Coexistence of spin ordering on ladders and spin dimer formation in a new-structure-type compound Sr2Co3S2O3

We report on the syntheses and characterizations of single crystalline and polycrystalline Sr₂Co₃S₂O₃ with a novel crystal structure type. It contains Co-O 2-leg rectangular ladders and necklace ladders. The two ladders share common legs and construct a hybrid spin ladder. A rare meridional heteroleptic octahedral coordination is found for the Co²⁺ ions in the 2-leg ladder. Within the necklace ladders, the Co²⁺ ions are in trans-octahedral coordination. An antiferromagnetic order is observed at T_N ~ 267 K, while a broad maximum in magnetic susceptibility is found below T_N. This relatively high ordering temperature among Co-based ladder compounds is related to the highly anisotropic mer-coordination of the Co²⁺ ions. The trans-octahedrally coordinated Co²⁺ ions, on the other hand, corresponds to the possible short-range magnetic correlations through dimers with an effective . This results in a rare situation that spin ordering and spin dimers coexist down to 2 K.

Engineering 1D Quantum Stripes from Superlattices of 2D Layered Materials

Dimensional tunability from two dimensions to one dimension is demonstrated for the first time using an artificial superlattice method in synthesizing 1D stripes from 2D layered materials. The 1D confinement of layered Sr₂ IrO₄ induces distinct 1D quantum-confined electronic states, as observed from optical spectroscopy and resonant inelastic X-ray scattering. This 1D superlattice approach is generalizable to a wide range of layered materials.

An efficient density matrix renormalization group algorithm for chains with periodic boundary condition

The Density Matrix Renormalization Group (DMRG) is a state-of-the-art numerical technique for a one dimensional quantum many-body system; but calculating accurate results for a system with Periodic Boundary Condition (PBC) from the conventional DMRG has been a challenging job from the inception of DMRG. The recent development of the Matrix Product State (MPS) algorithm gives a new approach to find accurate results for the one dimensional PBC system. The most efficient implementation of the MPS algorithm can scale as O($p \times m^3$), where $p$ can vary from 4 to $m^2$. In this paper, we propose a new DMRG algorithm, which is very similar to the conventional DMRG and gives comparable accuracy to that of MPS. The computation effort of the new algorithm goes as O($m^3$) and the conventional DMRG code can be easily modified for the new algorithm.Received: 2 August 2016,  Accepted: 12 October 2016; Edited by: K. Hallberg; DOI: http://dx.doi.org/10.4279/PIP.080006Cite as: D Dey, D Maiti, M Kumar, Papers in Physics 8, 080006 (2016)This paper, by D Dey, D Maiti, M Kumar, is licensed under the Creative Commons Attribution License 3.0. 

Spectral functions with the density matrix renormalization group: Krylov-space approach for correction vectors

Frequency-dependent correlations, such as the spectral function and the dynamical structure factor, help illustrate condensed matter experiments. Within the density matrix renormalization group (DMRG) framework, an accurate method for calculating spectral functions directly in frequency is the correction-vector method. The correction vector can be computed by solving a linear equation or by minimizing a functional. This paper proposes an alternative to calculate the correction vector: to use the Krylov-space approach. This paper then studies the accuracy and performance of the Krylov-space approach, when applied to the Heisenberg, the t-J, and the Hubbard models. The cases studied indicate that the Krylov-space approach can be more accurate and efficient than the conjugate gradient, and that the error of the former integrates best when a Krylov-space decomposition is also used for ground state DMRG.

Spin-orbit coupled molecular quantum magnetism realized in inorganic solid

Molecular quantum magnetism involving an isolated spin state is of particular interest due to the characteristic quantum phenomena underlying spin qubits or molecular spintronics for quantum information devices, as demonstrated in magnetic metal-organic molecular systems, the so-called molecular magnets. Here we report the molecular quantum magnetism realized in an inorganic solid Ba3Yb2Zn5O11 with spin-orbit coupled pseudospin-½ Yb(3+) ions. The magnetization represents the magnetic quantum values of an isolated Yb4 tetrahedron with a total (pseudo)spin 0, 1 and 2. Inelastic neutron scattering results reveal that a large Dzyaloshinsky-Moriya interaction originating from strong spin-orbit coupling of Yb 4f is a key ingredient to explain magnetic excitations of the molecular magnet states. The Dzyaloshinsky-Moriya interaction allows a non-adiabatic quantum transition between avoided crossing energy levels, and also results in unexpected magnetic behaviours in conventional molecular magnets.

