Search Results (300 total)

The issues of single particle coherence and its interplay with singlet pairing are studied within the slave boson gauge theory of a doped Mott insulator. Prior work by one of us [T. Senthil, Phys. Rev. B 78, 045109 (2008)] showed that the coherence scale below which Landau quasiparticles emerge is parametrically lower than that identified in the slave boson mean field theory. Here we study the resulting new non-Fermi liquid intermediate temperature regime characterized by a single particle scattering rate that is linear in temperature (T). In the presence of a d-wave pair amplitude, this leads to a pseudogap state with T-dependent Fermi arcs near the nodal direction. Implications for understanding the cuprates are discussed.

Phase diagram of Na_xCoO₂(x≳0.71) has been reinvestigated using electrochemically fine-tuned single crystals. Both phase-separation and staging phenomena as a result of sodium multivacancy cluster ordering have been found. Phase-separation phenomenon is observed in the narrow ranges of 0.76≲x≲0.82 and 0.83≲x≲0.86. While x=0.820 shows A-type antiferromagnetic (A-AF) ordering below 22 K, x=0.833 is confirmed to have a magnetic ground state of A-AF ordering below ∼8 K and is only reachable through slow cooling. In addition, x=0.859 is found to be responsible for the highest A-AF transition temperature at about 29 K. Staging model based on ordered stacking of multivacancy layers is proposed to explain the hysteretic behavior and A-AF correlation length for x ∼0.82–0.86.

The underdoped cuprates have a number of interesting and unusual properties that often seem hard to reconcile with one another. In this paper we show how many of these diverse phenomena can be synthesized into a single coherent theoretical picture. Specifically we present a description where a pseudogap and gapless Fermi arcs exist in the normal state above the superconducting transition temperature (T_c) but give way to the observed quantum oscillations and other phenomena at low temperature when the superconductivity is suppressed by a magnetic field. We show the consistency between these phenomena and observations of enhanced Nernst and diamagnetic signals above T_c. We also develop a description of the vortex core inside the superconducting state and discuss its relation with the high-field phenomena.

A Chern-Simons theory for the doped spin-1/2 kagome system is constructed, from which it is shown that the system is an exotic superconductor that breaks time-reversal symmetry. It is also shown that the system carries minimal vortices of flux hc/4e (as opposed to the usual hc/2e in conventional superconductors) and contains fractional quasiparticles (including fermionic quasiparticles with semionic mutual statistics and spin-1/2 quasiparticles with bosonic self-statistics) in addition to the usual spin-1/2 fermionic Bogoliubov quasiparticles. Two Chern-Simons theories—one with an auxiliary gauge field kept and one with the auxiliary field and a redundant matter field directly eliminated—are presented and shown to be consistent with each other.

We numerically study the quantum oscillations in superconducting vortex-mixed states with d-wave pairing. We show that in the parameter range of an underdoped cuprate superconductor, the commonly held assumption that the period is given by the underlying Fermi-surface area using the Onsager relation becomes invalid. Using this result, we conclude that the interpretation of the recent experimental data on YBCO as a signal of an underlying Fermi surface with four hole pockets created by a (π,π) folding cannot be ruled out.

The Dirac spin liquid was proposed to be the ground state of the spin-1/2 kagomé antiferromagnets. In a magnetic field B, we show that the state with Fermi pocket is unstable to the Landau level (LL) state. The LL state breaks the spin rotation around the axis of the magnetic field. We find that the LL state has an in-plane 120° q=0 magnetization M which scales with the external field M∼B^α, where α is a universal number of the Dirac spin liquid. We discuss the related experimental implications which can be used to detect the possible Dirac spin liquid phase in herbertsmithite ZnCu₃(OH)₆Cl₂.

