XV Training Course in the Physics of Strongly Correlated Systems

Vietri sul Mare (Salerno) Italy

October 4 - 15, 2010

Participant Seminar Abstracts

Miss Amy Briffa

School of Physics and Astronomy, University of Birmingham, United Kingdom

Generalising spin ice: the magnetic ground state of pyrochlore Gd2Ti2O7

Abstract: Recently there has been much interest in the magnetic monopole excitation of spin ice. This is a consequence of the 'ice rules' which give rise to a large low energy degeneracy. Both spin ice and gadolinium titanate have a pyrochlore lattice of corner sharing tetrahedra. In spin ice the strong crystal field interactions mean that the spins have an Ising character: they are constrained to point along the local tetrahedral axes. The degenerate ground state configurations consist of all tetrahedra containing two spins pointing inwards and the remaining two outwards (and, therefore, each tetrahedron has a ferromagnetic component). In contrast, in gadolinium titanate the antiferromagnetic nearest neighbour Heisenberg interaction is dominant, which is extremely frustrated on the pyrochlore lattice. This gives rise to a similar but much larger degeneracy than is seen in spin ice. We investigate the residual degeneracy in gadolinium titanate. Firstly we enforce the Mössbauer experimental restriction that the spins are confined to planes perpendicular to the local crystal directions in order to reduce the degeneracy to a manageable level. This XY-like anisotropy can be seen to be a result of minimising the dipolar interactions. Our assumptions generate a more complex analogue of the ice rules for gadolinium titanate. Theoretically the frustrated antiferromagnetic interaction leads to the absence of classical long range magnetic order in the ground state. Gd2Ti2O7 shows precisely this behaviour by magnetically ordering well below the Curie-Weiss temperature as a result of other much weaker interactions becoming relevant. We require that our rules discussed above are also consistent with the neutron scattering experiments, which show a magnetic scattering vector k = (½ ½ ½). This turns out to lead to two distinct magnetic states which are unrelated to previous proposals. The energetics are complicated. The residual degeneracy appears to be lifted by a compromise between longer range Heisenberg and direct dipolar interactions of a similar energy scale.

Dr. Valentina Brosco

Dipartimento di Fisica, Università degli Studi di Roma "La Sapienza", Italy

Low-temperature conductivity of a low-density Rashba electron gas.

Abstract: Besides its fundamental relevance to the transport properties of a variety materials, the interplay between spin and orbital degrees of freedom is of crucial importance for spintronics experiments and applications. In these regards Rashba spin-orbit interaction arising in low dimensional heterostructures is of special interest. Indeed, as it has been demonstrated in recent experiments carried on LaAlO3/SrTiO3 interfaces and on surface states in surface alloys, it is possible to tune its strength via external gates and material engineering. Motivated by these experiments, in the present work we investigate the consequences of low charge carrier densities on the low-temperature conductivity of a two-dimensional electron gas with strong Rashba spin-orbit coupling. We show how the small value of the Fermi energy compared to Rashba spin-orbit coupling leads to an effective reduction of the dimensionality. We analyze both the conductivity in Born approximation and the weak-localization correction.

Mr. Wojciech Brzezicki

M. Smoluchowski Institute of Physics, Jagiellonian University, Krakow, Poland

Hidden Dimer Order in the Orbital Compass Model

Abstract: We introduce an exact spin transformation that maps frustrated $Z_{i,j}Z_{i,j+1}$ and $X_{i,j}X_{i+1,j}$ spin interactions along the rows and columns of the orbital compass model (OCM) on an $L\times L$ square lattice to $(L-1)\times (L-1)$ quantum spin models with $2(L-1)$ classical spins. Using the symmetry properties we unravel the hidden dimer order in the OCM, with equal two-dimer correlations $\langle X_{i,i}X_{i+1,i}X_{k,l}X_{k+1,l}\rangle$ and $\langle X_{i,i}X_{i+1,i}X_{l,k}X_{l+1,k}\rangle$ in the ground state, which is independent of the actual interactions. This order coexists with Ising-like spin correlations which decay with distance.

Mr. Stefanos Kourtis

Leibniz Institute for Solid State and Materials Research, Dresden, Germany

Coexistence of superconducting and charge density wave states

Abstract: Phase coexistence is a subject relevant to several systems of interest in the scope of condensed matter physics. From optical lattices loaded with ultra-cold atoms to high temperature superconductors, the interplay between condensed phases is a decisive factor for the properties of the physical system under consideration. Here we consider conventional and unconventional charge density wave and superconducting states with or without the presence of spontaneous d-wave Fermi surface deformation (also called Pomeranchuk instability). In a numerical mean-field treatment we find that under certain circumstances these states can either compete or coexist, possibly giving rise to a staggered superconducting state.

