Quantum spin liquids may be considered "quantum disordered" ground states of spin systems, in which zero point fluctuations are so strong that they prevent conventional magnetic long range order. More interestingly, quantum spin liquids are prototypical examples of ground states with massive many-body entanglement, of a degree sufficient to render these states distinct phases of matter. Their highly entangled nature imbues quantum spin liquids with unique physical aspects, such as non-local excitations, topological properties, and more. In this review, we discuss the nature of such phases and their properties based on paradigmatic models and general arguments, and introduce theoretical technology such as gauge theory and partons that are conveniently used in the study of quantum spin liquids. An overview is given of the different types of quantum spin liquids and the models and theories used to describe them. We also provide a guide to the current status of experiments to study quantum spin liquids, and to the diverse probes used therein.

Quantum Spin Liquids

Quantum spin liquids may be considered 'quantum disordered' ground states of spin systems, in which zero-point fluctuations are so strong that they prevent conventional magnetic long-range order. More interestingly, quantum spin liquids are prototypical examples of ground states with massive many-body entanglement, which is of a degree sufficient to render these states distinct phases of matter. Their highly entangled nature imbues quantum spin liquids with unique physical aspects, such as non-local excitations, topological properties, and more. In this review, we discuss the nature of such phases and their properties based on paradigmatic models and general arguments, and introduce theoretical technology such as gauge theory and partons, which are conveniently used in the study of quantum spin liquids. An overview is given of the different types of quantum spin liquids and the models and theories used to describe them. We also provide a guide to the current status of experiments in relation to study quantum spin liquids, and to the diverse probes used therein.

https://iopscience.iop.org/article/10.1088/0034-4885/80/1/016502/pdf

Quantum spin liquids: a review.

This is an introductory review of the physics of quantum spin liquid states. Quantum magnetism is a rapidly evolving field, and recent developments reveal that the ground states and low-energy physics of frustrated spin systems may develop many exotic behaviors once we leave the regime of semiclassical approaches. The purpose of this article is to introduce these developments. The article begins by explaining how semiclassical approaches fail once quantum mechanics become important and then describe the alternative approaches for addressing the problem. Mainly spin-1/2 systems are discussed, and most of the time is spent in this article on one particular set of plausible spin liquid states in which spins are represented by fermions. These states are spin-singlet states and may be viewed as an extension of Fermi liquid states to Mott insulators, and they are usually classified in the category of so-called SU(2), U(1), or Z2 spin liquid states. A review is given of the basic theory regarding these states and the extensions of these states to include the effect of spin-orbit coupling and to higher spin (S>1/2) systems. Two other important approaches with strong influences on the understanding of spin liquid states are also introduced: (i) matrix product states and projected entangled pair states and (ii) the Kitaev honeycomb model. Experimental progress concerning spin liquid states in realistic materials, including anisotropic triangular-lattice systems [κ-(ET)2Cu2(CN)3 and EtMe3Sb[Pd(dmit)2]2], kagome-lattice system [ZnCu3(OH)6Cl2], and hyperkagome lattice system (Na4Ir3O8), is reviewed and compared against the corresponding theories.

Quantum spin liquid states

Tunneling spectroscopy has played a central role in the experimental verification of the microscopic theory of superconductivity in classical superconductors. Initial attempts to apply the same approach to high-temperature superconductors were hampered by various problems related to the complexity of these materials. The use of scanning tunneling microscopy and spectroscopy (STM and STS) on these compounds allowed the main difficulties to be overcome. This success motivated a rapidly growing scientific community to apply this technique to high-temperature superconductors. This paper reviews the experimental highlights obtained over the last decade. The crucial efforts to gain control over the technique and to obtain reproducible results are first recalled. Then a discussion on how the STM and STS techniques have contributed to the study of some of the most unusual and remarkable properties of high-temperature superconductors is presented: the unusually large gap values and the absence of scaling with the critical temperature, the pseudogap and its relation to superconductivity, the unprecedented small size of the vortex cores and its influence on vortex matter, the unexpected electronic properties of the vortex cores, and the combination of atomic resolution and spectroscopy leading to the observation of periodic local density of states modulations in the superconducting and pseudogap states and in the vortex cores.

