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We study the electronic structure of Ta2NiSe5 in its low-temperature semiconducting phase, using resonant inelastic x-ray scattering (RIXS) at the Ta L3 edge. We also investigate the electronic properties of Ta2NiSe5 within the density-functional theory using the generalized gradient approximation in the framework of the fully relativistic spin-polarized Dirac linear muffin-tin orbital band-structure method. While ARPES, dc transport, and optical measurements indicate that Ta2NiSe5 is a small band-gap semiconductor, DFT gives a metallic nonmagnetic solution in Ta2NiSe5 . To obtain the semiconducting ground state in Ta2 NiSe5 we use a self-interaction correction (SIC) procedure by introducing an orbital-dependent potential Vl into the Hamiltonian. We investigate theoretically the x-ray absorption spectroscopy (XAS) and RIXS spectra at the Ni and Ta L3 edges and analyze the spectra in terms of interband transitions. We investigate the RIXS spectra as a function of momentum transfer vector Q and incident photon energy. Because Ta2 NiSe5 possesses only fully occupied (Ni 3d and Se 4p) and completely empty (Ta 5d) shells with the formal valencies Ta5+ (5d0), Ni0 (3d10 ), and Se2- (4p6 ), both the Ni and Ta L3 RIXS spectra belong to a charge transfer type with ligand-to-metal excitations., Comment: 11 pages, 15 figures

A surprising "planar" thermal Hall effect, whereby the field is parallel to the current, has recently been observed in a few magnetic insulators, and this has been attributed to exotic excitations such as Majorana fermions or chiral magnons. Here we investigate the possibility of a planar thermal Hall effect in three different cuprate materials, in which the conventional thermal Hall conductivity $\kappa_{\rm {xy}}$ (with an out-of-plane field perpendicular to the current) is dominated by either electrons or phonons. Our measurements show that the planar $\kappa_{\rm {xy}}$ from electrons in cuprates is zero, as expected from the absence of a Lorentz force in the planar configuration. By contrast, we observe a sizable planar $\kappa_{\rm {xy}}$ in those samples where the thermal Hall response is due to phonons, even though it should in principle be forbidden by the high crystal symmetry. Our findings call for a careful re-examination of the mechanisms responsible for the phonon thermal Hall effect in insulators.

We have carried out a comprehensive high-resolution angle-resolved photoemission spectroscopy (ARPES) study of the pseudogap interplay with superconductivity in La-based cuprates. The three systems La$_{2-x}$Sr$_x$CuO$_4$, La$_{1.6-x}$Nd$_{0.4}$Sr$_x$CuO$_4$, and La$_{1.8-x}$Eu$_{0.2}$Sr$_x$CuO$_4$ display slightly different pseudogap critical points in the temperature versus doping phase diagram. We have studied the pseudogap evolution into the superconducting state for doping concentrations just below the critical point. In this setting, near optimal doping for superconductivity and in the presence of the weakest possible pseudogap, we uncover how the pseudogap is partially suppressed inside the superconducting state. This conclusion is based on the direct observation of a reduced pseudogap energy scale and re-emergence of spectral weight suppressed by the pseudogap. Altogether these observations suggest that the pseudogap phenomenon in La-based cuprates is in competition with superconductivity for anti-nodal spectral weight.

In high-temperature cuprate superconductors, stripe order refers broadly to a coupled spin and charge modulation with a commensuration of eight and four lattice units, respectively. How this stripe order evolves across optimal doping remains a controversial question. Here we present a systematic resonant inelastic x-ray scattering (RIXS) study of weak charge correlations in La2-xSrxCuO4 (LSCO) and La1.8-xEu0.2SrxCuO4 (LESCO). Ultra high energy resolution experiments demonstrate the importance of the separation of inelastic and elastic scattering processes. Upon increasing doping x, the long-range temperature dependent stripe order is found to be replaced by short-range temperature independent correlations at a critical point xc = 0.15 distinct from the pseudogap critical doping. We argue that the doping and temperature independent short-range correlations originate from unresolved electron-phonon coupling that broadly peaks at the stripe ordering vector. In LSCO, long-range static stripe order vanishes in a quantum critical point around optimal doping.

When spin-orbit-entangled $d$-electrons reside on a honeycomb lattice, rich quantum states are anticipated to emerge, as exemplified by the $d^5$ Kitaev materials. Distinct yet equally intriguing physics may be realized with a $d$-electron count other than $d^5$. We found that the layered ruthenate Ag$_3$LiRu$_2$O$_6$ with $d^4$ Ru$^{4+}$ ions at ambient pressure forms a honeycomb lattice of spin-orbit-entangled singlets, which is a playground for frustrated excitonic magnetism. Under pressure, the singlet state does not develop the expected excitonic magnetism but experiences two successive transitions to other nonmagnetic phases, first to an intermediate phase with moderate distortion of honeycomb lattice, and eventually to a high-pressure phase with very short Ru-Ru dimer bonds. While the strong dimerization in the high-pressure phase originates from a molecular orbital formation as in the sister compound Li$_2$RuO$_3$, the intermediate phase represents a spin-orbit-coupled $J$-dimer state which is stabilized by the admixture of upper-lying $J_{\rm eff} = 1$-derived states. We argue that the $J$-dimer state is induced by a pseudo-Jahn-Teller effect associated with the low-lying spin-orbital excited states and is unique to spin-orbit-entangled $d^4$ systems. The discovery of competing singlet phases demonstrates rich spin-orbital physics of $d^4$ honeycomb compounds and paves the way for realization of unconventional magnetism., Comment: 12 pages, 5 figures

