The zero temperature Mott transition region in antiferromagnetic, spin S=5/2 MnO is probed using the correlated band theory LSDA+U method. The first transition encountered is an insulator-insulator volume collapse within the rocksalt structure that is characterized by an unexpected Hund's rule violating `spin-flip' moment collapse. This spin-flip to S=1/2 takes fullest advantage of the anisotropy of the Coulomb repulsion, allowing gain in the kinetic energy (which increases with decreasing volume) while retaining a sizable amount of the magnetic exchange energy. While transition pressures vary with the interaction strength, the spin-flip state is robust over a range of interaction strengths and for both B1 and B8 structures.
We report results of tunneling studies on the charge ordering compound Nd_{0.5}Sr_{0.5}MnO_{3} in a magnetic field up to 6T and for temperature down to 25K.We show that a gap (2Δ_{CO} \approx 0.5eV opens up in the density of state (DOS) at the Fermilevel (E_F) on charge ordering (T_{CO}=150K) which collapses in an applied magnetic field when the charge ordered state melts. There is a clear correspondence between the behavior of the resistivity and the gap formation and its collapse in an applied magnetic field. We conclude that a gap in the DOS at E_F is necessary for the stability of the char…
The pressure dependence of the Curie temperature T$_{C}(P)$ in La$_{0.75}$Ca$_{0.25}$MnO$_{3}$ was determined by neutron diffraction up to 8 GPa, and compared with the metallization temperature T$_{IM}(P)$ \cite{irprl}. The behavior of the two temperatures appears similar over the whole pressure range suggesting a key role of magnetic double exchange also in the pressure regime where the superexchange interaction is dominant. Coexistence of antiferromagnetic and ferromagnetic peaks at high pressure and low temperature indicates a phase separated regime which is well reproduced with a dynamical…
We report on a high-pressure Raman study on two members of the La$_{1-x}$Ca$_x$MnO$_{3-δ}$ manganite family ($x=0.20$, $δ=0$ and $δ=0.08$). The results obtained for the $δ=0$ sample show a different behavior in the low and high pressure regimes ascribed to the onset of a new pressure-activated interaction previously invoked in other manganite compounds. The comparison of our results with literature data gives further support to the identification of the Jahn-Teller active stretching mode and shows that pressure-induced octahedral symmetrization is more effective in systems exhibiting a lower m…
We perform a comprehensive theoretical study of the pressure-induced evolution of the electronic structure, magnetic state, and phase stability of the late transition metal monoxides MnO, FeO, CoO, and NiO using a fully charge self-consistent DFT+dynamical mean-field theory method. Our results reveal that the pressure-induced Mott insulator-to-metal phase transition in MnO-NiO is accompanied by a simultaneous collapse of local magnetic moments and lattice volume, implying a complex interplay between chemical bonding and electronic correlations. We compute the pressure-induced evolution of rela…
We have investigated the dielectric properties of Gd$_{1-x}$Tb$_{x}$MnO$_{3}$ crystals in magnetic fields to clarify the crossover between two distinct ferroelectric phases, GdMnO$_{3}$-type (first-order displacive) and TbMnO$_{3}$-type (second-order order-disorder). We have found that the compositional phase boundary between the phases exists around 0.3$<$$x$$<$0.4. For the $x$=0.1 sample exposed to magnetic fields above 3T, we have discovered successive ferroelectric phase transitions that occur at two different temperatures. We have determined the electric and magnetic phase diagram for $x$…