We have recently demonstrated that x=0.71 and 0.84 are the two most stable single-phase compounds above x=0.5 in γ-Na_xCoO₂ [G. J. Shu , Phys. Rev. B 76, 184115 (2007); F. C. Chou , Phys. Rev. Lett. 101, 127404 (2008)], and this structural investigation was performed on the single crystals and pulverized samples elaborately synthesized therein. Using the complementary techniques of x-ray and electron diffractions, we unambiguously established the existence of superlattices in x=0.71 and 0.84, sqrt[12]a_p×sqrt[12]a_p×3c_p, and sqrt[13]a_p×sqrt[13]a_p×3c_p (a_p and c_p, the hexagonal primitive cell parameters), respectively. The exceptionally large superlattice for x=0.71 arises from the long-range three-dimensional sodium ordering, consisting of the spiral-like sodium-trimer chain screwing along c axis and being decorated with alternating truncated-triangle and honeycomb sodium sublattices in ab plane. The trimers in the neighboring sodium planes show corner-shared-like characteristics along the chain direction. A larger interplane separation of the trimers that was expected to minimize the trimer interlayer Coulomb repulsion was, however, not observed, and all Co ions in the superlattice show similar charge characteristics. In x=0.84, the large superlattice is rather a result of the long-range ordering of sodium monomers both in ab plane and along c axis. The differences in the superlattice structure and the corresponding long-range sodium ordering between x=0.71 and 0.84 may provide the critical information in understanding their distinctly different physical properties from structural aspects.

Spin liquid states for the spin-1 / 2 antiferromagnetic Heisenberg model on a hyperkagome lattice are studied. We classify and study flux states according to symmetries. Applying this model to Na₄Ir₃O₈, we propose that the high temperature state may be described by a spinon Fermi surface, which forms a paired state with line nodes below 20 K. The possible mixed spin singlet and spin triplet pairing states are discussed according to the lattice symmetry which breaks inversion.

We examine the possibility that the superconductivity in the newly discovered FeAs materials may be caused by the Coulomb interaction between d electrons of the iron atoms. We find that when the Hund’s rule ferromagnetic interaction is strong enough, the leading pairing instability is in spin-triplet p-wave channel in the weak-coupling limit. The resulting superconducting gap has nodal points on the two-dimensional Fermi surfaces. The k dependent hybridization of several orbitals around a Fermi pocket is the key for the appearance of the spin-triplet p-wave pairing.

A method for calculating free-energy differences based on a free-energy perturbation (FEP) formalism in an alloy system described by two different Hamiltonians is reported. The intended application is the calculation of solid-liquid phase equilibria in alloys with the accuracy of first-principles electronic density-functional theory (DFT). For this purpose free energies are derived with a classical interatomic potential, and FEP calculations are used to compute corrections to these reference values. For practical applications of this approach, due to the relatively high computational cost of DFT calculations, it is critical that the FEP calculations converge rapidly in terms of the number of samples used to estimate relevant ensemble averages. This issue is investigated in the current study employing two classical interatomic-potential models for Ni-Cu. These models yield differences in predicted phase-boundary temperatures of approximately 100 K, comparable to those that might be expected between a DFT Hamiltonian and a well-fit classical potential. We show that for pure elements the FEP calculations converge rapidly with the number of samples, yielding free-energy differences converged to within a fraction of a meV/atom in a few dozen energy calculations. For a concentrated equiatomic alloy similar precision requires roughly a hundred samples. The results suggest that the proposed methodology could provide a computationally tractable framework for calculating solid-liquid phase equilibria in concentrated alloys with DFT accuracy.

In the rich phase diagram of Na_xCoO₂, x=0.71 enjoys special stability and is called the Curie-Weiss metal due to its anomalous properties. Similarly, x=0.84 prepared from high temperature melt is a special end point beyond which the system phase separates. Using synchrotron x-ray diffraction on single crystals, we discovered sqrt[12]a and sqrt[13]a superlattice structures which we interpret as the ordering of Na (vacancy) clusters. These results lead to a picture of coexisting local moments and itinerant carriers and form the first step towards understanding the many anomalous properties of cobaltates.

