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arXiv2009

Role of spin orbit coupling and electron correlation in the ground state properties of 5d electronic states

Electron correlation strength is predicted to be weak in 5d transition metal oxides and hence, various anomalous electronic properties observed in these systems are often attributed to the large spin-orbit interaction strength. Employing first principles approaches, we studied the electronic structure of Y2Ir2O7, which is insulating and exhibits ferromagnetic phase below 150 K. The results reveal breakdown of both the above paradigms. The role of spin-orbit interaction is found to be marginal in determining the ground state properties of Y2Ir2O7. A large electron correlation strength is required to derive the experimental bulk spectrum that is consistent with the bulk properties.

cond-mat.str-el↗
arXiv2009

Exotic phases and quantum phase transitions: model systems and experiments

I survey theoretical advances in our understanding of the quantum phases and phase transitions of Mott insulators, and of allied conducting systems obtained by doping charge carriers. A number of new experimental examples of Mott insulators have appeared in recent years, and I critically compare their observed properties with the theoretical expectations.

cond-mat.str-el↗
arXiv2009

Getting into Shape: Efficient Cooling Scheme for Fermionic Atoms in Optical Lattices

We propose an experimental procedure to cool fermionic atoms loaded into an optical lattice. The central idea is to spatially divide the system into entropy-rich and -poor regions by shaping the confining potential profile. Atoms in regions of high entropy per particle are subsequently isolated from the system. We discuss how to experimentally carry out this proposal, and perform a quantitative study of its efficiency. We find that the entropy per particle, $s$, can typically be reduced by a factor of 10 such that entropies lower than $s/k_B \sim 0.2$ can be reached. Cooling into highly sought-after quantum phases (such as an antiferromagnet) can thus be achieved. We show that this procedure is robust against variations of the experimental conditions.

cond-mat.str-el↗
arXiv2009

Excitation spectra, spin structures, and entanglement characteristics of four-electron double-quantum-dot artificial molecules

Energy spectra, spin configurations, and entanglement characteristics of a system of four electrons in lateral double quantum dots are investigated using exact diagonalization (EXD), as a function of interdot separation, applied magnetic field, and strength of interelectron repulsion. A distinctly different quantum behavior is found compared to that of circular single quantum dots. As a function of the magnetic field, the energy spectra exhibit a low-energy band consisting of a group of six states, with the number six being a consequence of the conservation of the total spin of the four electrons and the ensuing spin degeneracies. These six states appear to cross at a single value of the magnetic field, with the crossing point becoming sharper for larger interdot distances. As the strength of the Coulomb repulsion increases, the six states tend to become degenerate and a well defined energy gap separates them from the higher-in-energy excited states. The appearance of the low-energy band is a consequence of the formation of a Wigner supermolecule, with the four electrons (two in each dot) being localized at the vertices of a rectangular parallelogram. Using the spin-resolved pair-correlation functions, one can map the EXD many-body wave functions onto the spin functions associated with the four localized electrons. Analogies with nanomagnets, such as finite Heisenberg clusters, are discussed. The ability to determine associated spin functions enables investigations concerning entanglement properties of the system of four electrons. In particular, the formation of Wigner supermolecules generates EXD solutions belonging to the class of strongly entangled states referred to as N-qubit Dicke states in the quantum-information literature.

cond-mat.mes-hall↗
arXiv2009

From magnetism to one-dimensional spin liquid in the anisotropic triangular lattice

We investigate the anisotropic triangular lattice that interpolates from decoupled one-dimensional chains to the isotropic triangular lattice and has been suggested to be relevant for various quasi-two-dimensional materials, such as Cs$_2$CuCl$_4$ or $κ$-(ET)$_2$Cu$_2$(CN)$_3$, an organic material that shows intriguing magnetic properties. We obtain an excellent accuracy by means of a novel representation for the resonating valence bond wave function with both singlet and triplet pairing. This approach allows us to establish that the magnetic order is rapidly destroyed away from the pure triangular lattice and incommensurate spin correlations are short range. A non-magnetic spin liquid naturally emerges in a wide range of the phase diagram, with strong one-dimensional character. The relevance of the triplet pairing for $κ$-(ET)$_2$Cu$_2$(CN)$_3$ is also discussed.

cond-mat.str-el↗
arXiv2009

The f-spin physics of rare-earth iron pnictides: influence of d-electron antiferromagnetic order on the heavy fermion phase diagram

