Effect of spin crossovers on the Mott-Hubbard transition at high pressures

The effect of spin crossovers in d _n terms on the effective Hubbard parameter (U _eff) determining the gap between the lower and upper Hubbard bands is analyzed using a many-electron approach to describe the electron structure of Mott insulators. A new mechanism of the insulator-metal transition is established for d ⁵ ions, which is related to a decrease in U _eff caused by spin crossover. For other ions, the U _eff value is either independent of pressure (d ², d ⁴, d ⁷) or it exhibits nonmonotonic growth (d ³, d ⁶, d ⁸).     

Luminescence study of the 4f5d configuration of Nd in LiYF4, LiLuF4 and BaY2F8 crystals

Ultraviolet fluorescence of Nd³⁺ ions induced by triphotonic excitation process was studied in Nd-doped LiYF₄, LiLuF₄ and BaY₂F₈ crystals using a technique of time-resolved spectroscopy. The observed ultraviolet luminescence was due to transitions between the bottom of 4f²5d configuration and 4f³ states of Nd³⁺ ions. Narrow emission lines superposed to the broadband emissions were observed. A detailed analysis of luminescence spectrum revealed that the narrow emissions are due to parity and spin allowed radiative transitions from the Stark levels of ⁴K_11/2(5d) state created by the electrostatic interaction between the 5d electron and the two electrons of the 4f² configuration. The narrow emissions are related to the high spin state (S=3/2) which gives f-f characteristics to the f-d broadband emissions. The narrow emissions superposed to the wide emission correspond to 18%, 34% and 43% of the integrated broadband emission at 262 nm observed in LiYF₄, LiLuF₄ and BaY₂F₈ crystals, respectively. Although the 5d-4f² interaction is observed to be weaker than 5d-crystal field interaction, it is stronger enough to select only the radiative transitions from 4f²5d configuration to 4f³ states that preserves the total spin S=3/2.     

Spin crossovers in Mott–Hubbard insulators at high pressures

The high-pressure induced phase transitions initiated by electronic transition in 3d ions from the high-spin (HS) to the low-spin (LS) state (HS-LS spin-crossover) are considered. Behavior of the system with d⁶ electronic configuration is investigated in the ground state of zero temperature and critical pressure P_c. Magnetic properties of the Mott–Hubbard insulator (Mg_1−xFe_x)O are studied in the vicinity of the quantum critical point (T=0, P_c). At the critical pressure of spin crossover P_c, the spin gap energy ε_S between HS and LS states is zero. The quantum spins fluctuations HS⇔LS do not require any energy, and the antiferromagnetism is destroyed in the quantum critical point by the first order transition.     

Pressure and temperature induced modification of luminescence from tetracene single crystals

Fluorescence spectra of crystalline tetracene have been recorded in the temperature range 120 to 300 K under hydrostatic pressure up to 600 MPa. From discontinuities in both emission spectra and spectral intensities it is concluded that two phase transitions occur. The room temperature phase is transformed to a low temperature phase/high pressure phase I at T^I_t (p = 0) = 182 K, the temperature coefficient being dT^I_t/dp = 0.395 K/MPa. The phase transition is induced by a decrease of the specific volume under pressure and/or upon cooling. Lack of a significant shift of the origin of the fluorescence band near T^I_t at constant pressure is an artifact resulting from the neglect of reabsorption effects. The Stokes shift is 260 cm^-1, independent of temperature and crystal modification. In accord with previous Raman data a second phase transition occurs at T^II_t (p) = 143 K, the pressure shift being dT^II_t/dp = 0.088 K/MPa.In addition, the shift of the triplet energy as a function of pressure as well as the pressure-dependence of the rate constants governing fission of a singlet exciton into a pair of triplets is discussed utilizing their magnetic field dependences.     

Pressure and temperature induced high spin–low spin phase transition: Macroscopic and microscopic consideration

The behavior under pressure of the high spin–low spin phase transition in the coordination compounds containing 3d ions is analyzed using thermodynamic and microscopic approaches. For thermodynamic approach the mean field model with interactions between spin-crossover molecules is considered. Microscopic model takes into account the interaction of d electrons of the transition metal ions with full symmetric distortions of the ligands. The relationship of the thermodynamic interaction parameters with microscopic ones is installed and shown how the quantum–mechanical interactions form the cooperativity of the system. Within the microscopic model the temperature and pressure dependences of the high spin fraction in 2-D compounds {Fe(3-Fpy)₂[M(CN)₄]} (M=Pd, Pt) are simulated and microscopic parameters are evaluated. It is concluded that different experimental behaviors of the temperature and pressure induced spin transitions are determined by different variations of the inelastic and elastic energies under pressure, and vibrational component of the free energy drives the ST equally with electronic part.     

