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Low-dimensional quantum magnets are a versatile materials platform for studying the emergent many-body physics and collective excitations that can arise even in systems with only short-range interactions. Understanding their low-temperature structure and spin Hamiltonian is key to explaining their magnetic properties, including unconventional quantum phases, phase transitions, and excited states. We study the metal-organic coordination compound (C$_5$H$_9$NH$_3$)$_2$CuBr$_4$ and its deuterated counterpart, which upon its discovery was identified as a candidate two-leg quantum ($S = 1/2$) spin ladder in the strong-leg coupling regime. By growing large single crystals and probing them with both bulk and microscopic techniques, we deduce that two previously unknown structural phase transitions take place between 136 K and 113 K. The low-temperature structure has a monoclinic unit cell giving rise to two inequivalent spin ladders. We further confirm the absence of long-range magnetic order down to 30 mK and discuss the implications of this two-ladder structure for the magnetic properties of (C$_5$H$_9$NH$_3$)$_2$CuBr$_4$.

Appearance of quantum oscillations (QO) in both thermodynamic and transport properties of metals at low temperatures is the most striking experimental consequence of the existence of a Fermi surface (FS). The frequency of these oscillations and the temperature dependence of their amplitude provides essential information about the FS topology and fermionic quasiparticle properties. Here, we report the observation of an anomalous suppression of the QO amplitude seen in resistivity (Shubnikov de-Haas effect) at sub-kelvin temperatures in ZrTe5 samples with a single small FS sheet comprising less than 5% of the first Brillouin zone. By comparing these results with measurements of the magneto-acoustic QO and the recovery of the usual Lifshitz-Kosevich behavior of the Shubnikov de-Haas (SdH) effect in ZrTe$_5$ samples with a multi-sheet FS, we show that the suppression of the SdH effect originates from a decoupling of the electron liquid from the lattice. On crossing the so-called Bloch-Gr\"uneisen temperature, T$_BG$, electron-phonon scattering becomes strongly suppressed and in the absence of Umklapp scattering the electronic liquid regains Galilean invariance. In addition, we show, using a combination of zero-field electrical conductivity and ultrasonic-absorption measurements, that entering this regime leads to an abrupt increase of electronic viscosity.

The recent synthesis of the chromium selenide compound LiCrSe$_2$ constitutes a valuable addition to the ensemble of two-dimensional triangular lattice antiferromagnets (2D-TLA). In this work we present the very first comprehensive study of the combined low temperature nuclear and magnetic structure established in this material. Details on the connection between Li-ion dynamics and structural changes are also presented along with a direct link between atomic structure and spin order via a strong magnetoelastic coupling. LiCrSe$_2$ was found to undergo a first order structural transition from a trigonal crystal system with space group $P\bar{3}m1$ to a monoclinic one with space group $C2/m$ at $T_{\rm s}=30$~K. Such restructuring of the lattice is accompanied by a magnetic transition at $T_{\rm N}=30$~K, with the formation of a complex spin arrangement for the Cr$^{3+}$ moments. Refinement of the magnetic structure with neutron diffraction data and complementary muon spin rotation analysis reveal the presence of two incommensurate magnetic domains with a up-up-down-down arrangement of the spins with ferromagnetic (FM) double chains coupled antiferromagnetically (AFM). In addition to this unusual arrangement, the spin axial vector is modulated both in direction and modulus, resulting in a spin density wave-like order with periodic suppression of the Cr moment along the chains. This behavior is believed to appear as a result of strong competition between direct exchange AFM and superexchange FM couplings established between both nearest neighbor and next nearest neighbor Cr$^{3+}$ ions. We finally conjecture that the resulting magnetic order is stabilized via subtle vacancy/charge order within the Li layers, potentially causing a mix of two different magnetic phases within the sample.