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Scanning tunneling spectroscopy of high-temperature superconductors

We review studies of the electromagnetic response of various classes of correlated electron materials including transition metal oxides, organic and molecular conductors, intermetallic compounds with $d$- and $f$-electrons as well as magnetic semiconductors. Optical inquiry into correlations in all these diverse systems is enabled by experimental access to the fundamental characteristics of an ensemble of electrons including their self-energy and kinetic energy. Steady-state spectroscopy carried out over a broad range of frequencies from microwaves to UV light and fast optics time-resolved techniques provide complimentary prospectives on correlations. Because the theoretical understanding of strong correlations is still evolving, the review is focused on the analysis of the universal trends that are emerging out of a large body of experimental data augmented where possible with insights from numerical studies.

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Electrodynamics of correlated electron materials

Low-temperature thermal-transport measurements of a frustrated organic magnet in which a quantum spin-liquid is believed to exist, suggest that the emergence of this state is accompanied by a spin-gap. This contradicts previous studies conducted at higher temperatures, suggesting that our understanding of this system should be re-evaluated.

Thermal-transport measurements in a quantum spin-liquid state of the frustrated triangular magnet -(BEDT-TTF) 2 Cu 2 (CN) 3

We have measured and analyzed the dielectric constant of the dimer Mott insulator $\ensuremath{\kappa}\text{\ensuremath{-}}{(\text{BEDT-TTF})}_{2}{\text{Cu}}_{2}{(\text{CN})}_{3}$, which is known as a playground for a spin-liquid state. Most unexpectedly, this particular organic salt has nontrivial charge degrees of freedom, being characterized by a relaxor-like dielectric relaxation below around 60 K. This is ascribed to the charge disproportionation within the dimer due to the intersite Coulomb repulsion. A possible microscopic model is suggested and discussed.

Anomalous dielectric response in the dimer Mott insulator κ − ( BEDT-TTF ) 2 Cu 2 ( CN ) 3

Magnetization measurements on untwinned YBa2Cu3Oy are performed in magnetic fields up to 27 T using the Hall probe magnetometry and the anomalous second peak is found near the multicritical point. Below the second peak, the vortex pinning force shows a steep increase at a characteristic field H*(T), which is connected both with the first-order vortex lattice melting line Tm(H) and with the second-order vortex glass transition line Tg(H) at the multicritical point. The field-driven transition from the ordered Bragg glass to the disordered vortex glass phase is discussed as a possible origin of H*(T). @S0163-1829~98!00841-8#

Anomalous magnetization and field-driven disordering transition of a vortex lattice in untwinned yba2cu3oy

Human-assisted reproductive technologies (ART) are a widely accepted treatment for infertile couples. At the same time, many studies have suggested the correlation between ART and increased incidences of normally rare imprinting disorders such as Beckwith-Wiedemann syndrome (BWS), Angelman syndrome (AS), Prader-Willi syndrome (PWS), and Silver-Russell syndrome (SRS). Major methylation dynamics take place during cell development and the preimplantation stages of embryonic development. ART may prevent the proper erasure, establishment, and maintenance of DNA methylation. However, the causes and ART risk factors for these disorders are not well understood. A nationwide epidemiological study in Japan in 2015 in which 2777 pediatrics departments were contacted and a total of 931 patients with imprinting disorders including 117 BWS, 227 AS, 520 PWS, and 67 SRS patients, were recruited. We found 4.46- and 8.91-fold increased frequencies of BWS and SRS associated with ART, respectively. Most of these patients were conceived via in vitro fertilization (IVF) and intracytoplasmic sperm injection (ICSI), and showed aberrant imprinted DNA methylation. We also found that ART-conceived SRS (ART-SRS) patients had incomplete and more widespread DNA methylation variations than spontaneously conceived SRS patients, especially in sperm-specific methylated regions using reduced representation bisulfite sequencing to compare DNA methylomes. In addition, we found that the ART patients with one of three imprinting disorders, PWS, AS, and SRS, displayed additional minor phenotypes and lack of the phenotypes. The frequency of ART-conceived Prader-Willi syndrome (ART-PWS) was 3.44-fold higher than anticipated. When maternal age was 37 years or less, the rate of DNA methylation errors in ART-PWS patients was significantly increased compared with spontaneously conceived PWS patients. We reconfirmed the association between ART and imprinting disorders. In addition, we found unique methylation patterns in ART-SRS patients, therefore, concluded that the imprinting disorders related to ART might tend to take place just after fertilization at a time when the epigenome is most vulnerable and might be affected by the techniques of manipulation used for IVF or ICSI and the culture medium of the fertilized egg.