The pyrochlore ruthenate In$_2$Ru$_2$O$_7$ displays a subtle competition between spin-orbital entanglement and molecular orbital formation. At room temperature, a spin-orbit-entangled singlet state was identified. With decreasing temperature, In$_2$Ru$_2$O$_7$ undergoes multiple structural transitions and eventually forms a nonmagnetic ground state with semi-isolated Ru$_2$O units on the pyrochlore lattice. The dominant hopping through the Ru-O-Ru linkage leads to molecular orbital formation within the Ru$_2$O units. This molecular orbital formation is unique in that it involves the O$^{2-}$ anions, unlike the transition-metal dimers observed in systems with edge-sharing octahedra. We argue that the covalent character of In-O bonds plays a pivotal role in the structural transitions and molecular orbital formation and such bonding character of "$A$-site" ions is an important ingredient for electronic phase competition in complex transition metal oxides.

Spin-orbit Mott insulators composed of $t_{2g}^4$ transition metal ions may host excitonic magnetism due to the condensation of spin-orbital $J=1$ triplons. Prior experiments suggest that the $4d$ antiferromagnet Ca$_{2}$RuO$_{4}$ embodies this notion, but a $J = 0$ nonmagnetic state as a basis of the excitonic picture remains to be confirmed. We use Ru $L_3$-edge resonant inelastic x-ray scattering to reveal archetypal $J$ multiplets with a $J=0$ ground state in the cubic compound K$_{2}$RuCl$_{6}$, which are well described within the $LS$-coupling scheme. This result highlights the critical role of unquenched orbital moments in $4d$-electron compounds and calls for investigations of quantum criticality and excitonic magnetism on various crystal lattices.

We report an ultrahigh resolution resonant inelastic x-ray scattering (RIXS) study of the in-plane bond-stretching phonon mode in stripe-ordered cuprate La$_{1.675}$Eu$_{0.2}$Sr$_{0.125}$CuO$_4$. Phonon softening and lifetime shortening are found around the charge ordering wave vector. In addition to these self-energy effects, the electron-phonon coupling is probed by its proportionality to the RIXS cross section. We find an enhancement of the electron-phonon coupling around the charge-stripe ordering wave vector upon cooling into the low-temperature tetragonal structure phase. These results suggest that in addition to electronic correlations, electron-phonon coupling contributes significantly to the emergence of long-range charge-stripe order in cuprates., Comment: Supplemental Material available on request

We report thermopower measurements under hydrostatic pressure on the cuprate superconductor La$_{1.6-x}$Nd$_{0.4}$Sr$_x$CuO$_4$ (Nd-LSCO), at low-temperature in the normal state accessed by suppressing superconductivity with a magnetic field up to $H = 31$ T. Using a newly developed AC thermopower measurement technique suitable for high pressure and high field, we track the pressure evolution of the Seebeck coefficient $S$. At ambient pressure and low temperature, $S/T$ was recently found to suddenly increase in Nd-LSCO at the pseudogap critical doping $p^{\star} = 0.23$, consistent with a drop in carrier density $n$ from $n = 1 + p$ above $p^{\star}$ to $n = p$ below. Under a pressure of 2.0 GPa, we observe that this jump in $S/T$ is suppressed. This confirms a previous pressure study based on electrical resistivity and Hall effect which found $dp^{\star}/dP \simeq - 0.01$ holes/GPa, thereby reinforcing the interpretation that this effect is driven by the pressure-induced shift of the van Hove point. It implies that the pseudogap only exists when the Fermi surface is hole-like, which puts strong constraints on theories of the pseudogap phase. We also report thermopower measurements on Nd-LSCO and La$_{1.8-x}$Eu$_{0.2}$Sr$_x$CuO$_4$ in the charge density-wave phase near $p \sim 1/8$, which reveals a weakening of this phase under pressure., Comment: 9 pages, 5 figures

We present a microscopic study of a doped quantum spin liquid candidate, the hyperkagome Na$_3$Ir$_3$O$_8$ compound by using $^{23}$Na NMR. We determine the intrinsic behavior of the uniform \textbf{q} $ = 0$ susceptibility via shift measurements and the dynamical response by probing the spin-lattice relaxation rate. Throughout the studied temperature range, the susceptibility is consistent with a semimetal behavior, though with electronic bands substantially modified by correlations. Remarkably, the antiferromagnetic fluctuations present in the insulating parent compound Na$_4$Ir$_3$O$_8$ survive in the studied compound. The spin dynamics are consistent with 120$^o$ excitations modes displaying short-range correlations.