Two$-$dimensional (2D) triangular lattices antiferromagnets (2D$-$TLA) often manifest intriguing physical and technological properties, due to the strong interplay between lattice geometry and electronic properties. The recently synthesized 2$-$dimensional transition metal dichalcogenide LiCrTe$_2$, being a 2D$-$TLA, enriched the range of materials which can present such properties. In this work, muon spin rotation ($\mu^+$SR) and neutron powder diffraction (NPD) have been utilized to reveal the true magnetic nature and ground state of LiCrTe$_2$. From high$-$resolution NPD the magnetic spin order at base$-$temperature is not, as previously suggested, helical, but rather collinear antiferromagnetic (AFM) with ferromagnetic (FM) spin coupling within the $ab-$plane and AFM coupling along the $c-$axis. The ordered magnetic Cr moment is established as $\mu_{\rm Cr}=$ 2.36 $\mu_{\rm B}$. From detailed $\mu^+$SR measurements we observe an AFM ordering temperature $T_{\rm N}\approx$ 125 K. This value is remarkably higher than the one previously reported by magnetic bulk measurements. From $\mu^+$SR we are able to extract the magnetic order parameter, whose critical exponent allows us to categorize LiCrTe$_2$ in the 3D Heisenberg AFM universality class. Finally, by combining our magnetic studies with high$-$resolution synchrotron X$-$ray diffraction (XRD), we find a clear coupling between the nuclear and magnetic spin lattices. This suggests the possibility for a strong magnon$-$phonon coupling, similar to what has been previously observed in the closely related compound LiCrO$_2$.

The coupling of spin, charge and lattice degrees of freedom results in the emergence of novel states of matter across many classes of strongly correlated electron materials. A model example is unconventional superconductivity, which is widely believed to arise from the coupling of electrons via spin excitations. In cuprate high-temperature superconductors, the interplay of charge and spin degrees of freedom is also reflected in a zoo of charge and spin-density wave orders that are intertwined with superconductivity. A key question is whether the different types of density waves merely coexist or are indeed directly coupled. Here we use a novel neutron diffraction technique with superior beam-focusing that allows us to probe the subtle spin-density wave order in the prototypical high-temperature superconductor La1.88Sr0.12CuO4 under applied uniaxial pressure to demonstrate that it is immediately coupled with charge-density wave order. Our result shows that suitable models for high-temperature superconductivity must equally account for charge and spin degrees of freedom via uniaxial charge-spin stripe fluctuations.

Static stripe order is detrimental to superconductivity. Yet, it has been proposed that transverse stripe fluctuations may enhance the inter-stripe Josephson coupling and thus promote superconductivity. Direct experimental studies of stripe dynamics, however, remain difficult. From a strong-coupling perspective, transverse stripe fluctuations are realized in the form of dynamic "kinks" -- sideways shifting stripe sections. Here, we show how modest uniaxial pressure tuning reorganizes directional kink alignment. Our starting point is La$_{1.88}$Sr$_{0.12}$CuO$_4$, where transverse kink ordering results in a rotation of stripe order away from the crystal axis. Application of mild uniaxial pressure changes the ordering pattern and pins the stripe order to the crystal axis. This reordering occurs at a much weaker pressure than that to detwin the stripe domains and suggests a rather weak transverse stripe stiffness. Weak spatial stiffness and transverse quantum fluctuations are likely key prerequisites for stripes to coexist with superconductivity., Comment: Accepted in Nature Communications

We investigate the critical properties of the spin-$1$ honeycomb antiferromagnet BaNi$_2$V$_2$O$_8$, both below and above the ordering temperature $T_N$ using neutron diffraction and muon spin rotation measurements. Our results characterize BaNi$_2$V$_2$O$_8$ as a two-dimensional (2D) antiferromagnet across the entire temperature range, displaying a series of crossovers from 2D Ising-like to 2D XY and then to 2D Heisenberg behavior with increasing temperature. In particular, the extracted critical exponent of the order parameter reveals a narrow temperature regime close to $T_N$, in which the system behaves as a 2D XY antiferromagnet. Above $T_N$, evidence for Berezinsky-Kosterlitz-Thouless behavior driven by vortex excitations is obtained from the scaling of the correlation length. Our experimental results are in accord with classical and quantum Monte Carlo simulations performed for microscopic magnetic model Hamiltonians for BaNi$_2$V$_2$O$_8$., Comment: 16 pages, 17 figures

