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The $S=1$ Haldane state is constructed from a product of local singlet dimers in the bulk and topological states at the edges of a chain. It is a fundamental representative of topological quantum matter. Its well-known representative, the quasi-one-dimensional SrNi$_2$V$_2$O$_8$ shows both conventional as well as unconventional magnetic Raman scattering. The former is observed as one- and two-triplet excitations with small linewidths and energies corresponding to the Haldane gap $\Delta_H$ and the exchange coupling $J_c$ along the chain, respectively. Well-defined magnetic quasiparticles are assumed to be stabilized by interchain interactions and uniaxial single-ion anisotropy. Unconventional scattering exists as broad continua of scattering with an intensity $I(T)$ that shows a mixed bosonic / fermionic statistic. Such a mixed statistic has also been observed in Kitaev spin liquids and could point to a non-Abelian symmetry. As the ground state in the bulk of SrNi$_2$V$_2$O$_8$ is topologically trivial, we suggest its fractionalization to be due to light-induced interchain exchange processes. These processes are supposed to be enhanced due to a proximity to an Ising ordered state with a quantum critical point. A comparison with SrCo$_2$V$_2$O$_8$, the $S=1/2$ analogue to our title compound, supports these statements., Comment: 3 figures, 1 table

We present a concept for an ultra-thin solar fuel device with a nanostructured back contact. Using rigorous simulations we show that the nanostructuring significantly increases the absorption in the semiconductor, CuBi$_2$O$_4$ in this case, by 47\% (5.2~mAcm$^{-2}$) through the excitation of plasmonic modes. We are able to attribute the resonances in the device to metal-insulator-metal plasmons coupled to either localised surface plasmon resonances or surface plasmon polaritons. Rounding applied to the metallic corners leads to a blueshift in the resonance wavelength while maintaining absorption enhancement, thus supporting the possibility for a successful realization of the device. For a 2D array, the tolerance of the polarization-dependent absorption enhancement is investigated and compared to a planar structure. The device maintains an absorption enhancement up to incident angles of 75$^{\circ}$. The study highlights the high potential for plasmonics in ultra-thin opto-electronic devices such as in solar fuel generation., 4 Figures 18 Pages. Supporting Information Included (7 Figures 11 pages)

We present new experimental low-temperature heat capacity and detailed dynamical spin-structure factor data for the quantum spin-liquid candidate material ${\mathrm{Ca}}_{10}{\mathrm{Cr}}_{7}{\mathrm{O}}_{28}$. The measured heat capacity shows an almost-perfect linear temperature dependence in the range $0.1\phantom{\rule{0.28em}{0ex}}\mathrm{K}\phantom{\rule{4pt}{0ex}}\ensuremath{\lesssim}T\ensuremath{\lesssim}0.5\phantom{\rule{0.28em}{0ex}}\mathrm{K}$, reminiscent of fermionic spinon degrees of freedom. The spin-structure factor exhibits two energy regimes of strong signal which display rather different but solely diffuse scattering features. We theoretically describe these findings by an effective spinon-hopping model which crucially relies on the existence of strong ferromagnetically coupled triangles in the system. Our spinon theory is shown to naturally reproduce the overall weight distribution of the measured spin-structure factor. Particularly, we argue that various different observed characteristic properties of the spin-structure factor and the heat capacity consistently indicate the existence of a spinon Fermi surface. A closer analysis of the heat capacity at the lowest accessible temperatures hints toward the presence of weak $f$-wave spinon-pairing terms inducing a small partial gap along the Fermi surface (except for discrete nodal Dirac points) and suggesting an overall ${\mathbb{Z}}_{2}$ quantum spin-liquid scenario for ${\mathrm{Ca}}_{10}{\mathrm{Cr}}_{7}{\mathrm{O}}_{28}$.

Complex bound states of magnetic excitations, known as Bethe string, were predicted almost a century ago to exist in one-dimensional quantum magnets 1. The dispersions of the string states have so far remained the subject of intensive theoretical studies 2-7. By performing neutron scattering experiments on the one-dimensional Heisenberg-Ising antiferromagnet SrCo2V2O8 in high longitudinal magnetic fields, we reveal in detail the dispersion relations of the string states over the full Brillouin zone, as well as their magnetic field dependences. Furthermore the characteristic energy, the scattering intensity and linewidth of the observed string states exhibit excellent agreement with our precise Bethe Ansatz calculations. Our results establish the important role of string states in the quantum spin dynamics of one-dimensional systems, and will invoke studies of their dynamical properties in more general many-body systems., Main manuscript: 6 pages, 4 figures, Supplementary information: 8 pages, 6 figures, 1 table