Quantum simulation of the Hubbard model with dopant atoms in silicon

In quantum simulation, many-body phenomena are probed in controllable quantum systems. Recently, simulation of Bose-Hubbard Hamiltonians using cold atoms revealed previously hidden local correlations. However, fermionic many-body Hubbard phenomena such as unconventional superconductivity and spin liquids are more difficult to simulate using cold atoms. To date the required single-site measurements and cooling remain problematic, while only ensemble measurements have been achieved. Here we simulate a two-site Hubbard Hamiltonian at low effective temperatures with single-site resolution using subsurface dopants in silicon. We measure quasi-particle tunnelling maps of spin-resolved states with atomic resolution, finding interference processes from which the entanglement entropy and Hubbard interactions are quantified. Entanglement, determined by spin and orbital degrees of freedom, increases with increasing valence bond length. We find separation-tunable Hubbard interaction strengths that are suitable for simulating strongly correlated phenomena in larger arrays of dopants, establishing dopants as a platform for quantum simulation of the Hubbard model.

Real-space localization and quantification of hole distribution in chain-ladder Sr3Ca11Cu24O41 superconductor

Understanding the physical properties of the chain-ladder Sr3Ca11Cu24O41 hole-doped superconductor has been precluded by the unknown hole distribution among chains and ladders. We use electron energy-loss spectrometry (EELS) in a scanning transmission electron microscope (STEM) at atomic resolution to directly separate the contributions of chains and ladders and to unravel the hole distribution from the atomic scale variations of the O-K near-edge structures. The experimental data unambiguously demonstrate that most of the holes lie within the chain layers. A quantitative interpretation supported by inelastic scattering calculations shows that about two holes are located in the ladders, and about four holes in the chains, shedding light on the electronic structure of Sr3Ca11Cu24O41. Combined atomic resolution STEM-EELS and inelastic scattering calculations is demonstrated as a powerful approach toward a quantitative understanding of the electronic structure of cuprate superconductors, offering new possibilities for elucidating their physical properties.

Two S = 1/2 one-dimensional barium copper phosphates showing antiferromagnetic and ferromagnetic intrachain interactions

Two barium copper phosphates, BaCu2(PO4)2(H2O) (1) and Ba2Cu(HPO4)(PO4)(OH) (2), were synthesized under mild hydrothermal conditions. The Cu cation in 1 adopts a CuO4(H2O) square pyramidal coordination configuration, forming alternating chains along the b axis through alternative corner and edge sharing, while the geometry of the Cu center in 2 is a CuO4(OH)2 octahedron which further connects each other by edge sharing to constitute uniform chains along the b axis. Magnetic behaviors of both compounds were analyzed by susceptibility, magnetization and heat capacity measurements. The dominant intrachain couplings are antiferromagnetic in 1 with a long-range ordering at 14 K and ferromagnetic in 2 without long-range ordering above 2 K. The first principles calculations indicate that the intrachain ferromagnetic couplings in 2 originate from Cu(1)-O(7)H-Cu(1) dpσ correlation superexchanges. The susceptibility data of compounds 1 and 2 are fitted by using suitable antiferromagnetic chain and ferromagnetic chain models, respectively. In addition, we report the results of the infrared and thermal measurements of both the compounds.

Heat Conductivity of the Heisenberg Spin-1/2 Ladder: From Weak to Strong Breaking of Integrability

We investigate the heat conductivity κ of the Heisenberg spin-1/2 ladder at finite temperature covering the entire range of interchain coupling J(⊥), by using several numerical methods and perturbation theory within the framework of linear response. We unveil that a perturbative prediction κ∝J(⊥)(-2), based on simple golden-rule arguments and valid in the strict limit J(⊥)→0, applies to a remarkably wide range of J(⊥), qualitatively and quantitatively. In the large J(⊥) limit, we show power-law scaling of opposite nature, namely, κ∝J(⊥)(2). Moreover, we demonstrate the weak and strong coupling regimes to be connected by a broad minimum, slightly below the isotropic point at J(⊥)=J(∥). Reducing temperature T, starting from T=∞, this minimum scales as κ∝T(-2) down to T on the order of the exchange coupling constant. These results provide for a comprehensive picture of κ(J(⊥),T) of spin ladders.