The high-pressure synthesized quasi-one-dimensional compounds NaMn$_2$O$_4$ and Li$_{0.92}$Mn$_2$O$_4$ are both antiferromagnetic insulators, and here their atomic and magnetic structures were investigated using neutron powder diffraction. The present crystal structural analyses of NaMn2O4 reveal that Mn3+/Mn4+ charge-ordering state exist even at low temperature (down to 1.5 K). It is evident from one of the Mn sites shows a strongly distorted Mn3+ octahedra due to the Jahn-Teller effect. Above TN = 39 K, a two-dimensional short-range correlation is observed, as indicated by an asymmetric diffuse scattering. Below TN, two antiferromagnetic transitions are observed (i) a commensurate long-range Mn3+ spin ordering below 39 K, and (ii) an incommensurate Mn4+ spin ordering below 10 K. The commensurate magnetic structure (kC = 0.5, -0.5, 0.5) follows the magnetic anisotropy of the local easy axes of Mn3+, while the incommensurate one shows a spin-density-wave order with kIC = (0,0,0.216). For Li$_{0.92}$Mn$_2$O$_4$, on the other hand, absence of a long-range spin ordered state down to 1.5 K is confirmed., Comment: 11 pages, 8 figures

In the quest of finding novel and efficient batteries, a great interest has raised in K-based honeycomb layer oxide materials both for their fundamental properties and potential applications. A key issue in the realization of efficient batteries based on such compounds, is to understand the K-ion diffusion mechanism. However, investigation of potassium-ion (K$^+$) dynamics in materials using magneto-spin properties has so far been challenging, due to its inherently weak nuclear magnetic moment, in contrast to other alkali ions such as lithium and sodium. Spin-polarised muons, having a high gyromagnetic ratio, make the muon spin rotation and relaxation ($\mu$+SR) technique ideal for probing ions dynamics in weak magneto-spin moment materials. Here we report the magnetic properties and K+ dynamics in honeycomb layered oxide material of the K$_2$Ni$_2$TeO$_6$ using $\mu$+SR measurements. Our low-temperature $\mu$+SR results together with, with complementary magnetic susceptibility, find an antiferromagnetic transition at 26 K. Further $\mu$+SR studies performed at higher temperatures reveal that potassium ions (K$^+$) become mobile above 250 K and the activation energy for the diffusion process is Ea = 121(13) meV. This is the first time that K+ dynamics in potassium-based battery materials has been measured using $\mu$+SR. Finally our results also indicate an interesting possibility that K-ion self diffusion occurs predominantly at the surface of the powder particles. This opens future possibilities for improving ion diffusion and device performance using nano-structuring., Comment: 12 pages, 12 figures

The magnetic ground state in highly ordered double perovskites LaSr$_{1-x}$Ca$_x$NiReO$_6$ ($x$ = 0.0, 0.5, 1.0) were studied in view of the Goodenough-Kanamori rules of superexchange interactions. In LaSrNiReO$_6$, Ni and Re sublattices are found to exhibit curious magnetic states, but do not show any long range magnetic ordering. The magnetic transition at $\sim$ 255 K is identified with the Re sublattic magnetic ordering. The sublattice interactions are tuned by modifying the Ni-O-Re bond angles via changing the lattice structure through Ca doping. Upon Ca doping, the Ni and Re sublattices start to display a ferrimagnetically ordered state at low temperature. The neutron powder diffraction reveals a canted alignment between the Ni and the Re sublattices, while the individual sublattice is ferromagnetic. The transition temperature of the ferrimagnetic phase increases monotonically with increasing Ca concentration., Comment: 18 pages, 8 figures

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.

In spinels ACr2O4 (A=Mg, Zn) realisation of the classical pyrochlore Heisenberg antiferromagnet model is complicated by a strong spin-lattice coupling: the extensive degeneracy of the ground state is lifted by a magneto-structural transition at TN=12.5 K. We study the resulting low-temperature low-symmetry crystal structure by synchrotron x-ray diffraction. The consistent features of x-ray low-temperature patterns are explained by the tetragonal model of Ehrenberg et. al (Pow. Diff. 17, 230( 2002)), while other features depend on sample or cooling protocol. Complex partially ordered magnetic state is studied by neutron diffraction and spherical neutron polarimetry. Multiple magnetic domains of configuration arms of the propagation vectors k1=(1/2 1/2 0), k2=(1 0 1/2) appear. The ordered moment reaches 1.94(3) muB/Cr3+ for k1 and 2.08(3) muB/Cr3+ for k2, if equal amount of the k1 and k2 phases is assumed. The magnetic arrangements have the dominant components along the [110] and [1-10] diagonals and a smaller c-component. By inelastic neutron scattering we investigate the spin excitations, which comprise a mixture of dispersive spin waves propagating from the magnetic Bragg peaks and resonance modes centered at equal energy steps of 4.5 meV. We interpret these as acoustic and optical spin wave branches, but show that the neutron scattering cross sections of transitions within a unit of two corner-sharing tetrahedra match the observed intensity distribution of the resonances. The distinctive fingerprint of cluster-like excitations in the optical spin wave branches suggests that propagating excitations are localized by the complex crystal structure and magnetic orders.

