Junichiro Wada

Bio: Junichiro Wada is an academic researcher from Tohoku University. The author has contributed to research in topics: Neutron scattering & Magnetic structure. The author has an hindex of 7, co-authored 9 publications receiving 747 citations.

Doping dependence of the spatially modulated dynamical spin correlations and the superconducting-transition temperature in La 2-x Sr x CuO 4

Abstract: Systematic low-energy neutron-scattering studies have been performed on float-zone-grown single crystals of ${\mathrm{La}}_{2\ensuremath{-}x}{\mathrm{Sr}}_{x}{\mathrm{CuO}}_{4}$ with $x$ extending from zero doping, $x=0$, to the overdoped, weakly superconducting regime, $x=025$ For $x$ beyond a critical doping value of ${x}_{c}\ensuremath{\approx}005$ the low-energy spin-fluctuation peak position shifts from ($\frac{1}{2}$, $\frac{1}{2}$) to ($\frac{1}{2}$\ifmmode\pm\else\textpm\fi{}\ensuremath{\delta}, $\frac{1}{2}$), and ($\frac{1}{2}$, $\frac{1}{2}$\ifmmode\pm\else\textpm\fi{}\ensuremath{\delta}); ${x}_{c}$ also represents the onset concentration for superconductivity For $006l~xl~012$ the incommensurability $\ensuremath{\delta}$ follows approximately the quantitative relation $\ensuremath{\delta}=x$ However, beyond $x\ensuremath{\approx}012$ the incommensurability tends to saturate around $\ensuremath{\delta}\ensuremath{\approx}1/8$ The superconducting-transition temperature ${T}_{c}(x)$ for stoichiometric samples at a given doping scales linearly with $\ensuremath{\delta}$ up to the optimal doping value of $x$ The peak momentum width of the spin fluctuations at low energies is small throughout the superconducting concentration region except in the strongly overdoped region An anomalously small width is observed for $x=\frac{1}{8}$ The incommensurate spatial modulation is found to be robust with respect to pair-breaking effects that lower ${T}_{c},$ such as deoxygenation of the sample or replacement of Cu by Zn

Spatial modulation of low-frequency spin fluctuations in hole-doped La2CuO4

Abstract: Systematic neutron scattering measurements have been performed on the Sr-doped La{2-x}SrxCuO4 to study the doping dependence of spatially modulated dynamical spin correlations or so-called incommensurate spin fluctuations. The modulated spin correlations appear beyond x ≈ 0.05 which is close to the lower boundary of the superconducting phase. First evidence was observed for the linear relation between the degree of spatial modulation or the incommensurability Σ(x) and the maximumTc atx. We present a universal curve for Σ(x) by adding data from other La2CuO2 systems such as oxygen-doped superconductors, oxygen-reduced or Zn-substituted nonsuperconductors, and La1.6-xNd0.4SrxCuO4.

Preparation of single crystal of Ca2CuO3 by TSFZ method

Abstract: A bulky (7×3×3 mm 3 ) single crystal fo Ca 2 CuO 3 has been successfully prepared by the traveling-solvent-floating-zone (TSFZ) method using CuO excess solvent. An optimum solvent composition for growth of the titled compound was found to be around 9 in CuO/CaO molar ratio R under 0.1 MPa oxygen partial pressure. The obtained R value considerably deviates from the estimated one based on the reported equilibrium phase diagrams. Anisotropic magnetic susceptibilities of the prepared single crystal were observed. In particular an anomalous thermal behaviour of the susceptibility under the magnetic field along [001] and [010] indicates an appearance of three-dimensional antiferromagnetic order below around 8 K.

Antiferromagnetic long range order of the linear chain cuprate Ca2CuO3

Abstract: Neutron scattering measurements have been performed on a single crystal of Ca 2 CuO 3 , an S = 1 2 quasi-one-dimensional (ID) antiferromagnet A three-dimensional (3D) antiferromagnetic long range order was observed below 8 K with an antiparallel Cu spin alignment within the CuO chain and between the nearest neighbor spins on the different chains The anomalously small ordered moment of about 005 μ B might originate from the large energy scale of the spin fluctuation

Neutron Scattering Study of Oxygen Ordering and Magnetic Correlations in La 2NiO 4.11 and La 2NiO 4.125

Abstract: We have performed neutron scattering experiments on single crystals of La 2 NiO 4+δ (δ=0.11, and 0.125) to study the effect of oxygen doping on the structural and magnetic properties. The degree of the excess oxygen order strongly influences the local crystal structure and interlayer magnetic correlation. The intralayer magnetic correlation is depressed by doping in the commensurate region (δ≤0.11), then turns to increase in the incommensurate region (δ>0.11). Our experimental results are compared with existing results for oxygen and Sr doped La 2 NiO 4 .