Pressure-Induced Mott Transition Followed by a 24-K Superconducting Phase in BaFe_{2}S_{3}

We performed high-pressure study for a Mott insulator BaFe_{2}S_{3}, by measuring dc resistivity and ac susceptibility up to 15 GPa. We found that the antiferromagnetic insulating state at the ambient pressure is transformed into a metallic state at the critical pressure, P_{c}=10  GPa, and the superconductivity with the optimum T_{c}=24  K emerges above P_{c}. Furthermore, we found that the metal-insulator transition (Mott transition) boundary terminates at a critical point around 10 GPa and 75 K. The obtained pressure-temperature (P-T) phase diagram is similar to those of the organic and fullerene compounds; namely, BaFe_{2}S_{3} is the first inorganic superconductor in the vicinity of bandwidth control type Mott transition.

Ultrafast electronic state conversion at room temperature utilizing hidden state in cuprate ladder system

Photo-control of material properties on femto- (10(-15)) and pico- (10(-12)) second timescales at room temperature has been a long-sought goal of materials science. Here we demonstrate a unique ultrafast conversion between the metallic and insulating state and the emergence of a hidden insulating state by tuning the carrier coherence in a wide temperature range in the two-leg ladder superconductor Sr(14-x)Ca(x)Cu24O41 through femtosecond time-resolved reflection spectroscopy. We also propose a theoretical scenario that can explain the experimental results. The calculations indicate that the holes injected by the ultrashort light reduce the coherence among the inherent hole pairs and result in suppression of conductivity, which is opposite to the conventional photocarrier-doping mechanism. By using trains of ultrashort laser pulses, we successively tune the carrier coherence to within 1 picosecond. Control of hole-pair coherence is shown to be a realistic strategy for tuning the electronic state on ultrafast timescales at room temperature.

Pressure-induced superconductivity in the iron-based ladder material BaFe2S3

All the iron-based superconductors identified so far share a square lattice composed of Fe atoms as a common feature, despite having different crystal structures. In copper-based materials, the superconducting phase emerges not only in square-lattice structures but also in ladder structures. Yet iron-based superconductors without a square-lattice motif have not been found, despite being actively sought out. Here, we report the discovery of pressure-induced superconductivity in the iron-based spin-ladder material BaFe2S3, a Mott insulator with striped-type magnetic ordering below ∼120 K. On the application of pressure this compound exhibits a metal-insulator transition at about 11 GPa, followed by the appearance of superconductivity below Tc = 14 K, right after the onset of the metallic phase. Our findings indicate that iron-based ladder compounds represent promising material platforms, in particular for studying the fundamentals of iron-based superconductivity.

Edge state magnetism in zigzag-interfaced graphene via spin susceptibility measurements

Development of graphene spintronic devices relies on transforming it into a material with a spin order. Attempts to make graphene magnetic by introducing zigzag edge states have failed due to energetically unstable structure of torn zigzag edges. Here, we report on the formation of nanoridges, i.e., stable crystallographically oriented fluorine monoatomic chains, and provide experimental evidence for strongly coupled magnetic states at the graphene-fluorographene interfaces. From the first principle calculations, the spins at the localized edge states are ferromagnetically ordered within each of the zigzag interface whereas the spin interaction across a nanoridge is antiferromagnetic. Magnetic susceptibility data agree with this physical picture and exhibit behaviour typical of quantum spin-ladder system with ferromagnetic legs and antiferromagnetic rungs. The exchange coupling constant along the rungs is measured to be 450 K. The coupling is strong enough to consider graphene with fluorine nanoridges as a candidate for a room temperature spintronics material.

Quantum phase transitions in the Kondo-necklace model: perturbative continuous unitary transformation approach

The Kondo-necklace model can describe magnetic low-energy limit of strongly correlated heavy fermion materials. There exist multiple energy scales in this model corresponding to each phase of the system. Here, we study quantum phase transition between the Kondo-singlet phase and the antiferromagnetic long-range ordered phase, and show the effect of anisotropies in terms of quantum information properties and vanishing energy gap. We employ the 'perturbative continuous unitary transformations' approach to calculate the energy gap and spin-spin correlations for the model in the thermodynamic limit of one, two, and three spatial dimensions as well as for spin ladders. In particular, we show that the method, although being perturbative, can predict the expected quantum critical point, where the gap of low-energy spectrum vanishes, which is in good agreement with results of other numerical and Green's function analyses. In addition, we employ concurrence, a bipartite entanglement measure, to study the criticality of the model. Absence of singularities in the derivative of concurrence in two and three dimensions in the Kondo-necklace model shows that this model features multipartite entanglement. We also discuss crossover from the one-dimensional to the two-dimensional model via the ladder structure.