Unlike conventional magnets where the magnetic moments are partially or completely static in the ground state, in a quantum spin liquid they remain in collective motion down to the lowest temperatures. The importance of this state is that it is coherent and highly entangled without breaking local symmetries. Such phenomena is usually sought in simple lattices where antiferromagnetic interactions and/or anisotropies that favor specific alignments of the magnetic moments are "frustrated" by lattice geometries incompatible with such order e.g. triangular structures. Despite an extensive search among such compounds, experimental realizations remain very few. Here we describe the investigation of a novel, unexplored magnetic system consisting of strong ferromagnetic and weaker antiferromagnetic isotropic interactions as realized by the compound Ca$_{10}$Cr$_7$O$_{28}$. Despite its exotic structure we show both experimentally and theoretically that it displays all the features expected of a quantum spin liquid including coherent spin dynamics in the ground state and the complete absence of static magnetism., Modified version accepted in Nature Physics

Many-body two- and three-string states are realized experimentally in the antiferromagnetic Heisenberg–Ising chain SrCo2V2O8 in strong longitudinal magnetic fields. The spin of electrons is what gives most materials their magnetic properties, and these spins are often treated as if they are located at fixed points in a crystal lattice. When a single spin flips in a one-dimensional system, it can cause neighbouring spins to flip and thus propagate the flip along the chain. This type of magnetic excitation can be described as an elementary quasiparticle known as a magnon. In the 1930s, Hans Bethe predicted that multiple spins could flip and that the multi-spin flip could propagate, giving rise to complex multi-magnon states that look like strings. Using terahertz spectroscopy, Zhe Wang and colleagues measure such 'Bethe strings' directly in a one-dimensional Heisenberg–Ising antiferromagnet, and show that they dominate the quantum spin dynamics when the system is close to a quantum phase transition. Almost a century ago, string states—complex bound states of magnetic excitations—were predicted to exist in one-dimensional quantum magnets1. However, despite many theoretical studies2,3,4,5,6,7,8,9,10,11, the experimental realization and identification of string states in a condensed-matter system have yet to be achieved. Here we use high-resolution terahertz spectroscopy to resolve string states in the antiferromagnetic Heisenberg–Ising chain SrCo2V2O8 in strong longitudinal magnetic fields. In the field-induced quantum-critical regime, we identify strings and fractional magnetic excitations that are accurately described by the Bethe ansatz1,3,4. Close to quantum criticality, the string excitations govern the quantum spin dynamics, whereas the fractional excitations, which are dominant at low energies, reflect the antiferromagnetic quantum fluctuations. Today, Bethe’s result1 is important not only in the field of quantum magnetism but also more broadly, including in the study of cold atoms and in string theory; hence, we anticipate that our work will shed light on the study of complex many-body systems in general.

The quantum spin liquid is a highly entangled magnetic state characterized by the absence of static magnetism in its ground state. Instead, the spins fluctuate in a highly correlated way down to the lowest temperatures. Quantum spin liquids are very rare and are confined to a few specific cases where the interactions between the magnetic ions cannot be simultaneously satisfied (known as frustration). Lattices with magnetic ions in triangular or tetrahedral arrangements, which interact via isotropic antiferromagnetic interactions, can generate such a frustration. Three-dimensional isotropic spin liquids have mostly been sought in materials where the magnetic ions form pyrochlore or hyperkagome lattices. Here we present a three-dimensional lattice called the hyper-hyperkagome that enables spin liquid behaviour and manifests in the compound PbCuTe2O6. Using a combination of experiment and theory, we show that this system exhibits signs of being a quantum spin liquid with no detectable static magnetism together with the presence of diffuse continua in the magnetic spectrum suggestive of fractional spinon excitations., Quantum spin liquids have magnetic moments that do not form magnetic order even as the temperature approaches zero, leading to the dominance of quantum fluctuations. Chillal et al. present evidence that the hyper-hyperkagome lattice of PbCuTe2O6 hosts a three-dimensional quantum spin liquid.

