14,637 research outputs found

    Random Magnetic Interactions and Spin Glass Order Competing with Superconductivity: Interference of the Quantum Parisi Phase

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    We analyse the competition between spin glass (SG) order and local pairing superconductivity (SC) in the fermionic Ising spin glass with frustrated fermionic spin interaction and nonrandom attractive interaction. The phase diagram is presented for all temperatures T and chemical potentials \mu. SC-SG transitions are derived for the relevant ratios between attractive and frustrated-magnetic interaction. Characteristic features of pairbreaking caused by random magnetic interaction and/or by spin glass proximity are found. The existence of low-energy excitations, arising from replica permutation symmetry breaking (RPSB) in the Quantum Parisi Phase, is shown to be relevant for the SC-SG phase boundary. Complete 1-step RPSB-calculations for the SG-phase are presented together with a few results for infinity-step breaking. Suppression of reentrant SG - SC - SG transitions due to RPSB is found and discussed in context of ferromagnet - SG boundaries. The relative positioning of the SC and SG phases presents a theoretical landmark for comparison with experiments in heavy fermion systems and high T_c superconductors. We find a crossover line traversing the SG-phase with (\mu=0,T=0) as its quantum critical (end)point in complete RPSB, and scaling is proposed for its vicinity. We argue that this line indicates a random field instability and suggest Dotsenko-Mezard vector replica symmetry breaking to occur at low temperatures beyond.Comment: 24 pages, 14 figures replaced by published versio

    Magnetic interactions and spin dynamics in the bond-disordered pyrochlore fluoride NaCaCo2_2F7_7

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    We report high-frequency/high-field electron spin resonance (ESR) and high-field magnetization studies on single crystals of the bond-disordered pyrochlore NaCaCo2_2F7_7. Frequency- and temperature-dependent ESR investigations above the freezing temperature Tf2.4T_f \sim 2.4 K reveal the coexistence of two distinct magnetic phases. A cooperative paramagnetic phase, evidenced by a gapless excitation mode, is found as well as a spin-glass phase developing below 20 K which is associated with a gapped low-energy excitation. Effective gg-factors close to 2 are obtained for both modes in line with pulsed high-field magnetization measurements which show an unsaturated isotropic behavior up to 58 T at 2 K. In order to describe the field-dependent magnetization in high magnetic fields, we propose an empirical model accounting for highly anisotropic ionic gg-tensors expected for this material and taking into account the strongly competing interactions between the spins which lead to a frustrated ground state. As a detailed quantitative relation between effective gg-factors as determined from ESR and the local gg-tensors obtained by neutron scattering [Ross et al., Phys. Rev. B 93, 014433 (2016)] is still sought after, our work motivates further theoretical investigations of the low-energy excitations in bond-disordered pyrochlores.Comment: 9 pages, 6 figure

    Valence Bonds in Random Quantum Magnets: Theory and Application to YbMgGaO4

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    We analyze the effect of quenched disorder on spin-1/2 quantum magnets in which magnetic frustration promotes the formation of local singlets. Our results include a theory for 2d valence-bond solids subject to weak bond randomness, as well as extensions to stronger disorder regimes where we make connections with quantum spin liquids. We find, on various lattices, that the destruction of a valence-bond solid phase by weak quenched disorder leads inevitably to the nucleation of topological defects carrying spin-1/2 moments. This renormalizes the lattice into a strongly random spin network with interesting low-energy excitations. Similarly when short-ranged valence bonds would be pinned by stronger disorder, we find that this putative glass is unstable to defects that carry spin-1/2 magnetic moments, and whose residual interactions decide the ultimate low energy fate. Motivated by these results we conjecture Lieb-Schultz-Mattis-like restrictions on ground states for disordered magnets with spin-1/2 per statistical unit cell. These conjectures are supported by an argument for 1d spin chains. We apply insights from this study to the phenomenology of YbMgGaO4_4, a recently discovered triangular lattice spin-1/2 insulator which was proposed to be a quantum spin liquid. We instead explore a description based on the present theory. Experimental signatures, including unusual specific heat, thermal conductivity, and dynamical structure factor, and their behavior in a magnetic field, are predicted from the theory, and compare favorably with existing measurements on YbMgGaO4_4 and related materials.Comment: v2: Stylistic revisions to improve clarity. 22 pages, 8 figures, 2 tables main text; 13 pages, 3 figures appendice