Relativistic massless Dirac fermions can be probed with high-energy physics experiments, but appear also as low-energy quasi-particle excitations in electronic band structures. In condensed matter systems, their massless nature can be protected by crystal symmetries. Classification of such symmetry-protected relativistic band degeneracies has been fruitful, although many of the predicted quasi-particles still await their experimental discovery. Here we reveal, using angle-resolved photoemission spectroscopy, the existence of two-dimensional type-II Dirac fermions in the high-temperature superconductor La$_{1.77}$Sr$_{0.23}$CuO$_4$. The Dirac point, constituting the crossing of $d_{x^2-y^2}$ and $d_{z^2}$ bands, is found approximately one electronvolt below the Fermi level ($E_\mathrm{F}$) and is protected by mirror symmetry. If spin-orbit coupling is considered, the Dirac point degeneracy is lifted and the bands acquire a topologically non-trivial character. In certain nickelate systems, band structure calculations suggest that the same type-II Dirac fermions can be realised near $E_\mathrm{F}$.

The magnetic properties of the two-dimensional, S=1 honeycomb antiferromagnet BaNi$_2$V$_2$O$_8$ have been comprehensively studied using DC susceptibility measurements and inelastic neutron scattering techniques. The magnetic excitation spectrum is found to be dispersionless within experimental resolution between the honeycomb layers, while it disperses strongly within the honeycomb plane where it consists of two gapped spin-wave modes. The magnetic excitations are compared to linear spin-wave theory allowing the Hamiltonian to be determined. The first- and second-neighbour magnetic exchange interactions are antiferromagnetic and lie within the ranges 10.90meV$\le$J$_n$$\le$13.35 meV and 0.85meV$\le$J$_{nn}$$\le$1.65 meV respectively. The interplane coupling J$_{out}$ is four orders of magnitude weaker than the intraplane interactions, confirming the highly two-dimensional magnetic behaviour of this compound. The sizes of the energy gaps are used to extract the magnetic anisotropies and reveal substantial easy-plane anisotropy and a very weak in-plane easy-axis anisotropy. Together these results reveal that BaNi$_2$V$_2$O$_8$ is a candidate compound for the investigation of vortex excitations and Berezinsky-Kosterliz-Thouless phenomenona.

Neutron diffraction on a triple-axis spectrometer and a small-angle neutron scattering instrument is used to study the magnetic phase transition in tetragonal Ba$_2$CuGe$_2$O$_7$ at zero magnetic field. In addition to the incommensurate cycloidal antiferromagnetic (AFM) long-range order, we establish that weak incommensurate ferromagnetism (FM) also arises below the transition temperature $T_N$ identified by sharp Bragg peaks close to the $\Gamma$ point. The intensities of both the incommensurate AFM and FM Bragg peaks vanish abruptly at $T_N$ indicative of a weak first-order transition. Above $T_N$, evidence is presented that the magnetic intensity within the tetragonal $(a,b)$ plane is distributed on a ring in momentum space whose radius is determined by the incommensurate wavevector of the cycloidal order. We speculate that the associated soft fluctuations are at the origin of the weak first-order transition in the spirit of a scenario proposed by Brazovskii., Comment: 10 pages, 4 figures

Metal organic magnets have enormous potential to host a variety of electronic and magnetic phases that originate from a strong interplay between the spin, orbital and lattice degrees of freedom. We control this interplay in the quantum magnet CuF$_2$(D$_2$O)$_2$pyz by using high pressure to drive the system through a structural and magnetic phase transition. Using neutron scattering, we show that the low pressure state, which hosts a two-dimensional square lattice with spin-wave excitations and a dominant exchange coupling of 0.89 meV, transforms at high pressure into a one-dimensional spin-chain hallmarked by a spinon continuum and a reduced exchange interaction of 0.43 meV. This direct microscopic observation of a magnetic dimensional crossover as a function of pressure opens up new possibilities for studying the evolution of fractionalised excitations in low dimensional quantum magnets and eventually pressure-controlled metal--insulator transitions.