We have investigated the specific heat and resistivity of a single crystal of Pr_{0.63}Ca_{0.37}MnO_3 around the charge ordering (CO) transition temperature, T_{CO}, in the presence of high magnetic fields (<=12T) which can melt the charge ordered state. At low magnetic fields (<=10T), the manganite transforms from a charge-disordered paramagnetic insulating (PI) state to a charge-ordered insulating (COI) state as the temperature is lowered. The COI state becomes unstable beyond a threshold magnetic field and melts to a ferromagnetic metallic phase (FMM). This occurs for T < T_{CO}. However, a…
Dc magnetic measurements across the charge ordering (CO) transition temperature (T$_{CO}$) in polycrystalline Pr$_{0.5}$Ca$_{0.5}$Mn$_{0.975}$Al$_{0.025}$O$_3$ have been performed under simultaneous influence of external hydrostatic pressure (P) and magnetic field (H). We show the first experimental evidence that the melting of charge order instability obey an interesting scaling function, $δ$T$_{CO}$/P$^α$ = $f$(H/P$^β$) in H-P-T landscape, where $δ$T$_{CO}$ is the suppression of T$_{CO}$ by P and H. Corresponding values of the exponents, $α$ = 1.63 and $β$ = 0.33 have been extracted from dat…
The high-pressure phases of FeO and MnO were studied by the first principles calculations. The present theoretical study predicts that the high-pressure phase of MnO is a metallic normal B8 structure (nB8), while that of FeO should take the inverse B8 structure (iB8). The novel feature of the unique high-pressure phase of stoichiometric FeO is that the system should be a band insulator in the ordered antiferromagnetic (AF) state and that the existence of a band gap leads to special stability of the phase. The observed metallicity of the high-pressure and high-temperature phase of FeO may be ca…
We employ the density functional theory to study a structural transition of MnO from B1 (rocksalt) to B8 (NiAs) structures that was observed experimentally at pressures around 100 GPa. We utilize all-electron description as well as norm-conserving pseudopotentials and demonstrate that these two approaches can significantly differ in quantitative predictions. We explicitly show that even small-core pseudopotentials exhibit transferability inaccuracies for quantities sensitive to the energy differences between high- and low-spin polarizations of valence electrons.
We study combined effect of hydrostatic pressure and magnetic field on the magnetization of La${_{0.5}}$Ca${_{0.5}}$MnO${_{3}}$. We do not observe any significant effect of pressure on the paramagnetic to ferromagnetic transition. However, pressure asymmetrically affects the thermal hysteresis across the ferro-antiferromagnetic first-order transition, which has strong field dependence. Though the supercooling (T*) and superheating (T**) temperatures decrease and the value of magnetization at 5K (M$_{5K}$) increases with pressure, T* and M$_{5K}$ shows abrupt changes in tiny pressure of 0.68kba…
The metal-insulator transition in correlated electron systems, where electron states transform from itinerant to localized, has been one of the central themes of condensed matter physics for more than half a century. The persistence of this question has been a consequence both of the intricacy of the fundamental issues and the growing recognition of the complexities that arise in real materials, even when strong repulsive interactions play the primary role. The initial concept of Mott was based on the relative importance of kinetic hopping (measured by the bandwidth) and on-site repulsion of e…
The structure of La$_{1-x}$Ca$_{x}$MnO$_{3}$ solid solutions (x=0, 0.25, 0.50, 0.67, 1) under high pressure (up to 40-45 GPa) has been investigated by synchrotron X-ray powder diffraction (XRD) in order to characterize their volume \emph{vs.} pressure (P-V) equation of state. All the members of the solid solution, except the extreme compounds: LaMnO$_3$ and CaMnO$_{3}$, present low pressure orthorhombic P$nma$ phase evolving toward an high pressure tetragonal (I$4/mcm$) symmetry. The details of this transition are related to the Ca doping: on increasing the Ca content, the critical pressure of…
