Pressure Dependence Of Electronic Structure Research Articles

Structural correlations in $Cs_2CuCl_4$: Pressure dependence of electronic structures

We have investigated the crystal structure of $Cs_2CuCl_4$ in the 0-20 GPa range as a function of pressure and how pressure affects its electronic properties by means of optical absorption spectroscopy. In particular, we focused on the electronic properties in the low-pressure Pnma phase, which are mainly related to the tetrahedral $CuCl_4^{2-}$ units distorted by the Jahn-Teller effect. This study provides a complete characterization of the electronic structure of $Cs_2CuCl_4$ in the Pmna phase as a function of the cell volume and the $Cu-Cl$ bond length, $R_{Cu-Cl}$. Interestingly, the opposite shift of the charge-transfer band-gap and the $Cu^{2+}$ d-d crystal-field band shift with pressure are responsible for the strong piezochromism of $Cs_2CuCl_4$. We have also explored the high-pressure structure of $Cs_2CuCl_4$ above 4.9 GPa yielding structural transformations that are probably associated with a change of coordination around $Cu^{2+}$. Since the high-pressure phase appears largely amorphized, any structural information from X-ray diffraction is ruled out. We use electronic probes to get structural information of the high-pressure phase.
 Edited by: A. Goñi, A. Cantarero, J. S. Reparaz

Pressure-driven 5f localized-itinerant transition and valence fluctuation in cubic phase californium

A combination of the density functional theory and the single-site dynamical mean-field theory is employed to study the pressure dependence of electronic structure for cubic phase californium. We predict that its 5$f$ electrons could undergo an orbital-selective localized-itinerant transition under moderate pressure. The volume contraction causes remarkable charge redistribution and valence fluctuation behaviors, which are the driving forces of the divalent-trivalent transition. Additionally, we find that the angular momentum coupling mechanism is hardly affected by pressure. The 5$f$ orbital occupancy is well described by the intermediate coupling scheme.

Interesting pressure dependence of power factor in BiTeI

We investigate pressure dependence of electronic structures and thermoelectric properties in BiTeI by using a modified Becke and Johnson exchange potential. Spin–orbit coupling (SOC) effects are also included due to giant Rashba splitting. Thermoelectric properties are illuminated through solving Boltzmann transport equations within the constant scattering time approximation. The calculated energy band gap of 0.36 eV agrees well with the experimental value of 0.38 eV. As the pressure increases, the energy band gap first decreases, and then increases. The Rashba energy has the opposite trend with the energy band gap. SOC has obvious detrimental influence on the power factor in both n-type and p-type doping. For low doping concentration, the power factor has the same trend with the energy band gap with increasing pressure, but shows a monotonic changing trend in high doping. It is found that the pressure can induce a significantly enhanced power factor in high n-type doping, which can be understood as pressure leading to two-dimensional-like density of states in the conduction bands. These results suggest that BiTeI may be a potential candidate for efficient thermoelectricity in n-type doping by pressure, turning an ordinary insulator into a topological insulator.

Pressure dependence of electronic structure and superconductivity of the MnX (X = N, P, As, Sb).

A recently experimental discovered (Cheng et al., Phys. Rev. Lett. 114, 117001 (2015)) of superconductivity on the border of long-range magnetic order in the itinerant-electron helimagnet MnP via the application of high pressure makes MnP the first Mn-based superconductor. In this paper, we carry out first-principles calculations on MnX (X = N, P, As, Sb) and find superconducting critical temperature TC of MnP sharply increases near the critical pressure PC ≈ 8 GPa, which is in good agreement with the experiments. Electron-phonon coupling constant λ and electronic density of states at the Fermi level N (EF) are found to increase with pressure for MnP, which lead to the increase of TC of MnP. Moreover, we also find that the TC of MnAs and MnSb are higher than MnP, implying that the MnAs and MnSb may be the more potential Mn-based superconducting materials.