A spin liquid is a new state of matter with topological order where the spin moments continue to fluctuate coherently down to the lowest temperatures rather than develop static long-range magnetic order as found in conventional magnets. For spin liquid behavior to arise in a material the magnetic Hamiltonian must be ``frustrated'', where the combination of lattice geometry, interactions, and anisotropies gives rise to competing spin arrangements in the ground state. Theoretical Hamiltonians which produce spin liquids are spin ice, the Kitaev honeycomb model, and the kagome antiferromagnet. Spin liquid behavior, however, in real materials is rare because they can only approximate these Hamiltonians and often have weak higher-order terms that destroy the spin liquid state. ${\mathrm{Ca}}_{10}{\mathrm{Cr}}_{7}{\mathrm{O}}_{28}$ is a new quantum spin liquid candidate with magnetic ${\mathrm{Cr}}^{5+}$ ions that possess quantum spin number $S=\textonehalf{}$. The spins are entirely dynamic in the ground state and the excitation spectrum is broad and diffuse, as is typical of spinons which are the excitations of a spin liquid. In this paper we determine the Hamiltonian of ${\mathrm{Ca}}_{10}{\mathrm{Cr}}_{7}{\mathrm{O}}_{28}$ using inelastic neutron scattering under high magnetic field to induce a field-polarized paramagnetic ground state and spin-wave excitations that can be fitted to extract the interactions. We further explore the phase diagram by using inelastic neutron scattering and heat capacity measurements and establish the boundaries of the spin liquid phase as a function of magnetic field and temperature. Our results show that ${\mathrm{Ca}}_{10}{\mathrm{Cr}}_{7}{\mathrm{O}}_{28}$ consists of distorted kagome bilayers with several isotropic ferromagnetic and antiferromagnetic interactions where, unexpectedly, the ferromagnetic interactions are stronger than the antiferromagnetic ones. This complex Hamiltonian does not correspond to any known spin liquid model and points to new directions in the search for quantum spin liquid behavior.

A detailed diffraction study of Ca

A detailed diffraction study of Ca10Cr7O28 is presented which adds significant new insights into the structural and magnetic properties of this compound. A new crystal structure type was used where the a and b axes are doubled compared to previous models providing a more plausible structure where all crystallographic sites are fully occupied. The presence of two different valences of chromium was verified and the locations of the magnetic Cr5+ and non-magnetic Cr6+ ions were identified. The Cr5+ ions have spin-1/2 and form distorted kagome bilayers which are stacked in an ABC arrangement along the c axis. These results lay the foundation for understanding of the quantum spin liquid behavior in Ca10Cr7O28 which has recently been reported in [C. Balz et al., Nature Physics, 12, 942 (2016)].

We report on spectroscopy study of elementary magnetic excitations in an Ising-like antiferromagnetic chain compound SrCo$_2$V$_2$O$_8$ as a function of temperature and applied transverse magnetic field up to 25 T. An optical as well as an acoustic branch of confined spinons, the elementary excitations at zero field, are identified in the antiferromagnetic phase below the N\'{e}el temperature of 5 K and described by a one-dimensional Schr\"{o}dinger equation. The confinement can be suppressed by an applied transverse field and a quantum disordered phase is induced at 7 T. In this disordered paramagnetic phase, we observe three emergent fermionic excitations with different transverse-field dependencies. The nature of these modes is clarified by studying spin dynamic structure factor of a 1D transverse-field Heisenberg-Ising (XXZ) model using the method of infinite time evolving block decimation. Our work reveals emergent quantum phenomena and provides a concrete system for testifying theoretical predications of one-dimension quantum spin models., Comment: 8 pages and 6 figures

Ultrasound velocity measurements of the orbitally degenerate frustrated spinel ${\mathrm{MgV}}_{2}{\mathrm{O}}_{4}$ are performed in a high-purity single crystal which exhibits successive structural and antiferromagnetic phase transitions, and in the disorder-introduced single crystal which exhibits spin-glass-like behavior. The measurements reveal that two types of unusual temperature dependence of the elastic moduli coexist in the cubic paramagnetic phase, which are resolved by magnetic-field and disorder sensitivities: huge Curie-type softening with decreasing temperature (convex temperature dependence), and concave temperature dependence with a characteristic minimum. These elastic anomalies suggest the coupling of the lattice to coexisting orbital fluctuations and spin-cluster excitations.