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

Since the seminal ideas of Berezinskii, Kosterlitz and Thouless, topological excitations are at the heart of our understanding of a whole novel class of phase transitions. In most of the cases, those transitions are controlled by a single type of topological objects. There are however some situations, still poorly understood, where two dual topological excitations fight to control the phase diagram and the transition. Finding experimental realization of such cases is thus of considerable interest. We show here that this situation occurs in BaCo$_2$V$_2$O$_8$, a spin-1/2 Ising-like quasi-one dimensional antiferromagnet when subjected to a uniform magnetic field transverse to the Ising axis. Using neutron scattering experiments, we measure a drastic modification of the quantum excitations beyond a critical value of the magnetic field. This quantum phase transition is identified, through a comparison with theoretical calculations, to be a transition between two different types of solitonic topological objects, which are captured by different components of the dynamical structure factor., Comment: 9 pages, 4 figures; 10 additional pages of supplemental information containing 6 figures

Using angle-resolved photoemission spectroscopy it is revealed that in the vicinity of optimal doping the electronic structure of La2-xSrxCuO4 cuprate undergoes an electronic reconstruction associated with a wave vector q_a=(pi, 0). The reconstructed Fermi surface and folded band are distinct to the shadow bands observed in BSCCO cuprates and in underdoped La2-xSrxCuO4 with x <= 0.12, which shift the primary band along the zone diagonal direction. Furthermore the folded bands appear only with q_a=(pi, 0) vector, but not with q_b= (0, pi). We demonstrate that the absence of q_b reconstruction is not due to the matrix-element effects in the photoemission process, which indicates the four-fold symmetry is broken in the system.

We report ac susceptibility, specific heat, and neutron scattering measurements on a dipolar-coupled antiferromagnet LiYbF$_4$. For the thermal transition, the order-parameter critical exponent is found to be 0.20(1) and the specific-heat critical exponent -0.25(1). The exponents agree with the 2D XY/h$_4$ universality class despite the lack of apparent two-dimensionality in the structure. The order-parameter exponent for the quantum phase transitions is found to be 0.35(1) corresponding to (2+1)D. These results are in line with those found for LiErF$_4$ which has the same crystal structure, but largely different T$_N$, crystal field environment and hyperfine interactions. Our results therefore experimentally establish that the dimensional reduction is universal to quantum dipolar antiferromagnets on a distorted diamond lattice., 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

Superconductivity and magnetic order strongly compete in many conventional superconductors, at least partly because both tend to gap the Fermi surface. In magnetically-ordered conventional superconductors, the competition between these cooperative phenomena leads to anomalies at magnetic and superconducting phase boundaries. Here we reveal that in Pr2Pt3Ge5 superconducting and multiple magnetic order are intertwined within the same HT-phase space, but remain completely decoupled. Our thermal conductivity measurements provide evidence for normal electrons in the superconducting phase from which magnetic order emerges with negligible coupling to electron bands that contribute to superconductivity.

Strong spin-orbit coupling (SOC) effects of heavy $d$-orbital elements have long been neglected in describing the ground states of their compounds thereby overlooking a variety of fascinating and yet unexplored magnetic and electronic states, until recently. The spin-orbit entangled electrons in such compounds can get stabilized into unusual spin-orbit multiplet $J$-states which warrants severe investigations. Here we show using detailed magnetic and thermodynamic studies and theoretical calculations the ground state of Ba$_3$ZnIr$_2$O$_9$, a 6$H$ hexagonal perovskite is a close realisation of the elusive $J$~=~0 state. However, we find that local Ir moments are spontaneously generated due to the comparable energy scales of the singlet-triplet splitting driven by SOC and the superexchange interaction mediated by strong intra-dimer hopping. While the Ir ions within the structural Ir$_2$O$_9$ dimer prefers to form a spin-orbit singlet state (SOS) with no resultant moment, substantial interdimer exchange interactions from a frustrated lattice ensure quantum fluctuations till the lowest measured temperatures and stabilize a spin-orbital liquid phase., Comment: Main Text: 6 pages, 4 figures, 1 table; Supplementary: 4 pages, 2 figures, 2 tables