The mesoscopic chiral metal-insulator transition

Sharp localization transitions of chiral edge states in disordered quantum wires subject to a strong magnetic field are shown to be driven by crossovers from two-to one-dimensional localization of bulk states. As a result, the two-terminal conductance is found to exhibit discontinuous transitions at zero temperature between exactly integer plateau values and zero, reminiscent of first-order phase transitions. We discuss the corresponding phase diagram. The spin of the electrons is shown to result in a multitude of phases when the spin degeneracy is raised by the Zeeman energy. The width of conductance plateaus is found to depend sensitively on the spin flip rate 1/τ_s.     

The vapor-phase Raman spectra and the ring-puckering vibration of some deuterated analogs of trimethylene oxide

The ring-puckering vibration has been observed in the Raman spectra of the vapor phase of several deuterated species of trimethylene oxide (α - d₂, β - d₂, α, α′ - d₄, and d₆). Only Δv = 2 transitions have a measurable Raman intensity. The line positions agree with infrared values to better than 0.5 cm⁻¹ in most cases and better than 1 cm⁻¹ in all cases. For all molecules the interpretation of the spectra is consistent with an essentially planar configuration having only a small barrier to inversion.     

Mutual Composite Fermion and Composite Boson approaches to balanced and imbalanced bi-layer quantum Hall system: An electronic analogy of the Helium 4 system

We use both Mutual Composite Fermion (MCF) and Composite Boson (CB) approach to study balanced and imbalanced Bi-layer Quantum Hall systems (BLQH) and make critical comparisons between the two approaches. We find the CB approach is superior to the MCF approach in studying ground states with different kinds of broken symmetries. In the phase representation of the CB theory, we first study the Excitonic superfluid (ESF) state. The theory puts spin and charge degree freedoms in the same footing, explicitly bring out the spin-charge connection and classify all the possible excitations in a systematic way. Then in the dual density representation of the CB theory, we study possible intermediate phases as the distance increases. We propose there are two critical distances d_c1 < d_c2 and three phases as the distance increases. When 0 < d < d_c1, the system is in the ESF state which breaks the internal U(1) symmetry, when d_c1 < d < d_c2, the system is in an pseudo-spin density wave (PSDW) state which breaks the translational symmetry, there is a first-order transition at d_c1 driven by the collapsing of magneto-roton minimum at a finite wavevector in the pseudo-spin channel. When d_c2 < d < ∞, the system becomes two weakly coupled ν = 1/2 Composite Fermion Fermi Liquid (FL) state. There is also a first-order transition at d = d_c2. We construct a quantum Ginzburg Landau action to describe the transition from ESF to PSDW which break the two completely different symmetries. By using the QGL action, we explicitly show that the PSDW takes a square lattice and analyze in detail the properties of the PSDW at zero and finite temperature. We also suggest that the correlated hopping of vacancies in the active and passive layers in the PSDW state leads to very large and temperature-dependent drag consistent with the experimental data. Then we study the effects of imbalance on both ESF and PSDW. In the ESF side, the system supports continuously changing fractional charges as the imbalance changes. In the PSDW side, there are two quantum phase transitions from the commensurate excitonic solid to an incommensurate excitonic solid and then to the excitonic superfluid state. We also comment on the effects of disorders and compare our results with the previous work. The very rich and interesting phases and phase transitions in the pseudo-spin channel in the BLQH is quite similar to those in ⁴He system with the distance playing the role of the pressure. A BLQH system in a periodic potential is also discussed. The Quantum Hall state to Wigner crystal transition in single layer Quantum Hall system is studied.     

Quasi-cw laser action from Hg-III lines

Quasi-cw operation of three laser lines (λ = 650.1 nm, 521.0 nm, 479.7 nm) has been achieved in a pure mercury, low pressure discharge. These lines belong to the Hg-III 5d⁸6s²-5d⁹6p transitions. An output power of 40 W at 650.1 nm, 25 W at 479.7 nm and 5 W at 521.0 nm was observed.     