Orbital control of effective dimensionality: from spin-orbital fractionalization to confinement in the anisotropic ladder system CaCu(2)O(3)

Fractionalization of an electronic quasiparticle into spin, charge, and orbital parts is a fundamental and characteristic property of interacting electrons in one dimension. However, real materials are never strictly one dimensional and the fractionalization phenomena are hard to observe. Here we studied the spin and orbital excitations of the anisotropic ladder material CaCu_{2}O_{3}, whose electronic structure is not one dimensional. Combining high-resolution resonant inelastic x-ray scattering experiments with theoretical model calculations, we show that (i) spin-orbital fractionalization occurs in CaCu_{2}O_{3} along the leg direction x through the xz orbital channel as in a 1D system, and (ii) no fractionalization is observed for the xy orbital, which extends in both leg and rung direction, contrary to a 1D system. We conclude that the directional character of the orbital hopping can select different degrees of dimensionality. Using additional model calculations, we show that spin-orbital separation is generally far more robust than the spin-charge separation. This is not only due to the already mentioned selection realized by the orbital hopping, but also due to the fact that spinons are faster than the orbitons.

Insulating state in tetralayers reveals an even-odd interaction effect in multilayer graphene

Close to charge neutrality, the electronic properties of graphene and its multilayers are sensitive to electron-electron interactions. In bilayers, for instance, interactions are predicted to open a gap between valence and conduction bands, turning the system into an insulator. In mono and (Bernal-stacked) trilayers, which remain conducting at low temperature, interactions do not have equally drastic consequences. It is expected that interaction effects become weaker for thicker multilayers, whose behaviour should converge to that of graphite. Here we show that this expectation does not correspond to reality by revealing the occurrence of an insulating state close to charge neutrality in Bernal-stacked tetralayer graphene. The phenomenology-incompatible with the behaviour expected from the single-particle band structure-resembles that observed in bilayers, but the insulating state in tetralayers is visible at higher temperature. We explain our findings, and the systematic even-odd effect of interactions in Bernal-stacked layers of different thickness that emerges from experiments, in terms of a generalization of the interaction-driven, symmetry-broken states proposed for bilayers.

Control of orbital reconstruction in (LaAlO3)M/(SrTiO3)N(001) quantum wells by strain and confinement

The diverse functionality emerging at oxide interfaces calls for a fundamental understanding of the mechanisms and control parameters of electronic reconstructions. Here, we explore the evolution of electronic phases in (LaAlO₃)_M/(SrTiO₃)_N (001) superlattices as a function of strain and confinement of the SrTiO₃ quantum well. Density functional theory calculations including a Hubbard U term reveal a charge ordered Ti³⁺ and Ti⁴⁺ state for N = 2 with an unanticipated orbital reconstruction, displaying alternating d_xz and d_yz character at the Ti³⁺ sites, unlike the previously reported d_xy state, obtained only for reduced c-parameter at a_STO. At a_LAOc-compression leads to a Dimer-Mott insulator with alternating d_xz, d_yz sites and an almost zero band gap. Beyond a critical thickness of N = 3 (a_STO) and N = 4 (a_LAO) an insulator-to-metal transition takes place, where the extra e/2 electron at the interface is redistributed throughout the STO slab with a d_xy interface orbital occupation and a mixed d_xz + d_yz occupation in the inner layers. Chemical variation of the SrTiO₃ counterpart (LaAlO₃ vs. NdGaO₃) proves that the significant octahedral tilts and distortions in the SrTiO₃ quantum well are induced primarily by the electrostatic doping at the polar interface and not by variation of the SrTiO₃ counterpart.

Angle-resolved photoemission spectroscopy of tetragonal CuO: evidence for intralayer coupling between cupratelike sublattices

We investigate by angle-resolved photoemission the electronic structure of in situ grown tetragonal CuO, a synthetic quasi-two-dimensional edge-sharing cuprate. We show that, in spite of the very different nature of the copper oxide layers, with twice as many Cu in the CuO layers of tetragonal CuO as compared to the CuO(2) layers of the high-T(c) cuprates, the low-energy electronic excitations are surprisingly similar, with a Zhang-Rice singlet dispersing on weakly coupled cupratelike sublattices. This system should thus be considered as a member of the high-T(c) cuprate family, with, however, interesting differences due to the intralayer coupling between the cupratelike sublattices.