The nature of the pseudogap phase of cuprates remains a major puzzle. One of its new signatures is a large negative thermal Hall conductivity $\kappa_{\rm xy}$, which appears for dopings $p$ below the pseudogap critical doping $p^*$, but whose origin is as yet unknown. Because this large $\kappa_{\rm xy}$ is observed even in the undoped Mott insulator La$_2$CuO$_4$, it cannot come from charge carriers, these being localized at $p = 0$. Here we show that the thermal Hall conductivity of La$_2$CuO$_4$ is roughly isotropic, being nearly the same for heat transport parallel and normal to the CuO$_2$ planes, i.e. $\kappa_{\rm zy}(T) \approx \kappa_{\rm xy} (T)$. This shows that the Hall response must come from phonons, these being the only heat carriers able to move as easily normal and parallel to the planes . At $p > p^*$, in both La$_{\rm 1.6-x}$Nd$_{\rm 0.4}$Sr$_x$CuO$_4$ and La$_{\rm 1.8-x}$Eu$_{\rm 0.2}$Sr$_x$CuO$_4$ with $p = 0.24$, we observe no c-axis Hall signal, i.e. $\kappa_{\rm zy}(T) = 0$, showing that phonons have zero Hall response outside the pseudogap phase. The phonon Hall response appears immediately below $p^* = 0.23$, as confirmed by the large $\kappa_{\rm zy}(T)$ signal we find in La$_{1.6-x}$Nd$_{\rm 0.4}$Sr$_x$CuO$_4$ with $p = 0.21$. The microscopic mechanism by which phonons become chiral in cuprates remains to be identified. This mechanism must be intrinsic - from a coupling of phonons to their electronic environment - rather than extrinsic, from structural defects or impurities, as these are the same on both sides of $p^*$. This intrinsic phonon Hall effect provides a new window on quantum materials and it may explain the thermal Hall signal observed in other topologically nontrivial insulators.

H3LiIr2O6 is the first honeycomb-lattice system without any signs of long-range magnetic order down to the lowest temperatures, raising the hope for the realization of an ideal Kitaev quantum spin liquid. Its honeycomb layers are coupled by interlayer hydrogen bonds. Static or dynamic disorder of these hydrogen bonds was proposed to strongly affect the magnetic exchange and to make Kitaev-type interactions dominant. Using dielectric spectroscopy, here we provide experimental evidence for dipolar relaxations in H3LiIr2O6 and deuterated D3LiIr2O6, which mirror the dynamics of protons and deuterons within the double-well potentials of the hydrogen bonds. The detected hydrogen dynamics reveals glassy freezing, characterized by a strong slowing down under cooling, with a crossover from thermally-activated hopping to quantum-mechanical tunneling towards low temperatures. Thus, besides being Kitaev quantum-spin-liquid candidates, these materials also are quantum paraelectrics. However, the small relaxation rates in the mHz range, found at low temperatures, practically realize quasi-static hydrogen disorder, as assumed in recent theoretical works to explain the quantum-spin-liquid ground state of both compounds., Comment: 10 pages, 8 figures

We use resonant inelastic x-ray scattering to investigate charge-stripe correlations in La$_{1.675}$Eu$_{0.2}$Sr$_{0.125}$CuO$_4$. By differentiating elastic from inelastic scattering, it is demonstrated that charge-stripe correlations precede both the structural low-temperature tetragonal phase and the transport-defined pseudogap onset. The scattering peak amplitude from charge stripes decays approximately as $T^{-2}$ towards our detection limit. The in-plane integrated intensity, however, remains roughly temperature independent. Therefore, although the incommensurability shows a remarkably large increase at high temperature, our results are interpreted via a single scattering constituent. In fact, direct comparison to other stripe-ordered compounds (La$_{1.875}$Ba$_{0.125}$CuO$_4$, La$_{1.475}$Nd$_{0.4}$Sr$_{0.125}$CuO$_4$ and La$_{1.875}$Sr$_{0.125}$CuO$_4$) suggests a roughly constant integrated scattering intensity across all these compounds. Our results therefore provide a unifying picture for the charge-stripe ordering in La-based cuprates. As charge correlations in La$_{1.675}$Eu$_{0.2}$Sr$_{0.125}$CuO$_4$ extend beyond the low-temperature tetragonal and pseudogap phase, their emergence heralds a spontaneous symmetry breaking in this compound.

A pressure-induced collapse of magnetic ordering in $\beta$-Li$_2$IrO$_3$ at $P_m\sim1.5- 2$ GPa has previously been interpreted as evidence for possible emergence of spin liquid states in this hyperhoneycomb iridate, raising prospects for experimental realizations of the Kitaev model. Based on structural data obtained at \emph{room temperature}, this magnetic transition is believed to originate in small lattice perturbations that preserve crystal symmetry, and related changes in bond-directional anisotropic exchange interactions. Here we report on the evolution of the crystal structure of $\beta$-Li$_2$IrO$_3$ under pressure at low temperatures ($T\leq50$ K) and show that the suppression of magnetism coincides with a change in lattice symmetry involving Ir-Ir dimerization. The critical pressure for dimerization shifts from 4.4(2) GPa at room temperature to $\sim1.5-2$ GPa below 50 K. While a direct $Fddd \rightarrow C2/c$ transition is observed at room temperature, the low temperature transitions involve new as well as coexisting dimerized phases. Further investigation of the Ir ($L_3$/$L_2$) isotropic branching ratio in x-ray absorption spectra indicates that the previously reported departure of the electronic ground state from a $J_{\rm{eff}}=1/2$ state is closely related to the onset of dimerized phases. In essence, our results suggest that the predominant mechanism driving the collapse of magnetism in $\beta$-Li$_2$IrO$_3$ is the pressure-induced formation of Ir$_2$ dimers in the hyperhoneycomb network. The results further confirm the instability of the $J_{\rm{eff}}=1/2$ moments and related non-collinear spiral magnetic ordering against formation of dimers in the low-temperature phase of compressed $\beta$-Li$_2$IrO$_3$., Comment: 14 pages, 13 figures