Pressure dependence of electronic structure and magnetic properties in Fe 16N 2

The electronic structures and magnetic properties of Fe 16N 2 system and their pressure dependence were investigated by using first-principles calculations based on the density functional theory. It has been found that the total magnetic moment in Fe 16N 2 system decreases monotonically as increasing pressure from 0 to 14.6 GPa. A phase transition from ferromagnetic (FM) to non-magnetic (NM) occurs with a volume collapse of around 0.008 nm 3 at 14.6 GPa, The lattice constants a and c for magnetic results decrease monotonically as pressure increasing from 0 to 14.6 GPa, at 14.6 GPa, the lattice constant a decreases sharply, on the contrary, the lattice constant c increases abruptly. We think that the change of microscopic structure of Fe 16N 2 is responsible for the phase transition from FM to NM.

Pressure dependence of the electronic structure and spin state in Fe1.01Se superconductors probed by x-ray absorption and x-ray emission spectroscopy

Pressure dependence of electronic structures and spin states of iron-chalcogenide Fe${}_{1.01}$Se superconductors up to \ensuremath{\sim}66 GPa has been investigated with x-ray emission spectra and x-ray absorption spectra with partial-fluorescence yield. The intensity of the pre-edge peak at energy of \ensuremath{\sim}7112.7 eV of the Fe K-edge x-ray absorption spectrum of Fe${}_{1.01}$Se decreases progressively with pressure up to \ensuremath{\sim}10 GPa. A new prepeak at energy of \ensuremath{\sim}7113.7 eV develops for pressure above \ensuremath{\sim}13 GPa, indicating formation of a new phase. The experimental and the calculated Fe K-edge absorption spectra of Fe${}_{1.01}$Se using the FDMNES code agree satisfactorily. The larger compression accompanied by significant distortion around the Fe atoms along the c axis in Fe${}_{1.01}$Se upon applying pressure suppresses the Fe 3d-Se 4p and Fe 4p-Se 4d hybridization. The applied pressure suppresses the nearest-neighbor ferromagnetic superexchange interaction and enhances spin fluctuations on the Fe sites in Fe${}_{1.01}$Se. A discontinuous variation of the integrated absolute difference values of the K\ensuremath{\beta} emission line was observed, originating from a phase transition of Fe${}_{1.01}$Se for a pressure >12 GPa. Fe${}_{1.01}$Se shows a small net magnetic moment of Fe${}^{2+}$ at ambient pressure, probably arising from strong Fe-Fe spin fluctuations. The satellite line K\ensuremath{\beta}\ensuremath{'} was reduced in intensity upon applying pressure and became absent for pressure >52 GPa, indicating a continuous reduction of the spin moment of Fe in Fe${}_{1.01}$Se superconductors. The experimental results provide insight into the spin state of Fe${}_{1.01}$Se superconductors under pressure.

Pressure dependence of superconducting transition temperature of MgB2 up to 8 GPa

AbstractThe pressure dependence of electronic structure, phonon frequencies, electron–phonon coupling constant, and the superconducting transition temperature Tc in MgB2 are calculated via zone‐center frozen phonon modes calculations using the FP‐LAPW method. The calculations are performed from pressure dependent lattice parameters measured experimentally up to 8 GPa. Under quasi‐hydrostatic pressure, Tc is found to decrease with an approximately quadratic dependence, deviating significantly from the linearity. It is shown that the pressure‐induced anisotropic change of lattice parameters, the increasing E2g phonon frequency, and nonlinear electron–phonon coupling are three key factors of producing nonlinear Tc(P) observed in MgB2. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Pressure dependence of the Curie temperature inNi2MnSnHeusler alloy: A first-principles study

The pressure dependence of electronic structure, exchange interactions and Curie temperature in ferromagnetic Heusler alloy Ni2MnSn has been studied theoretically within the framework of the density-functional theory. The calculation of the exchange parameters is based on the frozen--magnon approach. The Curie temperature, Tc, is calculated within the mean-field approximation by solving the matrix equation for a multi-sublattice system. In agrement with experiment the Curie temperature increased from 362K at ambient pressure to 396 at 12 GPa. Extending the variation of the lattice parameter beyond the range studied experimentally we obtained non-monotonous pressure dependence of the Curie temperature and metamagnetic transition. We relate the theoretical dependence of Tc on the lattice constant to the corresponding dependence predicted by the empirical interaction curve. The Mn-Ni atomic interchange observed experimentally is simulated to study its influence on the Curie temperature.