We have demonstrated the effect of pressure on magnetization jump and its associated magnetostriction in Eu$_{0.58}$Sr$_{0.42}$MnO$_{3}$. The critical field lowered by the applied pressure is explained by a suppression of the metastable blocked state, which is inherent to the phase separated manganite system exhibiting the magnetic avalanche. To reduce a factor of the bad thermal coupling in the bulk sample, and to further examine the nature of the magnetization jump, we have performed the influence of pressure on the isothermal magnetization of the powder sample. There is no discrepancy in th…
Effect of hydrostatic pressure and magnetic field on electrical resistance of the Kondo-like perovskite manganese oxide, La$_{0.1}$Ce$_{0.4}$Sr$_{0.5}$MnO$_3$ with a ferrimagnetic ground state, have been investigated up to 2.1 GPa and 9 T. In this compound, the Mn-moments undergo double exchange mediated ferromagnetic ordering at $T_{\rm C}$ $\sim$ 280 K and there is a resistance maximum, $T_{\rm max}$ at about 130 K which is correlated with an antiferromagnetic ordering of {\it cerium} with respect to the Mn-sublattice moments. Under pressure, the $T_{\rm max}$ shifts to lower temperature at …
Atomically flat, epitaxial, and stoichiometric thin films of the electronically phase separated compound (La$_{0.5}$Pr$_{0.5}$)$_{0.67}$Ca$_{0.33}$MnO$_{3}$ were grown on as-received and treated NdGaO$_{3}$ substrates by fine tuning of oxygen pressure during deposition. Optimal thin films with step flow growth mode show superior physical properties compared to thin films grown in off-optimal oxygen pressures, {\em viz.} the highest maximum temperature coefficient of resistance, the highest peak-resistivity temperature, and reduced coercive fields. Transport, magnetization, and x-ray diffractio…
We report a detail theoretical study of the electronic structure and phase stability of transition metal oxides MnO, FeO, CoO, and NiO in their paramagnetic cubic B1 structure by employing dynamical mean-field theory of correlated electrons combined with \emph{ab initio} band structure methods (DFT+DMFT). Our calculations reveal that under pressure these materials exhibit a Mott insulator-metal transition (IMT) which is accompanied by a simultaneous collapse of local magnetic moments and lattice volume, implying a complex interplay between chemical bonding and electronic correlations. Moreover…
The electronic structure, magnetic moment, and volume collapse of MnO under pressure are obtained from four different correlated band theory methods; local density approximation + Hubbard U (LDA+U), pseudopotential self-interaction correction (pseudo-SIC), the hybrid functional (combined local exchange plus Hartree-Fock exchange), and the local spin density SIC (SIC-LSD) method. Each method treats correlation among the five Mn 3d orbitals (per spin), including their hybridization with three O $2p$ orbitals in the valence bands and their changes with pressure. The focus is on comparison of the …
Using X-ray absorption spectroscopy (XAS), we studied the local structure in LaMnO3 under applied pressure across and well above the insulator to metal (IM) transition. A hysteretic behavior points to the coexistence of two phases within a large pressure range (7 to 25 GPa). The ambient phase with highly Jahn-Teller (JT) distorted MnO6 octahedra is progressively substituted by a new phase with less-distorted JT MnO6 units. The electronic delocalization leading to the IM transition is finger-printed from the pre-edge XAS structure around 30 GPa. We observed that the phase transition takes place…
The effect of deposition oxygen pressure (P$_{O}$) on phase separation (PS) induced in epitaxial La$_{0.67}$Ca$_{0.33}$MnO$_{3}$/NdGaO$_{3}$(001) films was investigated. Fully oxygenated films grown at high P$_{O}$ are anisotropically strained. They exhibit PS over a wide temperature range, because of the large orthorhombicity of NdGaO$_{3}$ substrates. The paramagnetic insulator-to-ferromagnetic metal (FM) and FM-to-antiferromagnetic insulator (AFI) transitions gradually shift to lower temperatures with decreasing PO. The AFI state is initially weakened (P$_{O}$ >= 30 Pa), but then becomes mo…