    Emergent glassiness in disorder-free Kitaev model: Density matrix renormalization group study on a one-dimensional ladder setting

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    The complete phase diagram of the Kitaev model with a magnetic field remains elusive, as do the experimental results in the candidate material {\alpha}-RuCl3. Here, we study the Kitaev model on a one-dimensional ladder setting within the density-matrix renormalization group method in the presence of a magnetic field at zero temperature. We find five distinct phases with increasing magnetic field, which are characterized by a homogeneous flux phase, the Z2 vortex gas, solid and emergent glass phase, and finally, a spin-polarized phase. The emergent glassiness is confirmed by calculating correlation functions showing quasi-long-range behavior and ground state fidelity, showing a plethora of energetically accessible orthogonal saddle points corresponding to different flux configurations. This glassy behavior seems to arise from the slow dynamics of the Z2 fluxes, which is a consequence of the local constraints present in the underlying Hilbert space. This phenomenon can also be explored in other spin-liquid systems where the corresponding low-energy excitations are similarly retarded due to constraints.Comment: 12 pages including abstrac

    Static and Dynamic Magnetism in the Electron-Doped High-Temperature Superconductor Pr\u3csub\u3e0.88\u3c/sub\u3eLaCe\u3csub\u3e0.12\u3c/sub\u3eCuO\u3csub\u3e4-δ\u3c/sub\u3e and in the \u3cem\u3ef\u3c/em\u3e-electron, Non-Fermi Liquid Alloy Sc\u3csub\u3e1-x\u3c/sub\u3eU\u3csub\u3ex\u3c/sub\u3ePd\u3csub\u3e3\u3c/sub\u3e

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    Here we present detailed studies of the spin dynamics within the electron-doped high temperature superconductor Pr.88LaCe.12CuO4-δ (PLCCO) and within the non-Fermi liquid metal Sc1-xUxPd3 (ScUPd). Comprehensive neutron scattering experiments were carried out mapping the evolution of magnetism within these systems as they are tuned across their respective phase diagrams. The novel features of the magnetic spectra within these systems are correlated with known anomalies in their quasiparticle behaviors driven through either the appearance of a superconducting phase in PLCCO or the emergence of a non-Fermi liquid phase in ScUPd. For the high-Tc cuprate Pr.88LaCe.12CuO4-δ, a detailed study of the evolution of the low energy spin excitations in this system is presented. In the unannealed, nonsuperconducting parent compound of PLCCO, the magnon excitations are well modeled as spin wave excitations arising from the long-range antiferromagnetic (AF) order in the system. As the system is doped into the superconducting phase towards optimal superconductivity, long-range AF order in the system is suppressed and low energy spin excitations behave drastically different than those observed in the NSC parent system. Instead of following the simple Bose statistics expected for spin wave magnon modes, the low energy excitations in superconducting concentrations show a form of hyperscaling in which the dynamic susceptibility is observed to scale as a function of ω/T. This likely reflects the influence of quantum critical excitations coupling to the spin degrees of freedom that arise from the quantum critical point in the phase diagram of PLCCO (where AF order is suppressed to 0K near optimal doping). High energy spin excitations in an under-doped concentration of PLCCO (Tc=21K) are also reported. Our experiments show that in contrast to the seemingly universal pattern of dispersion reported the spin excitations of hole-doped cuprates, the high energy excitations in this n-type system instead resemble those observed in the parent compounds of the high-Tc cuprates. Rather than the “hourglass”-type dispersion observed in hole-doped cuprates, the dispersion in this n-type system remains a broadened commensurate spot at low energies that disperses outward into a ring-like excitation at higher energy transfers. The actual dispersion for this underdoped concentration at higher energies is anomalously sharper than that reported for the parent systems, Pr2CuO4 and La2CuO4. Another facet of the spin excitations in PLCCO given particular focus is the newly discovered resonance mode in nearly optimally doped concentrations of PLCCO (Tc=24K). We find that the resonance mode in this system follows the universal relation ER=5.8kBTc for the resonance energies observed in all classes of cuprates. The resonance, when taken with the known commensurate response and high energy dispersion in the electron-doped cuprates, is therefore shown to stand as the long unifying feature in magnetic spectra of the cuprates, regardless of doped carrier-type. Field-induced changes in the magnetic spectrum of this optimally doped concentration of PLCCO (Tc=24K) are also discussed. Particular focus is given to the suppression of the resonance mode and field-induced spin density wave order in this system. The suppression of the resonance mode is seen to track the condensation energy in relative magnitude as a function of applied magnetic field, thereby suggesting that the mode itself is fundamentally connected to the mechanism of high-Tc superconductivity. For our studies of Sc1-xUxPd3, systematic neutron measurements mapped out magnetic excitations over a broad doping range. Concentrations near x=0.30, where the non-Fermi liquid (NFL) phase first appears, exhibit a seemingly local and nearly temperature-independent response in the spin excitation spectra. This parallels earlier results reported for another NFL system, UCu4Pd, whose spin dynamics acquire a local temperature insensitive character near the NFL phase boundary. We find that this spin behavior is characteristic of the proximity of a spin-glass quantum critical point in the phase diagram of these systems and possibly intrinsic to the appearance of NFL dynamics in these classes of materials