Prospects of high-resolution resonant X-ray inelastic scattering studies on solid materials, liquids and gases at diffraction-limited storage rings

The spectroscopic technique of resonant inelastic X-ray scattering (RIXS) will particularly profit from immensely improved brilliance of diffraction-limited storage rings (DLSRs). In RIXS one measures the intensities of excitations as a function of energy and momentum transfer. DLSRs will allow for pushing the achievable energy resolution, signal intensity and the sampled spot size to new limits. With RIXS one nowadays probes a broad range of electronic systems reaching from simple molecules to complex materials displaying phenomena like peculiar magnetism, two-dimensional electron gases, superconductivity, photovoltaic energy conversion and heterogeneous catalysis. In this article the types of improved RIXS studies that will become possible with X-ray beams from DLSRs are envisioned.

Nature of strong hole pairing in doped Mott antiferromagnets

Cooper pairing instability in a Fermi liquid is well understood by the BCS theory, but pairing mechanism for doped Mott insulators still remains elusive. Previously it has been shown by density matrix renormalization group (DMRG) method that a single doped hole is always self-localized due to the quantum destructive interference of the phase string signs hidden in the t-J ladders. Here we report a DMRG investigation of hole binding in the same model, where a novel pairing-glue scheme beyond the BCS realm is discovered. Specifically, we show that, in addition to spin pairing due to superexchange interaction, the strong frustration of the phase string signs on the kinetic energy gets effectively removed by pairing the charges, which results in strong binding of two holes. By contrast, if the phase string signs are "switched off" artificially, the pairing strength diminishes significantly even if the superexchange coupling remains the same. In the latter, unpaired holes behave like coherent quasiparticles with pairing drastically weakened, whose sole origin may be attributed to the resonating-valence-bond (RVB) pairing of spins. Such non-BCS pairing mechanism is therefore beyond the RVB picture and may shed important light on the high-T(c) cuprate superconductors.

Universal behavior beyond multifractality in quantum many-body systems

How many states of a configuration space contribute to a wave function? Attempts to answer this ubiquitous question have a long history in physics and are keys to understanding, e.g., localization phenomena. Beyond single-particle physics, a quantitative study of the ground state complexity for interacting many-body quantum systems is notoriously difficult, mainly due to the exponential growth of the configuration (Hilbert) space with the number of particles. Here we develop quantum Monte Carlo schemes to overcome this issue, focusing on Shannon-Rényi entropies of ground states of large quantum many-body systems. Our simulations reveal a generic multifractal behavior while the very nature of quantum phases of matter and associated transitions is captured by universal subleading terms in these entropies.

Magnetization process, bipartite entanglement, and enhanced magnetocaloric effect of the exactly solved spin-1/2 Ising-Heisenberg tetrahedral chain

The frustrated spin-1/2 Ising-Heisenberg ladder with Heisenberg intra-rung and Ising inter-rung interactions is exactly solved in a longitudinal magnetic field by taking advantage of the local conservation of the total spin on each rung and the transfer-matrix method. We have rigorously calculated the ground-state phase diagram, magnetization process, magnetocaloric effect, and basic thermodynamic quantities for the model, which can be alternatively viewed as an Ising-Heisenberg tetrahedral chain. It is demonstrated that a stepwise magnetization curve with an intermediate plateau at half of the saturation magnetization is also reflected in respective stepwise changes of the concurrence serving as a measure of bipartite entanglement. The ground-state phase diagram and zero-temperature magnetization curves of the Ising-Heisenberg tetrahedral chain are contrasted with the analogous results of the purely quantum Heisenberg tetrahedral chain, which have been obtained through density-matrix renormalization group (DMRG) calculations. While both ground-state phase diagrams fully coincide in the regime of weak inter-rung interaction, the purely quantum Heisenberg tetrahedral chain develops Luttinger spin-liquid and Haldane phases for strongly coupled rungs, which are absent in the Ising-Heisenberg counterpart model.

A magnetically isolated cuprate spin-ladder system: synthesis, structures, and magnetic properties

Two magnetically isolated cuprate spin ladders have been synthesized. Their crystal structures and preliminary investigation into the magnetic properties were reported.