Pressure sensitivity (up to 1 GPa) of the electrical resistivity and of the ac susceptibility was measured for low electron doping levels of Ca$_{1-x}$Y$_x$MnO$_3$ (CYMO) and of Ca$_{1-x}$Sm$_x$MnO$_3$ (CSMO) ceramic samples (0.05 $\leq$ x $\leq$ 0.15). A very weak pressure dependence of the Curie temperature (T$_c$) was observed for both systems ($\sim$ 6 K GPa$^{-1}$), when compared to the hole-doped manganites of the same T$_c$ ($\sim$ 20 K GPa$^{-1}$). Our results can be interpreted within a modified Double Exchange scenario, where pressure alters the reduction of the bandwidth produced by…
We have investigated the effect of Ti doping on the transport properties coupled with the magnetic ones in Sm$_{0.55}$Sr$_{0.45}$Mn$_{1-η}$Ti$_η$O$_3$ ($0 \leq η\leq 0.04$). The parent compound, Sm$_{0.55}$Sr$_{0.45}$MnO$_3$, exhibits a first-order paramagnetic-insulator to ferromagnetic-metal transition just below $T_{\rm c}$ = 128 K. With substitution of Ti at Mn sites ($B$-site), $T_{\rm c}$ decreases approximately linearly at the rate of 22 K$\%^{-1}$ while the width of thermal hysteresis in magnetization and resistivity increases almost in an exponential fashion. The most spectacular effe…
We report on the pressure effects on the orbital polaron lattice in the lightly doped manganites $\mathrm{La_{1-x}Sr_xMnO_{3}}$, with $x\sim 1/8$. The dependence of the orbital polaron lattice on $negative$ chemical pressure is studied by substituting Pr for La in $\mathrm{(La_{1-y}Pr_y)_{7/8}Sr_{1/8}MnO_{3}}$. In addition, we have studied its hydrostatic pressure dependence in $\mathrm{(La_{0.9}Pr_{0.1})_{7/8}Sr_{1/8}MnO_{3}}$. Our results strongly indicate that the hopping $t$ significantly contributes to the stabilization of the orbital polaron lattice and that the orbital polarons are ferr…
Collapse of Jahn-Teller Phonons in La$_{1-x}$Sr$_{x}$MnO$_3$ with Weak Magnetoresistance
University of Colorado Boulder（科罗拉多大学博尔德分校）；Oak Ridge National Laboratory（橡树岭国家实验室）；Japan Atomic Energy Agency（日本原子能机构）；High Energy Accelerator Research Organization (KEK)（高能加速器研究机构）；Comprehensive Research Organization for Science and Society（综合科学研究组织）；Karlsruhe Institute of Technology（卡尔斯鲁厄理工学院）
In this article a detailed study of La$_{1-x}$Ca$_x$MnO$_3$ ($0\leq x \leq 0.23$) phase diagram using powder x-ray diffraction and magnetization measurements is presented. Unfortunately, in the related literature no properly characterized samples have been used, with consequence the smearing of the real physics in this complicated system. As the present results reveal, there are two families of samples. The first family concerns samples prepared in atmosphere ($P({\rm O}_2)=0.2$ Atm) which are all ferromagnetic with Curie temperature rising with $x$. The second family concerns samples, where a…
The crystal field splitting and d bandwidth of the 3d transition metal monoxides MnO, FeO, CoO and NiO are analyzed as a function of pressure within density functional theory. In all four cases the 3d bandwidth is significantly larger than the crystal field splitting over a wide range of compressions. The bandwidth actually increases more as pressure is increased than the crystal field splitting. Therefore the role of increasing bandwidth must be considered in any explanation of a possible spin collapse that these materials may exhibit under pressure.
GdMnO$_3$ shows an incommensurate antiferromagnetic order below $\simeq 42$ K, transforms into a canted A-type antiferromagnet below $\simeq 20$ K, and for finite magnetic fields along the b axis ferroelectric order occurs below $\simeq 12$ K. From high-resolution thermal expansion measurements along all three principal axes, we determine the uniaxial pressure dependencies of the various transition temperatures and discuss their correlation to changes of the magnetic exchange couplings in $R$MnO$_3$ ($R = {\rm La, ... Dy}$).