A comprehensive angle resolved photoemission spectroscopy study of the band structure in single layer cuprates is presented with the aim of uncovering universal trends across different materials. Five different hole- and electron-doped cuprate superconductors (La$_{1.59}$Eu$_{0.2}$Sr$_{0.21}$CuO$_4$, La$_{1.77}$Sr$_{0.23}$CuO$_4$, Bi$_{1.74}$Pb$_{0.38}$Sr$_{1.88}$CuO$_{6+\delta}$, Tl$_{2}$Ba$_{2}$CuO$_{6+\delta}$, and Pr$_{1.15}$La$_{0.7}$Ce$_{0.15}$CuO$_{4}$) have been studied with special focus on the bands with predominately $d$-orbital character. Using light polarization analysis, the $e_g$ and $t_{2g}$ bands are identified across these materials. A clear correlation between the $d_{3z^2-r^2}$ band energy and the apical oxygen distance $d_\mathrm{A}$ is demonstrated. Moreover, the compound dependence of the $d_{x^2-y^2}$ band bottom and the $t_{2g}$ band top is revealed. Direct comparison to density functional theory (DFT) calculations employing hybrid exchange-correlation functionals demonstrates excellent agreement. We thus conclude that the DFT methodology can be used to describe the global band structure of overdoped single layer cuprates on both the hole and electron doped side.

SrIrO$_3$ crystallizes in a monoclinic structure of distorted hexagonal perovskite at ambient pressure. The transport measurements show that the monoclinic SrIrO$_3$ is a low-carrier density semimetal, as in the orthorhombic perovskite polymorph. The electronic structure calculation indicates a semimetallic band structure with Dirac bands at two high-symmetry points of Brillouin zone only when spin-orbit coupling is incorporated, suggesting that the semimetallic state is produced by the strong spin-orbit coupling. We argue that the Dirac bands are protected by the non-symmorphic symmetry of lattice.

The pyrochlore iridate In$_2$Ir$_2$O$_7$ is a strong $J_{\mathrm{eff}} = 1/2$ Mott insulator with frustrated magnetism. Despite the large trigonal crystal field, a small admixture of $J_{\mathrm{eff}} = 3/2$ component in the $J_{\mathrm{eff}} = 1/2$ bands and a small splitting of $J_{\mathrm{eff}} = 3/2$ bands are observed as compared with other pyrochlore iridates A$_2$Ir$_2$O$_7$ (A: trivalent cation). We argue that the reduced inter-site hopping between the $J_{\mathrm{eff}} = 1/2$ and the $J_{\mathrm{eff}} = 3/2$ manifold plays a predominant role in the distinct behavior of In$_2$Ir$_2$O$_7$ compared with other A$_2$Ir$_2$O$_7$. The effect of the intersite hopping should not be dismissed in the local physics of spin-orbital-entangled $J_{\mathrm{eff}} = 1/2$ Mott insulators.

Hyperhoneycomb iridate $\beta$-Li$_2$IrO$_3$ is a three-dimensional analogue of two-dimensional honeycomb iridates, such as $\alpha$-Li$_2$IrO$_3$, which recently appeared as another playground for the physics of Kitaev-type spin liquid. $\beta$-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 crystal structure in the high-pressure phase of $\beta$-Li$_2$IrO$_3$ above $P_{\rm S}$ was refined, which indicates the formation of Ir$_2$ dimers on the zig-zag chains, with the Ir-Ir distance even shorter than that of metallic Ir. We argue that the strong dimerization stabilizes the bonding molecular orbital state comprising the two local $d_{zx}$-orbitals on the Ir-O$_2$-Ir bond plane, which conflicts with the equal superposition of $d_{xy}$-, $d_{yz}$- and $d_{zx}$- orbitals in the $J_{\rm eff}$ = 1/2 wave function produced by strong spin-orbit coupling. The results of resonant inelastic x-ray scattering (RIXS) measurements and the electronic structure calculations are fully consistent with the collapse of the $J_{\rm eff}$ = 1/2 state. A subtle competition of various electronic phases is universal in honeycomb-based Kitaev materials.

Using a combination of infrared ellipsometry, time-domain terahertz spectroscopy, and far-infrared reflectometry we have obtained the $ac$-plane complex dielectric function of monoclinic ($C2/c$) Ta$_2$NiSe$_5$ and orthorhombic ($Cmcm$) Ta$_2$NiS$_5$ single crystals. The identified dipole-active phonon modes polarized along $a$ and $c$ axes are in good agreement with density functional theory calculations. With increasing temperature the $a$-axis phonon modes of Ta$_2$NiSe$_5$ become poorly discernible, as they are superimposed on the electronic background which gradually fills the energy gap near the monoclinic-to-orthorhombic phase transition temperature $T_c$ = 326 K. In Ta$_2$NiS$_5$, which does not exhibit such a structural transition and remains orthorhombic down to low temperatures, the $a$-axis phonon modes are superimposed on a persistent broad electronic mode centered near 16 meV. We attribute this difference to strongly overlapping exciton-phonon complexes in Ta$_2$NiSe$_5$, as opposed to isolated instances of the same in Ta$_2$NiS$_5$, and find this to be in good agreement with an excitonic insulator state below $T_c$ in the former, as compared to the absence of one in the latter., Comment: 10 pages, 7 figures

The three central phenomena of cuprate superconductors are linked by a common doping $p^{\star}$, where the enigmatic pseudogap phase ends, around which the superconducting phase forms a dome, and at which the resistivity exhibits an anomalous linear dependence on temperature as $T \to 0$. However, the fundamental nature of $p^{\star}$ remains unclear, in particular whether it marks a true quantum phase transition. We have measured the specific heat $C$ of the cuprates Eu-LSCO and Nd-LSCO at low temperature in magnetic fields large enough to suppress superconductivity, over a wide doping range across $p^{\star}$. As a function of doping, we find that the electronic term $C_{\rm el}$ is strongly peaked at $p^{\star}$, where it exhibits a $-T$log$T$ dependence as $T \to 0$. These are the classic signatures of a quantum critical point, as observed in heavy-fermion and iron-based superconductors where their antiferromagnetic phase ends. We conclude that the pseudogap phase of cuprates ends at a quantum critical point, whose associated fluctuations are most likely involved in the $d$-wave pairing and the anomalous scattering., Comment: Includes Supplementary Information