Spectrum of a magnetized strong-leg quantum spin ladder

Inelastic neutron scattering is used to measure the spin excitation spectrum of the Heisenberg S=1/2 ladder material (C7H10N)2CuBr4 in its entirety, both in the gapped spin liquid and the magnetic field-induced Tomonaga-Luttinger spin liquid regimes. A fundamental change of the spin dynamics is observed between these two regimes. Density matrix renormalization group calculations quantitatively reproduce and help understand the observed commensurate and incommensurate excitations. The results validate long-standing quantum field-theoretical predictions but also test the limits of that approach.

Characterizing genuine multisite entanglement in isotropic spin lattices

We consider a class of large superposed states, obtained from dimer coverings on spin-1/2 isotropic lattices, whose potential usefulness ranges from organic molecules to quantum computation. We show that they are genuinely multiparty entangled, irrespective of the geometry and dimension of the isotropic lattice. We then present an efficient method to characterize the genuine multisite entanglement in the case of isotropic square spin-1/2 lattices, with short-range dimer coverings. We use this iterative analytical method to calculate the multisite entanglement of finite-sized lattices, which through finite-size scaling, enables us to obtain the estimate of the multisite entanglement of the infinite square lattice. The method can be a useful tool to investigate other single- and multisite properties of such states.

Unconventional magnetic and thermodynamic properties of S=1/2 spin ladder with ferromagnetic legs

We have succeeded in synthesizing single crystals of a new organic radical 3-Cl-4-F-V [3-(3-chloro-4-fluorophenyl)-1,5-diphenylverdazyl]. Through the ab initio molecular orbital calculation and the analysis of the magnetic properties, this compound was confirmed to be the first experimental realization of an S=1/2 spin-ladder system with ferromagnetic leg interactions. The field-temperature phase diagram indicated that the ground state is situated very close to the quantum critical point. Furthermore, we found an unexpected field-induced successive phase transition, which possibly originates from the interplay of low dimensionality and frustration.

Evidence for dimer crystal melting in the frustrated spin-ladder system BiCu2PO6

In the spin ladder compound BiCu(2)PO(6), there exists a decisive dynamics of spin excitations that we classify and characterize using inelastic light scattering. We observe an interladder singlet bound mode at 24  cm(-1) and two intraladder bound states at 62 and 108  cm(-1) in the leg (bb) and the rung (cc) polarization as well as a broad triplon continuum extending from 36  cm(-1) to 700  cm(-1). Though isolated spin ladder physics can roughly account for the observed excitations at high energies, frustration and interladder interactions need to be considered to fully describe the spectral distribution and scattering selection rules at low and intermediate energies. In addition, we attribute the rich spectrum of singlet bound modes to a melting of a dimer crystal. Our study provides evidence for a Z(2) quantum phase transition from a dimer to a resonating valence bond state driven by singlet fluctuations.

Tuning charge and spin excitations in zigzag edge nanographene ribbons

Graphene and its quasi-one-dimensional counterpart, graphene nanoribbons, present an ideal platform for tweaking their unique electronic, magnetic and mechanical properties by various means for potential next-generation device applications. However, such tweaking requires knowledge of the electron-electron interactions that play a crucial role in these confined geometries. Here, we have investigated the magnetic and conducting properties of zigzag edge graphene nanoribbons (ZGNRs) using the many-body configuration interaction (CI) method on the basis of the Hubbard Hamiltonian. For the half-filled case, the many-body ground state shows a ferromagnetic spin-spin correlation along the zigzag edge, which supports the picture obtained from one-electron theory. However, hole doping reduces the spin and charge excitation gap, making the ground state conducting and magnetic. We also provide a two-state model that explains the low-lying charge and spin excitation spectrum of ZGNRs. An experimental setup to confirm the hole-mediated conducting and magnetic states is discussed.

Dipolar gases in coupled one-dimensional lattices

We consider dipolar bosons in two tubes of one-dimensional lattices, where the dipoles are aligned to be maximally repulsive and the particle filling fraction is the same in each tube. In the classical limit of zero intersite hopping, the particles arrange themselves into an ordered crystal for any rational filling fraction, forming a complete devil's staircase like in the single tube case. Turning on hopping within each tube then gives rise to a competition between the crystalline Mott phases and a liquid of defects or solitons. However, for the two-tube case, we find that solitons from different tubes can bind into pairs for certain topologies of the filling fraction. This provides an intriguing example of pairing that is purely driven by correlations close to a Mott insulator.