The $A$MnO$_{2}$ delafossites ($A$=Na, Cu), are model frustrated antiferromagnets, with triangular layers of Mn$^{3+}$~spins. At low temperatures ($T_{N}$=65 K), a $C2/m \rightarrow P\overline{1}$ transition is found in CuMnO$_2$, which breaks frustration and establishes magnetic order. In contrast to this clean transition, $A$=Na only shows short-range distortions at $T_N$. Here we report a systematic crystallographic, spectroscopic, and theoretical investigation of CuMnO$_2$. We show that, even in stoichiometric samples, non-zero anisotropic Cu displacements co-exist with magnetic order. Usi…
We investigate the magnetic properties of archetypal transition-metal oxides MnO, FeO, CoO and NiO under very high pressure by x-ray emission spectroscopy at the Kβline. We observe a strong modification of the magnetism in the megabar range in all the samples except NiO. The results are analyzed within a multiplet approach including charge-transfer effects. The pressure dependence of the emission line is well accounted for by changes of the ligand field acting on the d electrons and allows us to extract parameters like local d-hybridization strength, O-2p bandwidth and ionic crystal field acro…
We report a Raman scattering study of the pressure-induced collapse of the Mott-like phases of Ca_3Ru_2O_7 (T_N=56 K) and Ca_2RuO_4 (T_N=110 K). The pressure-dependence of the phonon and two-magnon excitations in these materials indicate: (i) a pressure-induced collapse of the antiferromagnetic (AF) insulating phase above P* ~ 55 kbar in Ca_3Ru_2O_7 and P* ~ 5-10 kbar in Ca_2RuO_4, reflecting the importance of Ru-O octahedral distortions in stabilizing the AF insulating phase; and (ii) evidence for persistent AF correlations above the critical pressure of Ca_2RuO_4, suggestive of phase separat…
The low temperature behavior of Pr$_{1-x}$Ca$_{x}$MnO$_{3}$ manganites are known to undergo a transition at $x\sim$0.3, from an insulating ferromagnetic state, at low Ca concentration, to an insulating antiferromagnetic state. Above the onset concentration for the charge-ordering effect ($x\sim 0.3$), a metal-insulator transition induced by an external magnetic field is also observed and related to the collapse of the charge-ordered state. In this paper we show that the ferro-antiferromagnetic transition takes place in an inhomogeneous way: around the critical concentration the sample is a mix…
Electronic orderings of charges, orbitals and spins are observed in many strongly correlated electron materials, and revealing their dynamics is a critical step toward understanding the underlying physics of important emergent phenomena. Here we use time-resolved resonant soft x-ray scattering spectroscopy to probe the dynamics of antiferromagnetic spin ordering in the manganite Pr$_{0.7}$Ca$_{0.3}$MnO$_3$ following ultrafast photo-exitation. Our studies reveal a glass-like recovery of the spin ordering and a crossover in the dimensionality of the restoring interaction from quasi-1D at low pum…
We employ a combination of the \emph{ab initio} band structure methods and dynamical mean-field theory to determine the electronic structure and phase stability of paramagnetic FeO at high pressure and temperature. Our results reveal a high-spin to low-spin transition within the B1 crystal structure of FeO upon compression of the lattice volume above 73~GPa. The spin-state transition is accompanied by an orbital-selective Mott metal-insulator transition (MIT). The lattice volume is found to collapse by about 8.5~\% at the MIT, implying a complex interplay between electronic and lattice degrees…
Cerium-doped manganite thin films were grown epitaxially by pulsed laser deposition at $720 ^\circ$C and oxygen pressure $p_{O_2}=1-25 $Pa and were subjected to different annealing steps. According to x-ray diffraction (XRD) data, the formation of CeO$_2$ as a secondary phase could be avoided for $p_{O_2}\ge 8 $Pa. However, transmission electron microscopy shows the presence of CeO$_2$ nanoclusters, even in those films which appear to be single phase in XRD. With O$_2$ annealing, the metal-to-insulator transition temperature increases, while the saturation magnetization decreases and stays wel…
The thermal expansion of the heavy fermion compound CeRh2Si2 has been measured under pressure as a function of temperature using strain gages. A large anomaly associated to the Neel temperature has been detected even above the suspected critical pressure Pc = 1.05 GPa where no indication of antiferromagnetism has been observed in calorimetry experiments sensitive to the entropy change. An unexpected feature is the pressure slowdown of the antiferromagnetic-paramagnetic transition by comparison to the fast pressure collapse predicted for homogeneous first order quantum phase transition with one…