    Progress in Neutron Scattering Studies of Spin Excitations in High-Tc Cuprates

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    Neutron scattering experiments continue to improve our knowledge of spin fluctuations in layered cuprates, excitations that are symptomatic of the electronic correlations underlying high-temperature superconductivity. Time-of-flight spectrometers, together with new and varied single crystal samples, have provided a more complete characterization of the magnetic energy spectrum and its variation with carrier concentration. While the spin excitations appear anomalous in comparison with simple model systems, there is clear consistency among a variety of cuprate families. Focusing initially on hole-doped systems, we review the nature of the magnetic spectrum, and variations in magnetic spectral weight with doping. We consider connections with the phenomena of charge and spin stripe order, and the potential generality of such correlations as suggested by studies of magnetic-field and impurity induced order. We contrast the behavior of the hole-doped systems with the trends found in the electron-doped superconductors. Returning to hole-doped cuprates, studies of translation-symmetry-preserving magnetic order are discussed, along with efforts to explore new systems. We conclude with a discussion of future challenges.Comment: revised version, to be published in JPSJ, 20 pages, 21 figure

    Local excitations in mean field spin glasses

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    We address the question of geometrical as well as energetic properties of local excitations in mean field Ising spin glasses. We study analytically the Random Energy Model and numerically a dilute mean field model, first on tree-like graphs, equivalent to a replica symmetric computation, and then directly on finite connectivity random lattices. In the first model, characterized by a discontinuous replica symmetry breaking, we found that the energy of finite volume excitation is infinite whereas in the dilute mean field model, described by a continuous replica symmetry breaking, it slowly decreases with sizes and saturates at a finite value, in contrast with what would be naively expected. The geometrical properties of these excitations are similar to those of lattice animals or branched polymers. We discuss the meaning of these results in terms of replica symmetry breaking and also possible relevance in finite dimensional systems.Comment: 7 pages, 4 figures, accepted for publicatio

    The infinite-range quantum random Heisenberg magnet

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    We study with exact diagonalization techniques the Heisenberg model for a system of SU(2) spins with S=1/2 and random infinite-range exchange interactions. We calculate the critical temperature T_g for the spin-glass to paramagnetic transition. We obtain T_g ~ 0.13, in good agreement with previous quantum Monte Carlo and analytical estimates. We provide a detailed picture for the different kind of excitations which intervene in the dynamical response chi''(w,T) at T=0 and analyze their evolution as T increases. We also calculate the specific heat Cv(T). We find that it displays a smooth maximum at TM ~ 0.25, in good qualitative agreement with experiments. We argue that the fact that TM>Tg is due to a quantum disorder effect.Comment: 17 pages, 14 figure
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