Boundary-locality and perturbative structure of entanglement spectra in gapped systems

The entanglement between two parts of a many-body system can be characterized in detail by the entanglement spectrum. Focusing on gapped phases of several one-dimensional systems, we show how this spectrum is dominated by contributions from the boundary between the parts. This contradicts the view of an "entanglement Hamiltonian" as a bulk entity. The boundary-local nature of the entanglement spectrum is clarified through its hierarchical level structure, through the combination of two single-boundary spectra to form a two-boundary spectrum, and finally through consideration of dominant eigenfunctions of the entanglement Hamiltonian. We show consequences of boundary-locality for perturbative calculations of the entanglement spectrum.

Dominant ferromagnetism in the spin-1/2 half-twist ladder 334 compounds, Ba3Cu3In4O12 and Ba3Cu3Sc4O12

The magnetic properties of polycrystalline samples of Ba(3)Cu(3)In(4)O(12) (In-334) and Ba(3)Cu(3)Sc(4)O(12) (Sc-334) are reported. Both 334 phases have a structure derived from perovskite, with CuO(4) squares interconnected to form half-twist ladders along the c-axis. The Cu-O-Cu angles, ~90°, and the positive Weiss temperatures indicate the presence of significant ferromagnetic (FM) interactions along the Cu ladders. At low temperatures, T < 20 K, sharp transitions in the magnetic susceptibility and heat capacity measurements indicate three-dimensional (3D) antiferromagnetic (AFM) ordering at T(N). T(N) is suppressed on application of a field and a complex magnetic phase diagram with three distinct magnetic regimes below the upper critical field can be inferred from our measurements. The magnetic interactions are discussed in relation to a modified spin-1/2 FM-AFM model and the 334 half-twist ladder is compared to other two-rung ladder spin-1/2 systems.

Spectral and thermodynamic properties of a strong-leg quantum spin ladder

The strong-leg S=1/2 Heisenberg spin ladder system (C(7)H(10)N)(2)CuBr(4) is investigated using density matrix renormalization group calculations, inelastic neutron scattering, and bulk magnetothermodynamic measurements. Measurements showed qualitative differences compared to the strong-rung case. A long-lived two-triplon bound state is confirmed to persist across most of the Brillouin zone in a zero field. In applied fields, in the Tomonaga-Luttinger spin-liquid phase, elementary excitations are attractive, rather than repulsive. In the presence of weak interladder interactions, the strong-leg system is considerably more prone to three-dimensional ordering.

Continuous and discontinuous quantum phase transitions in a model two-dimensional magnet

The Shasty-Sutherland model, which consists of a set of spin 1/2 dimers on a 2D square lattice, is simple and soluble but captures a central theme of condensed matter physics by sitting precariously on the quantum edge between isolated, gapped excitations and collective, ordered ground states. We compress the model Shastry-Sutherland material, SrCu(2)(BO(3))(2), in a diamond anvil cell at cryogenic temperatures to continuously tune the coupling energies and induce changes in state. High-resolution X-ray measurements exploit what emerges as a remarkably strong spin-lattice coupling to both monitor the magnetic behavior and the absence or presence of structural discontinuities. In the low-pressure spin-singlet regime, the onset of magnetism results in an expansion of the lattice with decreasing temperature, which permits a determination of the pressure-dependent energy gap and the almost isotropic spin-lattice coupling energies. The singlet-triplet gap energy is suppressed continuously with increasing pressure, vanishing completely by 2 GPa. This continuous quantum phase transition is followed by a structural distortion at higher pressure.

A phenomenological theory of the anomalous pseudogap phase in underdoped cuprates

The theoretical description of the anomalous properties of the pseudogap phase in the underdoped region of the cuprate phase diagram lags behind the progress in spectroscopic and other experiments. A phenomenological ansatz, based on analogies to the approach to Mott localization at weak coupling in lower dimensional systems, has been proposed by Yang et al (2006 Phys. Rev. B 73 174501). This ansatz has had success in describing a range of experiments. The motivation underlying this ansatz is described and the comparisons with experiment are reviewed. Implications for a more microscopic theory are discussed together with the relation to theories that start directly from microscopic strongly coupled Hamiltonians.