We present a soft x-ray angle-resolved photoemission spectroscopy study of the overdoped high-temperature superconductors La$_{2-x}$Sr$_x$CuO$_4$ and La$_{1.8-x}$Eu$_{0.2}$Sr$_x$CuO$_4$. In-plane and out-of-plane components of the Fermi surface are mapped by varying the photoemission angle and the incident photon energy. No $k_z$ dispersion is observed along the nodal direction, whereas a significant antinodal $k_z$ dispersion is identified. Based on a tight-binding parametrization, we discuss the implications for the density of states near the van-Hove singularity. Our results suggest that the large electronic specific heat found in overdoped La$_{2-x}$Sr$_x$CuO$_4$ can not be assigned to the van-Hove singularity alone. We therefore propose quantum criticality induced by a collapsing pseudogap phase as a plausible explanation for observed enhancement of electronic specific heat.

We report a combined experimental and theoretical x-ray magnetic circular dichroism (XMCD) spectroscopy study at the Ir-$L_{2,3}$ edges on the Ir$^{5+}$ ions of the layered hybrid solid state oxide Sr$_2$Co$_{0.5}$Ir$_{0.5}$O$_4$ with the K$_2$NiF$_4$ structure. From theoretical simulation of the experimental Ir-$L_{2,3}$ XMCD spectrum, we found a deviation from a pure $J_{eff}=0$ ground state with an anisotropic orbital-to-spin moment ratio ($L_x/2S_x$ = 0.43 and $L_z/2S_z$ = 0.78). This deviation is mainly due to multiplet interactions being not small compared to the cubic crystal field and due to the presence of a large tetragonal crystal field associated with the crystal structure. Nevertheless, our calculations show that the energy gap between the singlet ground state and the triplet excited state is still large and that the magnetic properties of the Ir$^{5+}$ ions can be well described in terms of singlet Van Vleck paramagnetism.

The minimal ingredients to explain the essential physics of layered copper-oxide (cuprates= materials remains heavily debated. Effective low energy single-band models of the copper-oxygen orbitals are widely used because there exists no strong experimental evidence supporting multiband structures. Here we report angle-resolved photoelectron spectroscopy experiments on La-based cuprates that provide direct observation of a two-band structure. This electronic structure, qualitatively consistent with density functional theory, is parametrised by a two-orbital ($d_{x^2-y^2}$ and $d_{z^2}$) tight-binding model. We quantify the orbital hybridisation which provides an explanation for the Fermi surface topology and the proximity of the van-Hove singularity to the Fermi level. Our analysis leads to a unification of electronic hopping parameters for single-layer cuprates and we conclude that hybridisation, restraining d-wave pairing, is an important optimisation element for superconductivity., Comment: supplementary material available on request

We explore the response of Ir $5d$ orbitals to pressure in $\beta$-$\mathrm{Li_2IrO_3}$, a hyperhoneycomb iridate in proximity to a Kitaev quantum spin liquid (QSL) ground state. X-ray absorption spectroscopy reveals a reconstruction of the electronic ground state below 2 GPa, the same pressure range where x-ray magnetic circular dichroism shows an apparent collapse of magnetic order. The electronic reconstruction, which manifests a reduction in the effective spin-orbit (SO) interaction in $5d$ orbitals, pushes $\beta$-$\mathrm{Li_2IrO_3}$ further away from the pure $J_{\rm eff}=1/2$ limit. Although lattice symmetry is preserved across the electronic transition, x-ray diffraction shows a highly anisotropic compression of the hyperhoneycomb lattice which affects the balance of bond-directional Ir-Ir exchange interactions driven by spin-orbit coupling at Ir sites. An enhancement of symmetric anisotropic exchange over Kitaev and Heisenberg exchange interactions seen in theoretical calculations that use precisely this anisotropic Ir-Ir bond compression provides one possible route to realization of a QSL state in this hyperhoneycomb iridate at high pressures.

We use the Nernst effect to delineate the boundary of the pseudogap phase in the temperature-doping phase diagram of cuprate superconductors. New data for the Nernst coefficient $\nu(T)$ of YBa$_{2}$Cu$_{3}$O$_{y}$ (YBCO), La$_{1.8-x}$Eu$_{0.2}$Sr$_x$CuO$_4$ (Eu-LSCO) and La$_{1.6-x}$Nd$_{0.4}$Sr$_x$CuO$_4$ (Nd-LSCO) are presented and compared with previous data including La$_{2-x}$Sr$_x$CuO$_4$ (LSCO). The temperature $T_\nu$ at which $\nu/T$ deviates from its high-temperature behaviour is found to coincide with the temperature at which the resistivity deviates from its linear-$T$ dependence, which we take as the definition of the pseudogap temperature $T^\star$- in agreement with gap opening detected in ARPES data. We track $T^\star$ as a function of doping and find that it decreases linearly vs $p$ in all four materials, having the same value in the three LSCO-based cuprates, irrespective of their different crystal structures. At low $p$, $T^\star$ is higher than the onset temperature of the various orders observed in underdoped cuprates, suggesting that these orders are secondary instabilities of the pseudogap phase. A linear extrapolation of $T^\star(p)$ to $p=0$ yields $T^\star(p\to 0)\simeq T_N(0)$, the N\'eel temperature for the onset of antiferromagnetic order at $p=0$, suggesting that there is a link between pseudogap and antiferromagnetism. With increasing $p$, $T^\star(p)$ extrapolates linearly to zero at $p\simeq p_{\rm c2}$, the critical doping below which superconductivity emerges at high doping, suggesting that the conditions which favour pseudogap formation also favour pairing. We also use the Nernst effect to investigate how far superconducting fluctuations extend above $T_{\rm c}$, as a function of doping, and find that a narrow fluctuation regime tracks $T_{\rm c}$, and not $T^\star$. This confirms that the pseudogap phase is not a form of precursor superconductivity., Comment: 24 pages and 26 figures including Appendix