In recent work, we argued that a particular algebraic spin liquid (ASL) may be the ground state of the S=1/2 kagome lattice Heisenberg antiferromagnet. Furthermore, this state, which lacks a spin gap, is appealing in light of recent experiments on herbertsmithite [ZnCu₃(OH)₆Cl₂]. Here, we study the properties of this ASL in more detail using both the low-energy effective field theory and Gutzwiller-projected wave functions of fermionic spinons. We identify the competing orders of the ASL, which are observables having slowly decaying power-law correlations—among them we find a set of magnetic orders lying at the M points of the Brillouin zone, the familiar q=0 magnetic ordered state, the “Hastings” valence-bond solid (VBS) state, and a pattern of vector spin-chirality ordering corresponding to one of the Dzyaloshinskii–Moriya (DM) interaction terms present in herbertsmithite. Identification of some of these orders requires an understanding of the quantum numbers of magnetic monopole operators in the ASL. We discuss the detection of the magnetic and VBS competing orders in experiments. While we primarily focus on a clean system without DM interaction, we consider the effects of small DM interaction and argue that, surprisingly, it leads to spontaneously broken time-reversal symmetry (for DM interaction that preserves XY spin rotation symmetry, there is also XY magnetic order). Our analysis of the projected wave function provides an estimate of the “Fermi velocity” v_F that characterizes all low-energy excitations of the ASL—this allows us to estimate the specific heat, which compares favorably with experiments. We also study the spin and bond correlations of the projected wave function and compare these results with those of the effective field theory. While the spin correlations in the effective field theory and wave function seem to match rather well (although not completely), the bond correlations are more puzzling. We conclude with a discussion of experiments in herbertsmithite and make several predictions.

By means of optimal control techniques we model and optimize the manipulation of the external quantum state (center-of-mass motion) of atoms trapped in adjustable optical potentials. We consider in detail the cases of both noninteracting and interacting atoms moving between neighboring sites in a lattice of a double-well optical potentials. Such a lattice can perform interaction-mediated entanglement of atom pairs and can realize two-qubit quantum gates. The optimized control sequences for the optical potential allow transport faster and with significantly larger fidelity than is possible with processes based on adiabatic transport.

We load cold atoms into an optical lattice dramatically reshaped by radio-frequency coupling of state-dependent lattice potentials. This radio-frequency dressing changes the unit cell of the lattice at a subwavelength scale, such that its curvature and topology departs strongly from that of a simple sinusoidal lattice potential. Radio-frequency dressing has previously been performed at length scales from mm to tens of μm, but not at the single-optical-wavelength scale. At this length scale significant coupling between adiabatic potentials leads to nonadiabatic transitions, which we measure as a function of lattice depth and dressing amplitude. We also investigate the dressing by measuring changes in the momentum distribution of the dressed states.

Here we report the observation of Fermi surface (FS) pockets via the Shubnikov–de Haas effect in Na_xCoO₂ for x=0.71 and 0.84, respectively. Our observations indicate that the FS expected for each compound intersects their corresponding Brillouin zones, as defined by the previously reported superlattice structures, leading to small reconstructed FS pockets, but only if a precise number of holes per unit cell is localized. For 0.71≤x<0.75 the coexistence of itinerant carriers and localized S=1/2 spins on a paramagnetic triangular superlattice leads at low temperatures to the observation of a deviation from standard Fermi-liquid behavior in the electrical transport and heat capacity properties, suggesting the formation of some kind of quantum spin-liquid ground state.

With the organic compound κ-(BEDT-TTF)₂-Cu₂(CN)₃ in mind, we consider a spin liquid system where a spinon Fermi surface is coupled to a U(1) gauge field. Using the nonequilibrium Green’s function formalism, we derive the quantum Boltzmann equation for this system. In this system, however, one cannot a priori assume the existence of Landau quasiparticles. We show that even without this assumption, one can still derive a linearized equation for a generalized distribution function. We show that the divergence of the effective mass and of the finite temperature self-energy do not enter these transport coefficients and thus they are well defined. Moreover, using a variational method, we calculate the temperature dependence of the spin resistivity and thermal conductivity of this system.

The generalization of the Gutzwiller approximation to inhomogeneous systems is considered, with extra spin-and-site-dependent fugacity factors included. It is found that the inclusion of fugacity factors reconciles the seemingly contradictory choices of Gutzwiller factors used in the literature. Moreover, from the derivation of the Gutzwiller factors, it is shown that the Gutzwiller approximation breaks the rotational symmetry of the trial wave functions and that different components of the spin-spin interaction need to be renormalized differently under the approximation. Various schemes to restore the rotational symmetry are discussed and are compared with results from variational Monte Carlo calculations for the two-dimensional square-lattice antiferromagnet. Results along different paths within the full parameter space, which correspond to different choices of fugacity factors in the literature, are also compared.