In order to study the phase diagram from a microscopic viewpoint, we have measured wTF- and ZF-$\mu^+$SR spectra for the Sr$_{1-x}$Ca$_x$Co$_2$P$_2$ powder samples with $x=0$, 0.2, 0.4, 0.5, 0.6, 0.8, and 1. Due to a characteristic time window and spatial resolution of $\mu^+$SR, the obtained phase diagram was found to be rather different from that determined by magnetization measurements. That is, as $x$ increases from 0, a Pauli-paramagnetic phase is observed even at the lowest $T$ measured (1.8~K) until $x=0.4$, then, a spin-glass like phase appears at $0.5\leq x\leq0.6$, and then, a phase with wide field distribution probably due to incommensurate AF order is detected for $x=0.8$, and finally, a commensurate $A$-type AF ordered phase (for $x=1$) is stabilized below $T_{\rm N}\sim80~$K. Such change is most likely reasonable and connected to the shrink of the $c$-axis length with $x$, which naturally enhances the magnetic interaction between the two adjacent Co planes., Comment: 5 pages, 5 figures, conference paper of muSR2014 (accepted for publication)

The low-temperature microscopic magnetic properties of the quasi-2D heavyfermion compound, CePt2In7 are investigated by using a positive muon-spin rotation and relaxation (?muSR) technique. Clear evidence for the formation of a commensurate antiferromagnetic order below TN=5.40 K is presented. The magnetic order parameter is shown to fit well to a modified BSC gap-energy function in a strong-coupling scenario., Comment: Accepted in Journal of Physics: Conference Series (2014)

We have studied low-temperature magnetic properties as well as high-temperature lithium ion diffusion in the battery cathode materials LixNi1/3Co1/3Mn1/3O2 by the use of muon spin rotation/relaxation. Our data reveal that the samples enter into a 2D spin-glass state below TSG=12 K. We further show that lithium diffusion channels become active for T>Tdiff=125 K where the Li-ion hopping-rate [nu(T)] starts to increase exponentially. Further, nu(T) is found to fit very well to an Arrhenius type equation and the activation energy for the diffusion process is extracted as Ea=100 meV., Comment: Submitted to Journal of Physics: Conference Series (2014)

We demonstrate the existence of the spin nematic interactions in an easy-plane type antiferromagnet Ba$_{2}$CoGe$_{2}$O$_{7}$ by exploring the magnetic anisotropy and spin dynamics. Combination of neutron scattering and magnetic susceptibility measurements reveals that the origin of the in-plane anisotropy is an antiferro-type interaction of the spin nematic operator. The relation between the nematic operator and the electric polarization in the ligand symmetry of this compound is presented. The introduction of the spin nematic interaction is useful to understand the physics of spin and electric dipole in multiferroic compounds., Comment: 5 pages, 4 figures

The quantum spin liquid compound (C$_4$H$_{12}$N$_2$)Cu$_2$Cl$_6$ (PHCC) is studied by $\mu$SR under hydrostatic pressures up to 23.6 kbar. At low temperatures, pressure-induced incommensurate magnetic order is detected beyond a quantum critical point at $P_c\sim4.3$ kbar. An additional phase transition to a different ordered phase is observed at $P_1\sim 13.4$ kbar. The data indicate that the high-pressure phase may be a commensurate one. The established $(P-T)$ phase diagram reveals the corresponding pressure-induced multicritical point at $P_1$, $T_1=2.0$ K.

The S=1/2 spin chain material SrCuO2 doped with 1% S=1 Ni-impurities is studied by inelastic neutron scattering. At low temperatures, the spectrum shows a pseudogap \Delta ~ 8 meV, absent in the parent compound, and not related to any structural phase transition. The pseudogap is shown to be a generic feature of quantum spin chains with dilute defects. A simple model based on this idea quantitatively accounts for the exprimental data measured in the temperature range 2-300 K, and allows to represent the momentum-integrated dynamic structure factor in a universal scaling form., Comment: 5 pages, 3 figures