We have grown several single crystals of the highly frustrated $S$ = 1 spinel MgV${}_{2}$O${}_{4}$ using different starting compositions and growth conditions. From our study of their physical properties by magnetic susceptibility, heat capacity, and single-crystal neutron and powder-diffraction measurements, we observe that a minute amount of disorder suppresses the structural and magnetic phase transitions. As little as 3$%$ disorder in the octahedral site introduced random strain, which was enough to completely suppress the transitions and induce a spin-glass phase at low temperatures. We believe that the reason is spin-exchange disorder rather than disorder in the weakly diluted magnetic lattice. Our results also show that the MgO-V${}_{2}$O${}_{3}$ system is a solid solution that melts slightly incongruently, and we demonstrate that by using an optimized solvent in the traveling-solvent floating-zone configuration a large single crystal ($l$ = 25 mm, $d$ = 6 mm) with a homogeneous composition, free of site disorders can be grown.

The spin dynamics and magnetic excitations of the slightly distorted triangular s = 3/2 system alpha-CaCr2O4 are investigated by means of Raman spectroscopy and electron spin resonance (ESR) to elucidate its peculiar magnetic properties. Two-magnon excitations in circular RL symmetry show a multi-maximum structure with a dominant spectral weight at low energies. The temperature dependence of the ESR linewidth is described by a critical broadening DeltaHpp(T) ~ (T - T_N)^{-p} with the exponent p = 0.30(3) - 0.38(5) for temperatures above T_N = 42.6 K. The exponent is much smaller than that of other s = 3/2 triangular lattices. This is ascribed to soft roton-like modes, indicative of the instability of a helical 120{\deg} phase. As an origin we discuss a complex spin topology formed by four inequivalent nearest neighbor and sizable next-nearest neighbor interactions., Comment: 7 pages, 4 figures

We report on an extended fluctuation regime in the spin dimer system Sr${}_{3}$Cr${}_{2}$O${}_{8}$ based on anomalies in Raman active phonons and magnetic scattering. The compound has two characteristic temperatures. The Jahn-Teller transition at ${T}_{S}=275$ K is related to structural distortions and orbital ordering and the second instability at ${T}^{*}\ensuremath{\approx}150$ K is due to further changes in the orbital sector. Below ${T}_{S}$ quasielastic scattering marks strong fluctuations and in addition phonon anomalies are observed. For temperatures below ${T}^{*}$ we observe an exponential decrease of one phonon linewidth and determine a gap of the orbital excitations. At low temperatures the observation of two- and three-magnon scattering allows the determination of the spin excitation gap.

Intrinsic Josephson junction (IJJ) stacks of cuprate superconductors have potential to be implemented as intrinsic phase qubits working at relatively high temperatures. We report success in fabricating submicron La$_{2-x}$Sr$_{x}$CuO$_{4}$ (LSCO) IJJ stacks carved out of single crystals. We also show a new fabrication method in which argon ion etching is performed after focused ion beam etching. As a result, we obtained an LSCO IJJ stack in which resistive multi-branches appeared. It may be possible to control the number of stacked IJJs with an accuracy of a single IJJ by developing this method., 5 pages, 6 figures

We present a study of the frustrated spinel ${\text{MgV}}_{2}{\text{O}}_{4}$, which possesses highly coupled spin, lattice, and orbital degrees of freedom. Using large single crystal and powder samples, we find a distortion from spinel at room temperature (space group $F\overline{4}3m$) which allows for a greater trigonal distortion of the ${\text{VO}}_{6}$ octahedra and a low-temperature space group $(I\overline{4}m2)$ that maintains the mirror plane symmetry. The magnetic structure that develops below 42 K consists of antiferromagnetic chains with a strongly reduced moment while inelastic neutron scattering reveals one-dimensional behavior and a single band of excitations. The implications of these results are discussed in terms of various orbital-ordering scenarios. We conclude that although spin-orbit coupling must be significant to maintain the mirror plane symmetry, the trigonal distortion is large enough to mix the $3d$ levels leading to a wave function of mixed real and complex orbitals.

We have studied epitaxial strain effect on superconductivity in single-crystal ${\text{La}}_{2\ensuremath{-}x}{\text{Sr}}_{x}{\text{CuO}}_{4}$ films grown by liquid-phase epitaxy method on (001) ${\text{La}}_{2}{\text{CuO}}_{4}$ substrates. Due to lattice mismatch the as-grown films suffer a compressive strain in the $c$ axis and an orthorhombic tensile strain on the $ab$ plane with almost no relaxation up to several micrometers thickness. Our results show that ${\text{La}}_{2\ensuremath{-}x}{\text{Sr}}_{x}{\text{CuO}}_{4}$ $(0.10\ensuremath{\le}x\ensuremath{\le}0.15)$, which is superconducting in the bulk at low temperatures, transforms to an insulating state under such strain.