There are very few materials that exhibit zero thermal expansion (ZTE), and of these even fewer are appropriate for electronic and optoelectronic applications. We find that a multifunctional crystalline hybrid inorganic-organic semiconductor, β-ZnTe(en)_0.5 (en denotes ethylenediamine), shows uniaxial ZTE in a very broad temperature range of 4–400 K, and concurrently possesses superior electronic and optical properties. The ZTE behavior is a result of compensation of contraction and expansion of different segments along the inorganic-organic stacking axis. This work suggests an alternative route to designing materials in a nanoscopic scale with ZTE or any desired positive or negative thermal expansion (PTE or NTE), which is supported by preliminary data for ZnTe(pda)_0.5 (pda denotes 1,3-propanediamine) with a larger molecule.

The charge dynamics of spin-liquid states described by U(1) gauge theory coupling to fermionic spinons is discussed in this paper. We find that the gapless spinons give rise to a power-law optical conductivity inside the charge gap. The theory is applied to explain the unusual optical conductivity observed recently in the organic compound κ-(BEDT-TTF)₂Cu₂(CN)₃. We also propose an optical experiment to search for the in-gap excitations in the kagome spin-liquid insulator.

We study the theory of a U(1) gauge field coupled to a spinon Fermi surface. Recently this model has been proposed as a possible description of the organic compound κ-(BEDT-TTF)₂Cu₂(CN)₃. We calculate the susceptibility of this system and in particular examine the effect of pairing of the underlying spin liquid. We show that this proposed theory is consistent with the observed susceptibility measurements.

We load atoms into every site of an optical lattice and selectively spin flip atoms in a sublattice consisting of every other site. These selected atoms are separated from their unselected neighbors by less than an optical wavelength. We also show spin-dependent transport, where atomic wave packets are coherently separated into adjacent sites according to their internal state. These tools should be useful for quantum information processing and quantum simulation of lattice models with neutral atoms.

We describe the controlled loading and measurement of number-squeezed states and Poisson states of atoms in individual sites of a double well optical lattice. These states are input to an atom interferometer that is realized by symmetrically splitting individual lattice sites into double wells, allowing atoms in individual sites to evolve independently. The two paths then interfere, creating a matter-wave double-slit diffraction pattern. The time evolution of the double-slit diffraction pattern is used to measure the number statistics of the input state. The flexibility of our double well lattice provides a means to detect the presence of empty lattice sites, an important and so far unmeasured factor in determining the purity of a Mott state.

We report on the unusual nature of the ν=0 state in the integer quantum Hall effect (QHE) in graphene and show that electron transport in this regime is dominated by counterpropagating edge states. Such states, intrinsic to massless Dirac quasiparticles, manifest themselves in a large longitudinal resistivity ρ_xx≳h/e², in striking contrast to ρ_xx behavior in the standard QHE. The ν=0 state in graphene is also predicted to exhibit pronounced fluctuations in ρ_xy and ρ_xx and a smeared zero Hall plateau in σ_xy, in agreement with experiment. The existence of gapless edge states puts stringent constraints on possible theoretical models of the ν=0 state.

We study the Kondo-Heisenberg model on the honeycomb lattice at half filling. Due to the vanishing of the density of states at the fermi level, the Kondo insulator disappears at a finite Kondo coupling even in the absence of the Heisenberg exchange. We adopt a large-N formulation of this model and use the renormalization group machinery to study systematically the second order phase transition of the Kondo insulator (KI) to the algebraic spin liquid (ASL). We note that neither phase breaks any physical symmetry, so that the transition is not described by the standard Ginzburg-Landau-Wilson critical point. We find a stable Lorentz-invariant fixed point that controls this second order phase transition. We calculate the exponent ν of the diverging length scale near the transition. The quasiparticle weight of the conduction electron vanishes at this KI-ASL fixed point, indicating non-Fermi-liquid behavior. The algebraic decay exponent of the staggered spin correlation is calculated at the fixed point and in the ASL phase. We find a jump in this exponent at the transition point.

Valley-polarized quantum Hall states in graphene are described by a Heisenberg O(3) ferromagnet model, with the ordering type controlled by the strength and the sign of the valley anisotropy. A mechanism resulting from electron coupling to the strain-induced gauge field, giving a leading contribution to the anisotropy, is described in terms of an effective random magnetic field aligned with the ferromagnet z axis. We argue that such a random field stabilizes the XY ferromagnet state, which is a coherent equal-weight mixture of the K and K^′ valley states. The implications such as the Berezinskii-Kosterlitz-Thouless ordering transition and topological defects with half-integer charge are discussed.