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Association of four imprinting disorders and ART

Due to an error during the printing process, many of the figures in this paper were printed incorrectly, despite appearing correctly in the proofs that were checked by the authors and passed for press. All the figures from this paper are reprinted correctly below. Figure 1. The temperature dependence of the resistivity for untwinned YBa2Cu3Oy at various magnetic fields (H||c axis). Magnetic field increases from right to left. Inset, the field dependence of the resistivity around Hm(T) with hysteresis subloop. Figure 2. Magnetization as a function of temperature in ZFC and FCC modes at 5 T for untwinned YBa2Cu3Oy. The inset is an expanded view of the magnetization and the jump in the magnetization μ0(M - Ms) near the vortex-lattice melting temperature. Figure 3. The inverse logarithmic derivative of the resistivity near the vortex-glass transition temperature Tg for untwinned YBa2Cu3Oy. Inset, the temperature dependence of the resistivity above (broken curves) and below (full curves) the critical point. Figure 4. Magnetization curves at various temperatures for untwinned YBa2Cu3Oy. Inset, the field derivative dM/dH shows a sharp peak at a characteristic field H*. Figure 5. The magnetic-field dependence of the critical current density Jc for untwinned YBa2Cu3Oy at several temperatures. Figure 6. The magnetic-field dependence of the critical current density Jc for both untwinned and heavy-twinned YBa2Cu3Oy at 58, 63, and 70 K. Figure 7. The vortex-matter phase diagram in untwinned YBa2Cu3Oy. The transition lines Tm(H), Tg(H), and H*(T) terminate at the critical point and divide into three different phases of the vortex liquid, the vortex glass, and the Bragg glass. The full curve is a fit to the field-driven transition line Bdis(T). Figure 8. Magnetization curves of untwinned YBa2Cu3Oy before and after electron irradiation with different irradiation doses for different temperatures: (a) T Tcp. Figure 9. The magnetic-field dependence of the critical current density Jc for untwinned YBa2Cu3Oy before and after electron irradiation. Figure 10. (a) Normalized resistivity against temperature in untwinned YBa2Cu3Oy before and after electron irradiation. (b) The inverse logarithmic derivative of the resistivity at 13 T in the vicinity of the vortex-glass transition temperature Tg. Inset, the reduced temperature dependence of the activation energy at 13 T. Figure 11. The vortex-matter phase diagram in untwinned YBa2Cu3Oy before and after electron irradiation. The three different phases, i.e., the vortex liquid, the vortex glass, and the Bragg glass, are divided by transition lines Hm(T), Hg(T), and H*(T). The full curves are fits to the field-driven transition line Bdis(T).

http://iopscience.iop.org/article/10.1088/0953-2048/13/1/301/pdf

Norio Kobayashi

Papers

Thermal-transport measurements in a quantum spin-liquid state of the frustrated triangular magnet -(BEDT-TTF) 2 Cu 2 (CN) 3

Anomalous dielectric response in the dimer Mott insulator κ − ( BEDT-TTF ) 2 Cu 2 ( CN ) 3

Anomalous magnetization and field-driven disordering transition of a vortex lattice in untwinned yba2cu3oy

Association of four imprinting disorders and ART

Vortex-matter phase diagram in YBa2Cu3Oy