Using angle-resolved photoemission spectroscopy, we show that the recently-discovered surface state on SrTiO$_{3}$ consists of non-degenerate $t_{2g}$ states with different dimensional characters. While the $d_{xy}$ bands have quasi-2D dispersions with weak $k_{z}$ dependence, the lifted $d_{xz}$/$d_{yz}$ bands show 3D dispersions that differ significantly from bulk expectations and signal that electrons associated with those orbitals permeate the near-surface region. Like their more 2D counterparts, the size and character of the $d_{xz}$/$d_{yz}$ Fermi surface components are essentially the same for different sample preparations. Irradiating SrTiO$_{3}$ in ultrahigh vacuum is one method observed so far to induce the "universal" surface metallic state. We reveal that during this process, changes in the oxygen valence band spectral weight that coincide with the emergence of surface conductivity are disproportionate to any change in the total intensity of the O $1s$ core level spectrum. This signifies that the formation of the metallic surface goes beyond a straightforward chemical doping scenario and occurs in conjunction with profound changes in the initial states and/or spatial distribution of near-$E_{F}$ electrons in the surface region., Comment: Main text plus supplemental material. Total of 12 pages, 9 figures, 2 tables

We report the observation of a stepwise "melting" of the low-temperature Na-vacancy order in the layered transition metal oxide Na0.7CoO2. High-resolution neutron powder diffraction indicates the existence of two first-order structural transitions, one at T1 = 290 K, followed by a second at T2 = 400 K. Detailed analysis reveals that both transitions are linked to changes in the Na mobility. Our data are consistent with a two-step disappearance of Na-vacancy order through the successive opening of first quasi-1D (T1 > T > T2) and then 2D (T > T2) Na diffusion paths. These results shed new light on previous, seemingly incompatible, experimental interpretations regarding the relationship between Na-vacancy order and Na dynamics in this material. They also represent an important step towards the tuning of physical properties and the design of tailored functional materials through an improved control and understanding of ionic diffusion., Comment: 5 pages, 4 figures, accepted by PRL

We present results of inelastic neutron scattering study on two bond disordered quasi twodimensional quantum magnets (C$_4$H$_12$N$_2$)Cu$_2$(Cl$_{1-x}$Br$_x$)$_6$ with x=0.035 and 0.075. We observe the increase of spin gap, reduction of magnon bandwidth and a decrease of magnon lifetimes compared to x=0 sample. Additional magnon damping is observed at higher energies away from zone center which is found to follow the density of single particle states.

It is widely believed that magnetic excitations become increasingly incoherent as temperature is raised due to random collisions which limit their lifetime. This picture is based on spin-wave calculations for gapless magnets in 2 and 3 dimensions and is observed experimentally as a symmetric Lorentzian broadening in energy. Here, we investigate a three-dimensional dimer antiferromagnet and find unexpectedly that the broadening is asymmetric - indicating that far from thermal decoherence, the excitations behave collectively like a strongly correlated gas. This result suggests that a temperature activated coherent state of quasi-particles is not confined to special cases like the highly dimerized spin-1/2 chain but is found generally in dimerized antiferromagnets of all dimensionalities and perhaps gapped magnets in general.

Neutron diffraction and calorimetric measurements are used to study the field-induced quantum phase transition in piperazinium-Cu$_2$(Cl$_{1-x}$Br$_x$)$_6$ ($x=0$, x=3.5% and x=7.5%), a prototypical quantum antiferromagnet with random bonds. The critical indexes $\phi$ and $\beta$ are determined. The findings contradict some original predictions for Bose Glass based on the assumption $z=d$, but are consistent with recent theoretical results implying $z

The title compound belongs to a wide group of quasi-one-dimensional (Q1D) antiferromagnets (AF) and its Co2+ ions form Q1D screw-chains along the c-axis. We here present the first investigation of the microscopic magnetic nature of a single crystalline BaCo2V2O8 sample using {\mu}+SR. Our data reveal the presence of several clear muon frequencies below TN indicating the onset of a long-range order. Above 5 K, the {\mu}+SR spectra is well fitted to a simple power-exponential relaxing function. The temperature dependence of the relaxation-rate ({\lambda}) as well as the power (n) display a clear anomaly around T = 44 K, indicating the onset of short-range 1D correlations., Comment: Submitted to Physics Procedia (muSR2011 conference)