Electrical resistivity as a function of temperature, pressure, and magnetic field was measured in high and low purity single crystals of BaVS$_3$ close to the critical pressure value $p_{cr}$$\approx$2 GPa, associated with the zero temperature insulator-to-metal (MI) transition. In the 1.8-2.0 GPa range, where the MI transition is below $\approx$20 K, one can observe a sudden collapse of the MI phase boundary upon increasing pressure and at fixed pressure a magnetic field induced insulator to metal transition. In high quality samples these features are accompanied by hysteresis in all measured…
Dramatic volume collapses under pressure are fundamental to geochemistry and of increasing importance to fields as diverse as hydrogen storage and high-temperature superconductivity. In transition metal materials, collapses are usually driven by so-called spin-state transitions, the interplay between the single-ion crystal field and the size of the magnetic moment. Here we show that the classical S=5/2 mineral Hauerite undergoes an unprecedented 22 % collapse driven by a conceptually different magnetic mechanism. Using synchrotron x-ray diffraction we show that cold compression induces the for…
The dynamics of the charge ordered (CO) state under non-equilibrium conditions created by strong dc-electric field (~106 V/cm) and photo-illumination with short (~ 6 ns) laser pulses is investigated in Bi1-xCaxMnO3 (x > 0.5) epitaxial films. A pulsed laser deposition method was used to synthesize films on (100) LaAlO3 (LAO) and (100) SrTiO3 (STO) substrates. The crystallographic structure, temperature dependence of electrical resistivity and magnetization of the samples of different composition prepared under different oxygen partial pressure (pO2) and deposition temperature (TD) are studied. …
We have investigated dielectric properties in a series of crystals of $R$MnO$_{3}$ ($R$ is a rare earth ion) under magnetic fields and quasihydrostatic pressure. We have found that ferroelectric phase appeared in GdMnO$_3$ crystal below 13K. We have confirmed that a small spontaneous polarization exists along a axis ($P_a$) in the orthorhombic $Pbnm$ setting and that $P_a$ can be reversed by the dc electric field. The dielectric anomaly due to the ferroelectric transition accompanied thermal hysteresis and lattice striction. The ferroelectric transition temperature decreased with quasihydrosta…
The accuracy of LDA + U total energies is tested by predicting the high pressure behavior of rocksalt-structured FeO, including metal-insulator and magnetic transitions, equation of state, and lattice strain, as functions of the Coulomb repulsion U. For U = 4.6 eV, 6.0 eV and 8.0 eV, two self-consistent solutions, one with rhombohedral and one with monoclinic electronic symmetry, are found, and for U = 4.6 eV the monoclinic solution becomes more stable than the rhombohedral solution at 100 GPa, leading to an insulator-metal transition. With increasing U, metallization occurs at higher pressure…
We introduce an effective model for $e_g$ electrons to describe three-dimensional perovskite (La$_{1-x}$Sr$_{x}$MnO$_3$ and La$_{1-x}$Ca$_{x}$MnO$_3$) manganites and study the magnetic and orbital order on $4\times 4\times 4$ cluster using correlated wave functions. The model includes the kinetic energy, and on-site Coulomb interactions for $e_g$ electrons, antiferromagnetic superexchange interaction between $S=3/2$ core spins, and the coupling between $e_g$ electrons and Jahn-Teller modes. The model reproduces the experimentally observed magnetic order: ($i$) $A$-type antiferromagnetic phase …
The itinerant quasi-ferromagnetic metal MnSi has been studied by detailed thermal expansion measurements under pressures and magnetic fields. A sudden decrease of the volume at the critical pressure Pc ~1.6 GPa has been observed and is in good agreement with the pressure variation of the volume fraction of the spiral magnetic ordering. This confirms that the magnetic order disappears by a first order phase transition. The energy change estimated by the volume discontinuity on crossing Pc is of similar order as the Zeeman energy of the transition from the spiral ground state to a polarized para…
We present magnetization measurements carried out on polycrystalline and single-crystalline samples of $α$-Li$_2$IrO$_3$ under hydrostatic pressures up to 2 GPa and establish the temperature-pressure phase diagram of this material. The Néel temperature ($T_{\rm{N}}$) of $α$-Li$_2$IrO$_3$ is slightly enhanced upon compression with $dT_{\rm{N}}/dp$ = 1.5 K/GPa. Above 1.2 GPa, $α$-Li$_2$IrO$_3$ undergoes a first-order phase transition toward a nonmagnetic dimerized phase, with no traces of the magnetic phase observed above 1.8 GPa at low temperatures. The critical pressure of the structural dimer…
Recent investigations of the superconducting iron-arsenide families have highlighted the role of pressure, be it chemical or mechanical, in fostering superconductivity. Here we report that CaFe2As2 undergoes a pressure-induced transition to a non-magnetic, volume "collapsed" tetragonal phase, which becomes superconducting at lower temperature. Spin-polarized total-energy calculations on the collapsed structure reveal that the magnetic Fe moment itself collapses, consistent with the absence of magnetic order in neutron diffraction.