We report the complex dielectric function of the quasi-one-dimensional chalcogenide Ta$_2$NiSe$_5$, which exhibits a structural phase transition that has been attributed to exciton condensation below $T_c = 326$ K, and of the isostructural Ta$_2$NiS$_5$ which does not exhibit such a transition. Using spectroscopic ellipsometry, we have detected exciton doublets with pronounced Fano lineshapes in both the compounds. The exciton Fano resonances in Ta$_2$NiSe$_5$ display an order of magnitude higher intensity than those in Ta$_2$NiS$_5$. In conjunction with prior theoretical work by E. Rashba, we attribute this observation to the giant oscillator strength of spatially extended exciton-phonon bound states in Ta$_2$NiSe$_5$. The formation of exciton-phonon complexes in Ta$_2$NiS$_5$ and Ta$_2$NiSe$_5$ is confirmed by the pronounced temperature dependence of sharp interband transitions in the optical spectra, whose peak energies and widths scale with the thermal population of optical phonon modes. The description of the optically excited states in terms of strongly overlapping exciton complexes is in good agreement with the hypothesis of an EI ground state., Comment: 10 pages, 7 figures

We have investigated the electronic structure of Sr2IrO4 using core level resonant inelastic x-ray scattering. The experimental spectra can be well reproduced using ab initio density functional theory based multiplet ligand field theory calculations, thereby validating these calculations. We found that the low-energy, effective Ir t2g orbitals are practically degenerate in energy. We uncovered that covalency in Sr2IrO4, and generally in iridates, is very large with substantial oxygen ligand hole character in the Ir t2g Wannier orbitals. This has far reaching consequences, as not only the onsite crystal-field energies are determined by the long range crystal-structure, but, more significantly, magnetic exchange interactions will have long range distance dependent anisotropies in the spin direction. These findings set constraints and show pathways for the design of d^5 materials that can host compass-like magnetic interactions.

We report the complex dielectric function along and perpendicular to the IrO2 planes in the layered perovskite Sr2IrO4 determined by spectroscopic ellipsometry in the spectral range from 12 meV to 6 eV. Thin high quality single crystals were stacked to measure the c-axis optical conductivity. In the phonon response we identified 10 infrared-active modes polarized within the basal plane and only four modes polarized along the c-axis, in full agreement with first-principle lattice dynamics calculations. We also observed a strong optical anisotropy in the near-infrared spectra arising from direct transitions between Ir 5d t2g Jeff = 1/2 and Jeff = 3/2 bands, which transition probability is highly suppressed for light polarized along the c-axis. The spectra are analyzed and discussed in terms of relativistic LSDA+U band structure calculations., Comment: 9 pages, 7 figures

We report an angle-resolved photoemission study of the charge stripe ordered La$_{1.6-x}$Nd$_{0.4}$Sr$_x$CuO$_4$ system. A comparative and quantitative line shape analysis is presented as the system evolves from the overdoped regime into the charge ordered phase. On the overdoped side ($x=0.20$), a normal state anti-nodal spectral gap opens upon cooling below ~ 80 K. In this process spectral weight is preserved but redistributed to larger energies. A correlation between this spectral gap and electron scattering is found. A different lineshape is observed in the antinodal region of charge ordered Nd-LSCO $x=1/8$. Significant low-energy spectral weight appears to be lost. These observations are discussed in terms of spectral weight redistribution and gapping %of spectral weight originating from charge stripe ordering., Comment: Accepted by PRB

In this study, the magnetic ground state of iridium perovskites (Sr$_{n+1}$Ir$_{n}$O$_{3n+1}$, where $n=1$, 2, and $\infty$) was considered using muon spin spectroscopy ($\mu$SR). When probed by muons in Sr$_2$IrO$_4$ and Sr$_3$Ir$_2$O$_7$ ($n=1,2$), the internal field ($B_{\rm loc}$) showed clear sign of a magnetic order in two stages at transition temperatures $T_{\rm N}\simeq 230$ K and $T_{\rm m}\simeq 90$ K in Sr$_2$IrO$_4$ and $T_{\rm N}\simeq 280$ K and $T_{\rm m}\simeq 70$ K in Sr$_3$Ir$_2$O$_7$, respectively. In contrast, no long-range magnetic order was observed in orthorhombic SrIrO$_3$ ($n=\infty$). Based on the known magnetic structure in Sr$_2$IrO$_4$ and Sr$_3$Ir$_2$O$_7$, we successfully identified muon sites in these compounds from the magnitude of $B_{\rm loc}$ in the first stage ($T_{\rm m}\le T\le T_{\rm N}$). Below $T_{\rm m}$, $B_{\rm loc}$ probed by a fraction of muons occupying sites near the apical oxygen of IrO$_6$ octahedra exhibited a further increase but remained mostly unchanged for sites close to the in-plane oxygen. While such behavior cannot be explained by the alteration of the Ir spin structure, it is consistent with the selective appearance of ordered magnetic moments on the apical oxygen. The oxygen polarization was also in line with the reported magnetization anomalies in these compounds below $\sim T_{\rm m}$. A possible link between the oxygen polarization and ferroelectric (multiferroic) behavior in Sr$_2$IrO$_4$ was considered., Comment: 9 figures, 12 pages