Some of the high Tc iron pnictides contain magnetic rare-earth elements, raising the question of how the existence and tunability of a d-electron antiferromagnetic order influences the heavy fermion behavior of the f-moments. With CeOFeP and CeOFeAs in mind as prototypes, we derive an extended Anderson lattice model appropriate for these quaternary systems. We show that the Kondo screening of the f-moments are efficiently suppressed by the d-electron ordering. We also argue that, inside the d-electron ordered state (as in CeOFeAs), the f-moments provide a rare realization of a quantum frustrated magnet with competing J1-J2-J3 interactions in an effective square lattice. Implications for the heavy fermion physics in broader contexts are also discussed.

cond-mat.str-el↗
arXiv2009

Electron-conduction mechanism and specific heat above transition temperature in LaFeAsO and BaFe(2)As(2) superconductors

The ionization energy theory is used to calculate the evolution of the resistivity and specific heat curves with respect to different doping elements in the recently discovered superconducting Pnictide materials. Subsequently, the electron-conduction mechanism in the Pnictides above the structural transition temperature is explained unambiguously, which is also consistent with other strongly correlated matter, such as cuprates, manganites, titanates and magnetic semiconductors. Therefore, the superconductivity is not uniquely corresponds to the electronic properties above the structural transition temperature. Detailed prediction are given on these compounds for various doping elements, namely, La(Sm,Ce,Ba,Sr,Ca)FeAsO(F,Cl,Br) and Ba(Sr,Ca,K,Rb,Cs)Fe(2)As(2).

cond-mat.supr-con↗
arXiv2012

Superconductivity at 14K in the Co-doped SmFe0.9Co0.1AsO

We report superconductivity in the SmFe0.9Co0.1AsO system being prepared by most easy and versatile single step solid-state reaction route. The parent compound SmFeAsO is non-superconducting but shows the spin density wave (SDW) like antiferromagnetic ordering at around 140K. To destroy the antiferromagnetic ordering and to induce the superconductivity in the parent system, the Fe2+ is substituted partially by Co3+. Superconductivity appears in SmFe0.9Co0.1AsO system at around 14K. The Co doping suppresses the SDW anomaly in the parent compound and induces the superconductivity. Magnetization measurements show clearly the onset of superconductivity with Tcdia at 14K. The isothermal magnetization measurements exhibit the lower critical fields (Hc1) to be around 200Oe at 2 K. The bulk superconductivity of the studied SmFe0.9Co0.1AsO sample is further established by open diamagnetic M(H) loops at 2, and 5K. Normal state (above Tc) linear isothermal magnetization M(H) plots excluded presence of any ordered magnetic impurity in the studied compound.

cond-mat.supr-con↗
arXiv2009

CaFe2As2: a Springboard to Investigating Fe-Pnictide Superconductivity

We present detailed electronic structure calculations for CaFe2As2. We investigate in particular the `collapsed' tetragonal and orthorhombic regions of the temperature-pressure phase diagram and find properties that distinguish CaFe2As2 from other Fe-pnictide compounds. In contrast to the tetragonal phase of other Fe-pnictides the electronic structure in the `collapsed' tetragonal phase of CaFe2As2 is found to be strongly 3D. We discuss the influence of these properties on the formation of superconductivity and in particular we find evidence that both magnetic and lattice interactions may be important to the formation of superconductivity. We also find that the Local Spin Density Approximation is able to accurately predict the ordering moment in the low temperature orthorhombic phase.

cond-mat.str-el↗
arXiv2009

Accessing the Neel phase of ultracold fermionic atoms in a simple-cubic optical lattice

We examine the phase diagram of a simple-cubic optical lattice half-filled by two species of fermionic atoms with a repulsive s-wave contact interaction. We use the Hartree approximation in the regime of weaker interactions, and a Hubbard model approximation in the Mott insulating phase. The regime where the Neel phase of this system is likely to be most accessible to experiments is at intermediate lattice strengths and interactions, and our two approximations agree fairly well in this regime. We discuss the various issues that may determine where in this phase diagram the Neel phase is first produced and detected experimentally, and analyze the quantum phases that occur in the intermediate lattice strength regime.

cond-mat.str-el↗
arXiv2010

Green's Functions for the Anderson model: the Atomic Approximation

We consider the cumulant expansion of the PAM employing the hybridization as perturbation (Phys. Rev. B 50, 17933 (1994)), and we obtain formally exact one-electron Green's functions (GF). These GF contain effective cumulants that are as difficult to calculate as the original GF, and the Atomic Approximation consists in substituting the effective cumulants by the ones that correspond to the atomic case, namely by taking a conduction band of zeroth width and local hybridization. This approximation has already been used for the case of infinite electronic repulsion U (Phys. Rev. B 62, 7882 (2000)), and here we extend the treatment to the case of finite U. The method can also be applied to the single impurity Anderson model (SIAM), and we give explicit expressions of the approximate GF both for the PAM and the SIAM.