We present an angle-resolved photoelectron spectroscopy study of YBa2Cu3O7-delta films in situ grown by pulsed laser deposition. We have successfully produced underdoped surfaces with ordered oxygen vacancies within the CuO chains resulting in a clear ortho-II band folding of the Fermi surface. This indicates that order within the CuO chains affects the electronic properties of the CuO2 planes. Our results highlight the importance of having not only the correct surface carrier concentration, but also a very well ordered and clean surface in order that photoemission data on this compound be representative of the bulk., Comment: Accepted in Physical Review B, Rapid Communication

A new type of hemispherical electron energy analyzer that permits angle and spin resolved photoelectron spectroscopy has been developed. The analyzer permits standard angle resolved spectra to be recorded with a two-dimensional detector in parallel with spin detection using a mini-Mott polarimeter. General design considerations as well as technical solutions are discussed and test results from the Au(111) surface state are presented.

Angle-resolved photoemission on underdoped La$_{1.895}$Sr$_{0.105}$CuO$_4$ reveals that in the pseudogap phase, the dispersion has two branches located above and below the Fermi level with a minimum at the Fermi momentum. This is characteristic of the Bogoliubov dispersion in the superconducting state. We also observe that the superconducting and pseudogaps have the same d-wave form with the same amplitude. Our observations provide direct evidence for preformed Cooper pairs, implying that the pseudogap phase is a precursor to superconductivity.

We report an angle resolved photoemission study of the electronic structure of the pseudogap state in \NdLSCO ($T_c<7$ K). Two opposite dispersing Fermi arcs are the main result of this study. The several scenarios that can explain this observation are discussed., Comment: A high-resolution version can be found at http://lns.web.psi.ch/lns/download/Pockets/arXiv.pdf

We present angle-resolved photoelectron spectroscopy data probing the electronic structure of the Nd-substituted high-$T_c$ cuprate La$_{1.48}$Nd$_{0.4}$Sr$_{0.12}$CuO$_4$ (Nd-LSCO). Data have been acquired at low and high photon energies, $h\nu$ = 55 and 500 eV, respectively. Earlier comparable low-energy studies of La$_{1.4-x}$Nd$_{0.6}$Sr$_{x}$CuO$_4$ ($x = 0.10, 0.12, 0.15$) have shown strongly suppressed photoemission intensity, or absence thereof, in large parts of the Brillouin zone. Contrary to these findings we observe spectral weight at all points along the entire Fermi surface contour at low and high photon energies. No signs of strong charge modulations are found. At high photon energy, the Fermi surface shows obvious differences in shape as compared to the low-energy results presented here and in similar studies. The observed difference in shape and the high bulk-sensitivity at this photon energy suggest intrinsic electronic structure differences between the surface and bulk regions., Comment: 6 pages, 4 figures

We present angle-resolved photoemission spectroscopy (ARPES) data on moderately underdoped La$_{1.855}$Sr$_{0.145}$CuO$_4$ at temperatures below and above the superconducting transition temperature. Unlike previous studies of this material, we observe sharp spectral peaks along the entire underlying Fermi surface in the superconducting state. These peaks trace out an energy gap that follows a simple {\it d}-wave form, with a maximum superconducting gap of 14 meV. Our results are consistent with a single gap picture for the cuprates. Furthermore our data on the even more underdoped sample La$_{1.895}$Sr$_{0.105}$CuO$_4$ also show sharp spectral peaks, even at the antinode, with a maximum superconducting gap of 26 meV., Comment: Accepted by Phys. Rev. Lett

The existence of coherent quasiparticles near the Fermi energy in the low temperature state of high-temperature superconductors has been well established by angle-resolved photoemission spectroscopy (ARPES). This technique directly probes the momentum-resolved electronic excitation spectrum of the CuO$_2$ planes. We present a study of close to optimally doped La$_{1.83}$Sr$_{0.17}$CuO$_4$ in the superconducting state and report an abrupt change in the quasiparticle spectral function, as we follow the dispersion of the ARPES signal from the Fermi energy up to 0.6 eV. The interruption in the quasiparticle dispersion separates coherent quasiparticle peaks at low energies from broad incoherent excitations at high energies. We find that the boundary between these low-energy and high-energy features exhibits a cosine-shaped momentum dependence, reminiscent of the superconducting d-wave gap. Further intriguing similarities between characteristics of the incoherent excitations and quasiparticle properties (lifetime, Fermi arcs) suggest a close relation between the electronic response at high and low energies in cuprate superconductors.