Low temperature phase diagrams for quantum perturbations of classical spin systems

We consider a quantum spin system with Hamiltonian $$H = H^{(0)} + \lambda V,$$ whereH ⁽⁰⁾ is diagonal in a basis ∣s〉=? _x ∣s _x 〉 which may be labeled by the configurationss={s_x} of a suitable classical spin system on ? ^d , $$H^{(0)} |s\rangle = H^{(0)} (s)|s\rangle .$$ We assume thatH ⁽⁰⁾(s) is a finite range Hamiltonian with finitely many ground states and a suitable Peierls condition for excitation, whileV is a finite range or exponentially decaying quantum perturbation. Mapping thed dimensional quantum system onto aclassical contour system on ad+1 dimensional lattice, we use standard Pirogov-Sinai theory to show that the low temperature phase diagram of the quantum spin system is a small perturbation of the zero temperature phase diagram of the classical HamiltonianH ⁽⁰⁾, provided λ is sufficiently small. Our method can be applied to bosonic systems without substantial change. The extension to fermionic systems will be discussed in a subsequent paper.     

A CCD based detector for quick detection of pressure induced phase transitions

Abstract

A high sensitivity CCD based two dimensional angle dispersive X-ray are a detector has been developed for quick detection of pressure induced phase transitions for a laboratory X-ray source such as a rotating anode generator. The performance of this detector was tested by successfully carrying out powder X-ray diffraction measurements on element Pd, intermetallics AuIn², AuGa² and low Z scatterer adamantane (C₁₀H_l6) at ambient conditions. Its utility for quick detection of phase transitions at high pressures with diamond anvil cell (DAC) is demonstrated by reproducing the known pressure induced structural phase transitions in RbI and KI. The importance of this detector system in search of unknown phase transitions has been established by observing new structural phase transitions in In_0.25Sn_0.75 and AuGa₂. Various softwares have also been developed such as interactive location of centre of diffraction rings, radial integration and image enhancement to analyze data from this detector.     

Hyperfine-structure and isotope-shift measurements in the 6s 5d↔6p 5d transitions of Ba I in the far-red spectral region

Measurements of the hyperfine structures in the 6p5d ¹ D ₂,³ D ₁ and³ F _{2, 3, 4} states of¹³⁵Ba and¹³⁷Ba, and isotope-shifts in several far-red transitions between the 6s 5d and 6p 5d configurations, as well as the transition 6s ^{2 1} S ₀→6s6p ³ P ₁ at 7,911 Å have been performed using high-resolution laser spectroscopy on a collimated atomic beam of natural barium. An analysis of the magnetic-dipole interaction in the 6p 5d configuration using effective one- and two-body hyperfine operators is presented. In particular the contact interaction was studied with respect to the correlation between the two valence electrons. Effects of strong configuration interaction were found. From a King-plot analysis of the isotope shift term- andJ-dependence of the field shift have been evaluated for the transitions between the 6s 5d and 6p 5d configurations. Relativistic Hartree-Fock (RHF) calculations have been performed of electron densities at the nucleus for six different configurations in Ba I and Ba II. The RHF calculations reproduce the experimental King-plot slopes quite well, while the absolute values, of the changes in electron density at the nucleus for the studied transitions, are found to be 9% lower than the results derived from a muonic experiment.     

The thermodynamic limit for long-range random systems

Long-range spin systems with random interactions are considered. A simple argument is presented showing that the thermodynamic limit of the free energy exists and depends neither on the specific random configuration nor on the sample shape, provided there is no external field. The argument is valid for both classical and quantum spin systems, and can be applied to (a) spins randomly distributed on a lattice and interacting via dipolar interactions; and (b) spin systems with potentials of the formJ(x ₁,x ₂)/|x ₁ -x ₂| ^αd , where theJ(x ₁,x ₂) are independent random variables with mean zero,d is the dimension, and α > 1/2. The key to the proof is a (multidimensional) subadditive ergodic theorem. As a corollary we show that, for random ferromagnets, the correlation length is a nonrandom quantity.     