We address the kinetic competition between charge striped order and superconductivity in La$_{1.675}$Eu$_{0.2}$Sr$_{0.125}$CuO$_{4}$. Ultrafast optical excitation is tuned to a mid-infrared vibrational resonance that destroys charge order and promptly establishes transient coherent interlayer coupling in this material. This effect is evidenced by the appearance of a longitudinal plasma mode reminiscent of a Josephson plasma resonance. We find that coherent interlayer coupling can be generated up to the charge order transition $T_{CO} \approx$ 80 K, far above the equilibrium superconducting transition temperature of any lanthanide cuprate. Two key observations are extracted from the relaxation kinetics of the interlayer coupling. Firstly, the plasma mode relaxes through a collapse of its coherence length and not its density. Secondly, two distinct kinetic regimes are observed for this relaxation, above and below spin order transition $T_{SO} =$ 25 K. Especially, the temperature independent relaxation rate observed below $T_{SO}$ is anomalous and suggests coexistence of superconductivity and stripes rather than competition. Both observations support arguments that a low temperature coherent stripe (or pair density wave) phase suppresses c-axis tunnelling by disruptive interference rather than by depleting the condensate., Comment: 14 pages, 4 figures (main text), supplementary added to v2: 6 pages, 5 figures

Methods to generate spin-polarised electronic states in non-magnetic solids are strongly desired to enable all-electrical manipulation of electron spins for new quantum devices. This is generally accepted to require breaking global structural inversion symmetry. In contrast, here we present direct evidence from spin- and angle-resolved photoemission spectroscopy for a strong spin polarisation of bulk states in the centrosymmetric transition-metal dichalcogenide WSe$_2$. We show how this arises due to a lack of inversion symmetry in constituent structural units of the bulk crystal where the electronic states are localised, leading to enormous spin splittings up to $\sim\!0.5$ eV, with a spin texture that is strongly modulated in both real and momentum space. As well as providing the first experimental evidence for a recently-predicted `hidden' spin polarisation in inversion-symmetric materials, our study sheds new light on a putative spin-valley coupling in transition-metal dichalcogenides, of key importance for using these compounds in proposed valleytronic devices., Comment: 6 pages, 4 figures

This work is on the Physics of the B Factories. Part A of this book contains a brief description of the SLAC and KEK B Factories as well as their detectors, BaBar and Belle, and data taking related issues. Part B discusses tools and methods used by the experiments in order to obtain results. The results themselves can be found in Part C. Please note that version 3 on the archive is the auxiliary version of the Physics of the B Factories book. This uses the notation alpha, beta, gamma for the angles of the Unitarity Triangle. The nominal version uses the notation phi_1, phi_2 and phi_3. Please cite this work as Eur. Phys. J. C74 (2014) 3026., Comment: 928 pages, version 3 (arXiv:1406.6311v3) corresponds to the alpha, beta, gamma version of the book, the other versions use the phi1, phi2, phi3 notation

The realization of Kitaev spin liquid, where spins on a honeycomb lattice are coupled ferromagnetically by bond-dependent anisotropic interactions, has been a sought-after dream. 5d iridium oxides $\alpha$-Li2IrO3 and $\alpha$-Na2IrO3 with a honeycomb lattice of Jeff = 1/2 moments recently emerged as a possible materialization. Strong signature of Kitaev physics, however, was not captured. Here we report the discovery of a complex iridium oxide $\beta$-Li2IrO3 with Jeff = 1/2 moments on "hyper-honeycomb" lattice, a three-dimensional analogue of honeycomb lattice. A positive Curie-Weiss temperature $\theta_{CW}$ ~ 40 K indicated dominant ferromagnetic interactions among Jeff = 1/2 moments in $\beta$-Li2IrO3. A magnetic ordering with a small entropy change was observed at Tc = 38 K, which, with the application of magnetic field of only 3 T, changed to a fully polarized state of Jeff = 1/2 moments. Those results imply that hyper-honeycomb beta-Li2IrO3 is located in the vicinity to a Kitaev spin liquid.

We report a high-field electron spin resonance study in the sub-THz frequency domain of a single crystal of Sr$_2$IrO$_4$ that has been recently proposed as a prototypical spin-orbital Mott insulator. In the antiferromagnetically (AFM) ordered state with noncollinear spin structure that occurs in this material at $T_{\rm N} \approx 240$ K we observe both the "low" frequency mode due to the precession of weak ferromagnetic moments arising from a spin canting, and the "high" frequency modes due to the precession of the AFM sublattices. Surprisingly, the energy gap for the AFM excitations appears to be very small, amounting to 0.83 meV only. This suggests a rather isotropic Heisenberg dynamics of interacting Ir$^{4+}$ effective spins despite the spin-orbital entanglement in the ground state., Comment: revised version