In this Letter we report the first LDA+DMFT (method combining Local Density Approximation with Dynamical Mean-Field Theory) results of magnetic and spectral properties calculation for paramagnetic phases of FeO at ambient and high pressures (HP). At ambient pressure (AP) calculation gave FeO as a Mott insulator with Fe 3$d$-shell in high-spin state. Calculated spectral functions are in a good agreement with experimental PES and IPES data. Experimentally observed metal-insulator transition at high pressure is successfully reproduced in calculations. In contrast to MnO and Fe$_2$O$_3$ ($d^5$ con…
Properties of iron oxides at the extreme conditions are of essential importance in condensed matter physics and Geophysics. The recent discovery of a new type of iron oxide, Fe4O5, at high pressure and high temperature of Earth's deep interior attracts great interests. In this paper, we report the electronic structure and the magnetic properties of Fe4O5 predicted by the density functional theory plus dynamic mean field theory (DFT+DMFT) approach. We find that Fe4O5 stays metallic from ambient pressure to high pressure. The magnetic moments of iron atoms at the three different crystallographic…
We present uniaxial pressure structural relaxations for CaFe2As2 and BaFe2Ase2 within density functional theory and compare them with calculations under hydrostatic pressure conditions as well as available experimental results. We find that CaFe2As2 shows a unique phase transition from a magnetic orthorhombic phase to a nonmagnetic collapsed tetragonal phase for both pressure conditions and no indication of a tetragonal phase is observed at intermediate uniaxial pressures. In contrast, BaFe2As2 shows for both pressure conditions two phase transitions from a magnetic orthorhombic to a collapsed…
Hyperhoneycomb iridate $β$-Li$_2$IrO$_3$ is a three-dimensional analogue of two-dimensional honeycomb iridates, such as $α$-Li$_2$IrO$_3$, which recently appeared as another playground for the physics of Kitaev-type spin liquid. $β$-Li$_2$IrO$_3$ shows a non-collinear spiral ordering of spin-orbital-entangled $J_{\rm eff}$ = 1/2 moments at low temperature, which is known to be suppressed under a pressure of $\sim$2 GPa. With further increase of pressure, a structural transition is observed at $P_{\rm S}$ $\sim$ 4 GPa at room temperature. Using the neutron powder diffraction technique, the crys…
We have studied the bandwidth-temperature-magnetic field phase diagram of RE(0.55)Sr(0.45)MnO(3) colossal magnetoresistance manganites with ferromagnetic metallic (FM) ground state. The bandwidth (or equivalently the double exchange interaction) was controlled both via chemical substitution and hydrostatic pressure with a focus on the vicinity of the critical pressure p* where the character of the zero-field FM transition changes from first to second order. Below p* the first-order FM transition extends up to a critical magnetic field, H_cr. It is suppressed by pressure and approaches zero on …
Motivated by pressure experiments on FeAs-122 superconductors, we propose a scenario based on local-moment physics to explain the simultaneous disappearance of magnetism, reduction of the unit cell volume, and decrease in resistivity. In this scenario, the low-pressure magnetic phase derives from Fe moments, which become screened in the paramagnetic high-pressure phase. The quantum phase transition can be described as an orbital-selective Mott transition, which is rendered first order by coupling to the lattice, in analogy to a Kondo volume collapse. Spin-fluctuation driven superconductivity c…