Magnetic collapse and the change of electronic structure of FeBO3 antiferromagnet under high pressure

The effect of high pressures up to 60 GPa on single-crystal and polycrystalline samples of iron borate ⁵⁷FeBO₃ was studied by Mössbauer absorption spectroscopy (⁵⁷Fe nuclei) in a diamond anvil cell. Magnetic field H _hf at the ⁵⁷Fe nuclei increases with pressure but abruptly drops to zero at 46±2 GPa, indicating the crystal transition from the antiferromagnetic to nonmagnetic state. This is accompanied by an abrupt change in the isomer shift and quadrupole splitting. Their values in the high-pressure phase are evidence for the transition of Fe⁺³ ions from a high-spin (S=5/2, ⁶ A _1g ) to low-spin (S=1/2, ⁶ T2g) state (spin crossover). This correlates with an abrupt decrease in the unit-cell volume (by ～9%) and optical gap. The change of the magnetic and electronic structures is explained by Mott’s transition with rupturing of strong d-d-electron correlations.     

New half-metallic ferromagnets: Double perovskites SR(FeM)O<Subscript>3</Subscript> (M = Sn,Ti, Zr)

In calculations using the local spin density approximation, a new group of half-metallic ferromagnets is predicted. This group comprises double perovskites Sr₂FeSnO₆, Sr₂FeTiO₆, and Sr₂ FeZrO₆—systems with one type of magnetic ions that can be obtained by diluting the magnetic sublattice of SrFeO₃ with ions of nonmagnetic pⁿ and d⁰ metals.     

Optical and ESR analysis of the Fe acceptor in InP

Photoluminescence (PL), optical absorption, and electron spin resonance (ESR) have been applied to detect the different charge states of substitutional Fe in InP. The 3d⁶ one electron trap state is detected by the PL and optical absorption lines of the ⁵T₂?⁵E transitions, whereas the 3d⁵ neutral acceptor state is identified by its characteristics S = 5/2 ESR signal.     

Theoretical evaluation of the electron paramagnetic resonance spin Hamiltonian parameters for the impurity displacements for Fe3+ and Ru3+ in corundum

The impurity displacements for Fe³⁺ and Ru³⁺ in corundum (Al₂O₃) are theoretically studied using the perturbation formulas of the spin Hamiltonian parameters (zero-field splitting and anisotropic g factors) for a 3d⁵ (with high spin S = 5/2) and a 4d⁵ (with low spin S = 1/2) ion in trigonal symmetry, respectively. According to the investigations, the nd⁵ (n = 3 and 4) impurity ions may not locate at the ideal Al³⁺ site but undergo axial displacements by about 0.132 Å and 0.170 Å for Fe³⁺ and Ru³⁺, respectively, away from the center of the ligand octahedron along the C₃ axis. The calculated spin Hamiltonian parameters based on the above axial displacements show good agreement with the observed values. The validity of the results is discussed.     

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.     

Energy-scale phenomenology and pairing via resonant spin-charge motion in FeAs, CuO, heavy-fermion and other exotic superconductors

Muon spin relaxation (μSR) studies of the “1111” and “122” FeAs systems have detected static magnetism with variably sized ordered moments in their parent compounds. The phase diagrams of FeAs, CuO, organic BEDT, A₃C₆₀ and heavy-fermion systems indicate competition between static magnetism and superconductivity, associated with first-order phase transitions at quantum phase boundaries. In both FeAs and CuO systems, the superfluid density n_s/m^* at T→0 exhibits a nearly linear scaling with T_c. Analogous to the roton-minimum energy scaling with the lambda transition temperature in superfluid ⁴He, clear scaling with T_c was also found for the energy of the magnetic resonance mode in cuprates, (Ba,K)Fe₂As₂, CeCoIn₅ and CeCu₂Si₂, as well as the energy of the superconducting coherence peak observed by angle resolved photo emission (ARPES) in the cuprates near (π,0). Both the superfluid density and the energy of these pair-non-breaking soft-mode excitations determine the superconducting T_c via phase fluctuations of condensed bosons. Combining these observations and common dispersion relations of spin and charge collective excitations in the cuprates, we propose a resonant spin-charge motion/coupling, “traffic-light resonance,” expected when the charge energy scale ε_F becomes comparable to the spin fluctuation energy scale ?ω_SF~J, as the process which leads to pair formation in these correlated electron superconductors.