Doping a Mott insulator: Physics of high-temperature superconductivity

Abstract: This article reviews the physics of high-temperature superconductors from the point of view of the doping of a Mott insulator. The basic electronic structure of cuprates is reviewed, emphasizing the physics of strong correlation and establishing the model of a doped Mott insulator as a starting point. A variety of experiments are discussed, focusing on the region of the phase diagram close to the Mott insulator (the underdoped region) where the behavior is most anomalous. The normal state in this region exhibits pseudogap phenomenon. In contrast, the quasiparticles in the superconducting state are well defined and behave according to theory. This review introduces Anderson's idea of the resonating valence bond and argues that it gives a qualitative account of the data. The importance of phase fluctuations is discussed, leading to a theory of the transition temperature, which is driven by phase fluctuations and the thermal excitation of quasiparticles. However, an argument is made that phase fluctuations can only explain pseudogap phenomenology over a limited temperature range, and some additional physics is needed to explain the onset of singlet formation at very high temperatures. A description of the numerical method of the projected wave function is presented, which turns out to be a very useful technique for implementing the strong correlation constraint and leads to a number of predictions which are in agreement with experiments. The remainder of the paper deals with an analytic treatment of the $t\text{\ensuremath{-}}J$ model, with the goal of putting the resonating valence bond idea on a more formal footing. The slave boson is introduced to enforce the constraint againt double occupation and it is shown that the implementation of this local constraint leads naturally to gauge theories. This review follows the historical order by first examining the U(1) formulation of the gauge theory. Some inadequacies of this formulation for underdoping are discussed, leading to the SU(2) formulation. Here follows a rather thorough discussion of the role of gauge theory in describing the spin-liquid phase of the undoped Mott insulator. The difference between the high-energy gauge group in the formulation of the problem versus the low-energy gauge group, which is an emergent phenomenon, is emphasized. Several possible routes to deconfinement based on different emergent gauge groups are discussed, which leads to the physics of fractionalization and spin-charge separation. Next the extension of the SU(2) formulation to nonzero doping is described with a focus on a part of the mean-field phase diagram called the staggered flux liquid phase. It will be shown that inclusion of the gauge fluctuation provides a reasonable description of the pseudogap phase. It is emphasized that $d$-wave superconductivity can be considered as evolving from a stable U(1) spin liquid. These ideas are applied to the high-${T}_{c}$ cuprates, and their implications for the vortex structure and the phase diagram are discussed. A possible test of the topological structure of the pseudogap phase is described.

Doping a Mott Insulator: Physics of High Temperature Superconductivity

Abstract: This article reviews the effort to understand the physics of high temperature superconductors from the point of view of doping a Mott insulator. The basic electronic structure of the cuprates is reviewed, emphasizing the physics of strong correlation and establishing the model of a doped Mott insulator as a starting point. A variety of experiments are discussed, focusing on the region of the phase diagram close to the Mott insulator (the underdoped region) where the behavior is most anomalous. We introduce Anderson's idea of the resonating valence bond (RVB) and argue that it gives a qualitative account of the data. The importance of phase fluctuation is discussed, leading to a theory of the transition temperature which is driven by phase fluctuation and thermal excitation of quasiparticles. We then describe the numerical method of projected wavefunction which turns out to be a very useful technique to implement the strong correlation constraint, and leads to a number of predictions which are in agreement with experiments. The remainder of the paper deals with an analytic treatment of the t-J model, with the goal of putting the RVB idea on a more formal footing. The slave-boson is introduced to enforce the constraint of no double occupation. The implementation of the local constraint leads naturally to gauge theories. We give a rather thorough discussion of the role of gauge theory in describing the spin liquid phase of the undoped Mott insulator. We next describe the extension of the SU(2) formulation to nonzero doping. We show that inclusion of gauge fluctuation provides a reasonable description of the pseudogap phase.

How to detect fluctuating stripes in the high-temperature superconductors

Abstract: This article discusses fluctuating order in a quantum disordered phase proximate to a quantum critical point, with particular emphasis on fluctuating stripe order. Optimal strategies are derived for extracting information concerning such local order from experiments, with emphasis on neutron scattering and scanning tunneling microscopy. These ideas are tested by application to two model systems---an exactly solvable one-dimensional (1D) electron gas with an impurity, and a weakly interacting 2D electron gas. Experiments on the cuprate high-temperature superconductors which can be analyzed using these strategies are extensively reviewed. The authors adduce evidence that stripe correlations are widespread in the cuprates. They compare and contrast the advantages of two limiting perspectives on the high-temperature superconductor: weak coupling, in which correlation effects are treated as a perturbation on an underlying metallic (although renormalized) Fermi-liquid state, and strong coupling, in which the magnetism is associated with well-defined localized spins, and stripes are viewed as a form of micro phase separation. The authors present quantitative indicators that the latter view better accounts for the observed stripe phenomena in the cuprates.

Advances in the physics of high-temperature superconductivity

Abstract: The high-temperature copper oxide superconductors are of fundamental and enduring interest. They not only manifest superconducting transition temperatures inconceivable 15 years ago, but also exhibit many other properties apparently incompatible with conventional metal physics. The materials expand our notions of what is possible, and compel us to develop new experimental techniques and theoretical concepts. This article provides a perspective on recent developments and their implications for our understanding of interacting electrons in metals.

A cold-atom Fermi–Hubbard antiferromagnet

Abstract: Exotic phenomena in systems with strongly correlated electrons emerge from the interplay between spin and motional degrees of freedom. For example, doping an antiferromagnet is expected to give rise to pseudogap states and high-temperature superconductors. Quantum simulation using ultracold fermions in optical lattices could help to answer open questions about the doped Hubbard Hamiltonian, and has recently been advanced by quantum gas microscopy. Here we report the realization of an antiferromagnet in a repulsively interacting Fermi gas on a two-dimensional square lattice of about 80 sites at a temperature of 0.25 times the tunnelling energy. The antiferromagnetic long-range order manifests through the divergence of the correlation length, which reaches the size of the system, the development of a peak in the spin structure factor and a staggered magnetization that is close to the ground-state value. We hole-dope the system away from half-filling, towards a regime in which complex many-body states are expected, and find that strong magnetic correlations persist at the antiferromagnetic ordering vector up to dopings of about 15 per cent. In this regime, numerical simulations are challenging and so experiments provide a valuable benchmark. Our results demonstrate that microscopy of cold atoms in optical lattices can help us to understand the low-temperature Fermi-Hubbard model.