The pressure evolution of the Raman active electronic excitations of the transition metal dichalcogenides 2H-TaS$_2$ is followed through the pressure phase diagram embedding incommensurate charge-density-wave and superconducting states. At high pressure, the charge-density-wave is found to collapse at 8.5~GPa. In the coexisting charge-density-wave and superconducting orders, we unravel a strong in-gap superconducting mode, attributed to a Higgs mode, coexisting with the expected incoherent Cooper-pair breaking signature. The latter remains in the pure superconducting state reached above 8.5~GP…
We report the evolution of the electronic nematic susceptibility in FeSe via Raman scattering as a function of hydrostatic pressure up to 5.8 GPa where the superconducting transition temperature $T_{c}$ reaches its maximum. The critical nematic fluctuations observed at low pressure vanish above 1.6 GPa, indicating they play a marginal role in the four-fold enhancement of $T_{c}$ at higher pressures. The collapse of nematic fluctuations appears to be linked to a suppression of low energy electronic excitations which manifests itself by optical phonon anomalies at around 2 GPa, in agreement with…
The sudden decrease in molar volume exhibited by most lanthanides under high pressure is often attributed to changes in the degree of localization of their 4f-electrons. We give evidence, based on electrical resistivity measurements of dilute Y(Gd) and Y(Tb) alloys to 120 GPa, that the volume collapse transitions in Gd and Tb metals have different origins, despite their being neighbors in the periodic table. Remarkably, the change under pressure in the magnetic state of isolated Pr or Tb impurity ions in the nonmagnetic Y host appears to closely mirror corresponding changes in pure Pr or Tb me…
By means of hybrid density functional theory we investigate the evolution of the structural, electronic and magnetic properties of the colossal magnetoresistance (CMR) parent compound LaMnO$_3$ under pressure. We predict a transition from a low pressure antiferromagnetic (AFM) insulator to a high pressure ferromagnetic (FM) transport half-metal (tHM), characterized by a large spin polarization (~ 80-90 %). The FM-tHM transition is associated with a progressive quenching of the cooperative Jahn-Teller (JT) distortions which transform the $Pnma$ orthorhombic phase into a perfect cubic one (throu…
Superconductivity in PrIr$_{2}$Zn$_{20}$ appears at $T_{\rm c} = 0.05$ K in the presence of an antiferroquadrupolar order below $T_{\rm Q} = 0.11$ K. We have studied pressure dependences of $T_{\rm c}$, $T_{\rm Q}$, and non-Fermi liquid behaviors in the resistivity $ρ(T)$ by using two pressure transmitting media: argon maintaining highly hydrostatic pressure, and glycerol, which solidifies above 5 GPa producing nonhydrostatic pressure. Upon applying $P$ with argon up to 10.6 GPa, $T_{\rm c}$ hardly changes, while $T_{\rm Q}$ monotonically increases from 0.11 to 0.23 K. With glycerol, however, …
We perform Raman spectroscopy studies on $α$-RuCl$_3$ at room temperature to explore its phase transitions of magnetism and chemical bonding under pressures. The Raman measurements resolve two critical pressures, about $p_1=1.1$~GPa and $p_2=1.7$~GPa, involving very different intertwining behaviors between the structural and magnetic excitations. With increasing pressures, a stacking order phase transition of $α$-RuCl$_3$ layers develops at $p_1=1.1$~GPa, indicated by the new Raman phonon modes and the modest Raman magnetic susceptibility adjustment. The abnormal softening and splitting of the…
We report a new tetragonal ground-state for perovskite-structured PbCrO3 from DFT+U calculations, and explain its anomalously large volume. The new structure is stabilized due to orbital ordering of Cr-d in the presence of a large tetragonal crystal field, mainly due to off-centering of the Pb atom. At higher pressures (smaller volumes) there is a first-order transition to a cubic phase where the Cr-d orbitals are orbitally liquid. This phase-transition is accompanied by a ~11.5% volume collapse, one of the largest known for transition-metal oxides. The large ferroelasticity and its strong cou…