cond-mat.str-el↗
arXiv2009

Supersymmetry, lattice fermions, independence complexes and cohomology theory

We analyze the quantum ground state structure of a specific model of itinerant, strongly interacting lattice fermions. The interactions are tuned to make the model supersymmetric. Due to this, quantum ground states are in one-to-one correspondence with cohomology classes of the so-called independence complex of the lattice. Our main result is a complete description of the cohomology, and thereby of the quantum ground states, for a two-dimensional square lattice with periodic boundary conditions. Our work builds on results by J. Jonsson, who determined the Euler characteristic (Witten index) via a correspondence with rhombus tilings of the plane. We prove a theorem, first conjectured by P. Fendley, which relates dimensions of the cohomology at grade n to the number of rhombus tilings with n rhombi.

cond-mat.str-el↗
arXiv2009

Impurity-induced magnetic order in low dimensional spin gapped materials

We have studied the effect of non-magnetic Zn impurities in the coupled spin-ladder Bi(Cu1xZnx)2PO6 using 31P NMR, muSR and Quantum Monte Carlo simulations. Our results show that the impurities induce in their vicinity antiferromagnetic polarizations, extending over a few unit cells. At low temperature, these extended moments freeze in a process which is found universal among various other spin-gapped compounds: isolated ladders, Haldane or Spin-Peierls chains. This allows us to propose a simple common framework to explain the generic low-temperature impurity induced freezings observed in low dimensional spin-gapped materials.

cond-mat.str-el↗
arXiv2009

Impurity effects in multiferroic compounds

We investigate the effect of impurities in multiferroic materials using an equation of motion approach for the spin dynamics of the host multiferroic compound. We model the impurities as a two-level system and focus on the regime where the impurity spins relax slowly. When the impurity strength is weak the host spins oscillate with no decay and the electric polarization is not affected. However as the impurity strength is increased the host spin components get damped and the electrion polarization is suppressed. Since polarization in multiferroic materials is driven by magnetic ordering we conclude that the presence of impurities is detrimental to multiferroicity.

cond-mat.str-el↗
arXiv2009

Non-Fermi liquid properties of 2d symplectic fermions: the role of a dynamically generated (pseudo)-gap

The interacting symplectic fermion model in two spatial dimensions is further analyzed. As an effective low energy theory, the model is unitary. We show that a relativistic mass m is dynamically generated and derive a gap equation for it. By incorporating a finite temperature we study some fundamental properties of the model, such as the specific heat and spin response, which clearly show non-Fermi liquid properties. We find that various physical properties are suppressed at temperatures T< T* where the cross-over scale is T* = m. As a simplified, toy model of high Tc superconductivity, we thus identify the pseudogap energy scale with the zero temperature relativistic mass m, and show that this reproduces some qualitative aspects of the observed phenomenology of the pseudogap. The effects of the pseudogap and finite temperature on the d-wave gap equation are analyzed. In this model, the pseudogap is a distinct phenomenon from superconductivity and in fact competes with it. Our analysis of Tc suggests that the quantum critical point of our model, where the pseudogap vanishes, occurs inside the superconducting dome near optimal doping. For an antiferromagnetic exchange energy of J/k_B ~ 1350K, solutions of the d-wave gap equation give a maximum Tc of about 110K.

cond-mat.str-el↗
arXiv2009

Kondo physics versus spin gap physics in fully spin-split quantum wires

Linear and nonlinear transport of quantum wires is investigated at a magnetic field where spin-split one-dimensional (1D) subbands are equidistant in energy. In this seldom-studied regime, experiments are consistent with a density-dependent energy gap between spin subbands, and with a complete spin polarization of the first 1D subband under a large source-drain bias at zero field.

cond-mat.mes-hall↗
arXiv2009

Noise-induced phase transition in the electronic Mach-Zehnder interferometer

We consider dephasing in the electronic Mach-Zehnder interferometer strongly coupled to current noise created by a voltage biased quantum point contact (QPC). We find the visibility of Aharonov-Bohm oscillations as a function voltage bias and express it via the cumulant generating function of noise. In the large-bias regime, high-order cumulants of current add up to cancel the dilution effect of a QPC. This leads to an abrupt change in the dependence of the visibility on voltage bias which occurs at the QPC's transparency T=1/2. Quantum fluctuations in the vicinity of this point smear out the sharp transition.

cond-mat.mes-hall↗