We report the complex dielectric function of high-quality Na_3Ir_3O_8 single crystals determined by spectroscopic ellipsometry in the spectral range from 15 meV to 2 eV. The far-infrared phonon spectra exhibit highly asymmetric line shapes characteristic of Fano resonances. With decreasing temperature, we observe a sharp increase of the infrared intensity of the Fano-shaped phonon modes accompanied by concomitant changes in the low energy electronic background, formed by electronic transitions between Ir 5d t_2g bands of a mostly J_eff = 1/2 character. The role of the complex Hyper-Kagome lattice structure and strong spin-orbit coupling is considered., Comment: 6 pages, 3 figures, 1 table

The complex iridium oxide Na3Ir3O8 with a B-site ordered spinel structure was synthesized in single crystalline form, where the chiral hyper-kagome lattice of Ir atoms, as observed in the spin-liquid candidate Na4Ir3O8, was identified. The average valence of Ir is 4.33+ and, therefore, Na3Ir3O8 can be viewed as a doped analogue of the hyper-kagome spin liquid with Ir4+. The transport measurements showed that Na3Ir3O8 is in fact a semi-metal. The electronic structure calculation demonstrated that the strong spin-orbit coupling of Ir yields the semi-metallic state out of an otherwise band insulating state, which may harbor exotic topological effects embedded in the hyper-kagome lattice., Comment: Reference updated

La1.8-xEu0.2SrxCuO4 (LESCO) is the member of the 214 family which exhibits the largest intervals among the structural, charge ordering (CO), magnetic, and superconducting transition temperatures. By using new dc transport measurements and data in the literature we construct the phase diagram of LESCO between x = 0.8 and 0.20. This phase diagram has been further probed in ac, by measuring the optical conductivity {\sigma}1({\omega}) of three single crystals with x = 0.11, 0.125, and 0.16 between 10 and 300 K in order to associate the extra-Drude peaks often observed in the 214 family with a given phase. The far-infrared peak we detect in underdoped LESCO is the hardest among them, survives up to room temperature and is associated with charge localization rather than with ordering. At the CO transition for the commensurate doping x = 0.125 instead the extra-Drude peak hardens and a pseudogap opens in {\sigma}1({\omega}), approximately as wide as the maximum superconducting gap of LSCO., Comment: 6 pages, 6 figures

Ta2PdxS5 (x < 1.0) was found to show superconductivity at Tc ~ 6 K. The temperature dependent resistivity of single crystalline Ta2Pd0.92S5 showed that the system is strongly disordered due to Pd deficiencies and close to Anderson localized state. Superconductivity in the dirty limit as well as the temperature dependence of specific heat C(T) implies that superconductivity is s-wave. The upper critical field Hc2 at T = 0 K limit with the magnetic field parallel to the TaS6 chains was found to be as high as 31 T, exceeding the Pauli paramagnetic limit Hp = 10.2 T by a factor of 3. We argue that the absence of the paramagnetic pair-breaking originates from strong spin-orbit scattering due to Pd deficiencies embedded in the periodic lattice of heavy 5d Ta and 4d Pd.

We investigate the bilayer Ruddlesden-Popper iridate Sr$_3$Ir$_2$O$_7$ by temperature-dependent angle-resolved photoemission. We find a narrow-gap correlated insulator, with spectral features indicative of a polaronic ground state, strikingly similar to that observed previously for the parent compounds of the cuprate superconductors. We additionally observe similar behaviour for the single-layer cousin Sr$_2$IrO$_4$, indicating that strong electron-boson coupling dominates the low-energy excitations of this exotic family of materials, and providing a microscopic link between the insulating ground states of the seemingly-disparate 3d cuprates and 5d iridates., Comment: 6 pages, 4 figures. A high resolution version is available at http://www.st-andrews.ac.uk/~pdk6/pub_files/Ir327_cond_mat_HiRes.pdf

The transition temperature Tc of cuprate superconductors falls when the doping p is reduced below a certain optimal value. It is unclear whether this fall is due to strong phase fluctuations or to a decrease in the pairing gap. Different interpretations of photoemission data disagree on the evolution of the pairing gap and different estimates of the upper critical field Hc2 are in sharp contradiction. Here we resolve this contradiction by showing that superconducting fluctuations in the underdoped cuprate Eu-LSCO, measured via the Nernst effect, have a characteristic field scale that falls with underdoping. The critical field Hc2 dips at p = 0.11, showing that superconductivity is weak where stripe order is strong. In the archetypal cuprate superconductor YBCO, Hc2 extracted from other measurements has the same doping dependence, also with a minimum at p = 0.11, again where stripe order is present. We conclude that competing states such as stripe order weaken superconductivity and this, rather than phase fluctuations, causes Tc to fall as cuprates become underdoped., Comment: Supplementary Information file available upon request; Nature Physics (2012)

The antiferromagnetic structure of Sr3Ir2O7, the bilayer analogue of a spin-orbital Mott insulator Sr2IrO4, was revealed by resonant magnetic x-ray diffraction. Contrasting intensities of the magnetic diffraction at the Ir LIII and LII edges show a Jeff=1/2 character of the magnetic moment as is argued in Sr2IrO4. The magnitude of moment, however, was found to be smaller than that of Sr2IrO4 by a factor of 5-6, implying that Sr3Ir2O7 is no longer a Mott insulator but a weak antiferromagnet. An evident change of the temperature dependence of the resistivity at TN, from almost temperature-independent resistivity to insulating, strongly suggests that the emergent weak magnetism controls the charge gap. The magnetic structure was found to be an out-of-plane collinear antiferromagnetic ordering in contrast to the inplane canted antiferromagnetism in Sr2IrO4, originating from the strong bilayer antiferromagnetic coupling.