kohn-Sham exchange potential. It was found that calcium changes from normal metal to semimetal and back to normal metal as the lattice constant decreases, in good agreement with high-pressure experiments.

Shock Compression of Argon

Liquid argon has been shock compressed from two initial states at 86°K and 2 bar, and 148.2°K and 70 bar. The highest pressure of 700 kbar and temperature of 13 000°K were obtained by reflecting a shock wave with a pressure of 270 kbar from a tungsten wall. These shock loci allow a test of the interatomic potential over a wide region and show that the potential can be represented by a sum of pair interactions up to a pressure of 400 kbar. The pair potential is furthermore found to be considerably less repulsive at small interatomic distances than the 12-power repulsive law. In fact, an exponential form yields good results up to 360 kbar. Above this pressure range the reflected-shock data require a large change in the potential form which is interpreted to be due to a pressure-induced electronic transition to the conduction band. Calculations show that a discontinuity in the Hugoniot data at 50 kbar and 1200°K could be identified with the melting line. This temperature corresponds to three times the highest reduced melting temperature observed so far.