Mr. Neeraj Kumar

Department of Condensed Matter Physics and Material Science, Tata Institute of Fundamental Research, Mumbai, India

Magnetic properties of EuPtSi3 single crystals

Abstract: Single crystals of EuPtSi3, which crystallize in the BaNiSn3-type crystal structure, have been grown by high temperature solution growth method using molten Sn as the solvent. EuPtSi3 which lacks the inversion symmetry and has only one Eu site in the unit cell is found to be an antiferromagnet with two successive magnetic transitions at TN1 = 17 K and TN2 = 16 K, as inferred from magnetic susceptibility, heat capacity and 151Eu MÃ¶ssbauer measurements. The isothermal magnetization data for H // [001] reveal a metamagnetic transition at a critical field Hc = 1 T. The magnetization saturates to a moment value of 6.43 Î¼B/Eu above 5.9 T and 9.2 T for H // [001] and [100] respectively, indicating that these fields are spin-flip fields for the divalent Eu moments along the two axes. A magnetic (H, T) phase diagram has been constructed from the temperature dependence of isothermal magnetization data. In my talk Iâ€™ll show above mentioned results and discuss the possible origin of anisotropic behavior in EuPtSi3.

Mr. Nicolaus Parragh

Vienna University of Technology, Austria

LDA+DMFT study of fotoemission and optical properties of V2O3

Abstract: The metal-insulator transition (MIT) in Chromium-doped Vanadium Sesquioxide has received much attention since its discovery in 1969. After several attempts for describing it in the framework of single-band Hubbard models some key experiments revealed the intimate multi-band nature of this material. Here I will present the results of a recent experiment combining Infrared (IR), Scanning Photoemission Microscopy (SPEM) and X-ray Diffraction (XRD) and local density approximation + dynamical mean field theory (LDA+DMFT) calculations showing for the first time that microscopic islands characterized by bad metallic behaviour form close to the first order MIT. The bad metallicity is only partially reduced by applying pressure, pointing to a non-trivial interplay between the orbital degrees of freedom across different paths in the MIT of V2O3.

Mr. Christopher Thomas

Instituto de Fisica, Universidade Federal do Rio Grande do Sul, Porto Alegre - RS, Brazil

Application of the S=1 underscreened Anderson lattice model to Kondo uranium and neptunium compounds

Abstract: Magnetic properties of uranium and neptunium compounds showing the coexistence of Kondo screening effect and ferromagnetic order are investigated within the Anderson lattice Hamiltonian with a two-fold degenerate f-level in each site, corresponding to 5f2 electronic configuration with S=1 spins. A derivation of the Schrieffer-Wolff transformation is presented and the resulting Hamiltonian has an effective f-band term, in addition to the regular exchange Kondo interaction between the S=1 f-spins and the s=1/2 spins of the conduction electrons. The obtained effective Kondo lattice model can describe both the Kondo regime and a weak delocalization of 5f-electron. Then we calculate the Kondo and Curie temperatures as a function of the Kondo exchange interaction constant J_K, the magnetic intersite exchange interaction J_H and the effective f-bandwidth. We deduce, therefore, a phase diagram which can yield the coexistence of Kondo effect and ferromagnetic ordering and can account for the pressure dependence of the Curie temperature of uranium compounds such as Ute.

Mr. Lev Vidmar

Department for Theoretical Physics (F-1), Jozef Stefan Institute, Ljubljana, Slovenia

Gain of the kinetic energy of bipolarons in the t-J Holstein model based on electron-phonon coupling

Abstract: Not long after discovery of cuprates a conjecture appeared that the onset of superconducting order is connected to lowering of the kinetic energy of charge carriers [1,2]. This was supported by the measurements of optical conductivity, which show a decreased sum-rule (and correspondingly the total kinetic energy) when entering the superconducting state [3]. In my seminar I will show that the formation of a bipolaron with d-wave symmetry [4] in context of the t-J model does not lead to lowering of the kinetic energy, calculated per hole. However, in the t-J Holstein model at the values of EPH coupling in the cross-over between the weak-coupling and strong-coupling regime we find a bipolaron which develops a lower effective mass than two unbound polarons and consequently lowers the total kinetic energy [5]. This effect is mostly pronounced for the parameters of the t-J model, which correspond to the experimentally relevant values for the cuprates. [1] J. E. Hirsch and F. Marsiglio: Physica C 162, 591 (1989) [2] J. E. Hirsch: Science 295, 2226 (2002) [3] H. J. A. Molegraaf, C. Presura, D. van der Marel, et al.: Science 295, 2239 (2002) [4] L. Vidmar, J. Bonca, S. Maekawa and T. Tohyama: Physical Review Letters 103, 186401 (2009) [5] L. Vidmar and J. Bonca: submitted