XIII Training Course in the Physics of Strongly Correlated Systems

Vietri sul Mare (Salerno) Italy

6 - 17 October 2008

Participant Seminar Abstracts

Mr. Giacomo Coslovich

Università degli Studi di Trieste, Italy

Time-resolved ultrafast optical studies of High-Tc Superconductors

Abstract: Pump-probe experiments give insight into the relaxation dynamics of electrons and phonons in solids. The physics behind this process is governed by electron-electron and electron-phonon interactions. Ultrafast optical spectroscopy is a suitable tool to study strongly correlated systems and in particular High-temperature superconductors, where the interplay of these interactions leads to a rich phase diagram. The subject of this seminar is the possibility to photo-induce a purely electronic phase transition from the superconducting state of a Bi-based underdoped cuprate by means of an ultrashort laser pulse. We report on the experimental evidence, by means of time-resolved reflectivity measurements, of an abrupt transition of the optical response at a critical fluence of Ipump≅70 μJ/cm2 [1]. This value can be compared with the predictions of the available BCS models of non-equilibrium superconductivity [2,3,4]. A simple phenomenological model that accounts for the main observed features is proposed. The optical control of the electronic phase of a superconducting system opens the way towards the manipulation of matter based on the change of the thermodynamic potential along non-equilibrium pathways. This technique has possible application in other complex and strongly correlated materials. Further on, the possibility to follow the phase transition dynamics on the femtosecond timescale could help in the understanding of the key mechanisms at the base of the elusive superconducting and pseudogap phases in cuprates. [1] C. Giannetti, G. Coslovich, F. Cilento, G. Ferrini, H. Eisaki, N. Kaneko, M. Greven, F. Parmigiani, arXiv:0804.4822v1 (2008) [2] C. S. Owen and D. J. Scalapino, Phys. Rev. Lett. 28, 155 (1972) [3] W. H. Parker, Phys. Rev. B 12, 3667 (1975) [4] E. Nicol and J. Carbotte, Phys. Rev. B 67, 214506 (2003)

Mr. Tomislav Ivek

Institute of Physics, Zagreb, Croatia

Collective Charge Excitations below the Metal-to-Insulator Transition in BaVS3

Abstract: Collective Charge Excitations below the Metal-to-Insulator Transition in BaVS3 T. Ivek*, T.Vuletić*, S. Tomić*, A. Akrap^, H. Berger^, L. Forró^ *Institut za fiziku, Zagreb, Croatia ^Ecole Polytechnique Fédérale, Lausanne, Switzerland The perovskite-type sulfide the barium vanadium sulfide (BaVS3) consists of parallel VS3 spin chains separated by Ba atoms, which yields a quasi-one-dimensional structure. There are two electrons per unit cell which are shared between a broad A1g band derived from vanadium dz2 orbitals overlapping along the chain direction, and a quasi-degenerate narrow Eg1 band originating from e(t2g) orbitals with isotropic interactions via V-S-S-V bonds [1]. The spin degrees of freedom of the localized electrons together with coupling of itinerant and localized electrons give rise to a novel and complex physics, resulting in a MI transition [2,3] at about 70 K and a magnetic transition at about 30 K. In spite of a great deal of experimental efforts, no definite understanding has been reached yet on the detailed nature of the MI phase transition and the ground state in BaVS3. We have characterized charge response in BaVS3 single crystals by dc resistivity and low frequency dielectric spectroscopy [4]. A broad relaxation mode in MHz range with a huge dielectric constant ~= 10^6 emerges at the metal-to-insulator phase transition TMI ~= 67 K, weakens with lowering temperature and eventually levels off below the magnetic transition Tchi ~= 30 K. The mean relaxation time is thermally activated in a manner similar to the dc resistivity. These features are interpreted as signatures of short-wavelength charge excitations characteristic for the orbital ordering which sets in at TMI and develops a long-range order below Tchi. References: [1] F. Lechermann et al., Phys. Rev. B 76, 085101 (2007). [2] S. Fagot et al., Phys. Rev. Lett. 90, 196401 (2003). [3] S. Fagot et al., Phys. Rev. B 73, 033102 (2006). [4] T. Ivek et al., accepted for publication in Phys. Rev. B, arXiv:0706.2079v4 (2008).

Dr. Kyungwan Kim

Department of Physics, University of Fribourg, Switzerland

Optical response of insulating CuO4 plaquett network

Abstract: The high temperature superconductivity of cuprates appears with charge doping in insulating parent meterials. Those insulating cuprates become insulator thanks to the strong Coulomb repulsion at Cu d orbitals. They have charge gap between filled oxygen 2p and empty Cu 3d states categorized as charge transfer insulators. Although only Cu 3d and O 2p states are involved in low energy optical transitions, optical excitation spectra of insulating cuprates show rich structures depending on the form of CuO4 networks. One dimensional chain systems with corner sharing plaquettes and edge saring plaquettes provide basic charactors of these transitions. Comparison of available spectra with one dimensional chain compound Sr2CuO3 shows that excitonic effect is strong in insulating cuprates with CuO2 planes.

Mr. Guido Klingschat

Institute for Theoretical Physics and Astrophysics, University of Würzburg, Germany

SU(3)-Hubbard model with (strong) attractive coupling

Abstract: Recently, a quantum phase transition from a color superfluid to a col- orless phase with conglomerates of 3 fermions on a single site (’trions’) has been proposed to occur in the attractive SU(3)-Hubbard model [Rapp et al., Phys. Rev. Lett. 98, 160405 (2007)]. Here we analyze the properties of the trionic phase using exact diagonalization. We de- termine the spectral function of single particle and trionic excitations, and compute spatial correlations. This way we can characterize the effective quasiparticles of the strong coupling phase.

Dr. Nicola Magnani

European Commission, Joint Research Centre, Institute for Transuranium Elements, Karlsruhe, Germany

Hidden Order and Low-Energy Excitations in Neptunium Dioxide

Abstract: We investigate the nature of the hidden order parameter of NpO$_2$, which had been identified with a staggered arrangement of $\Gamma_5$ magnetic multipoles. By analyzing the existing experimental data, we show that the most likely driving order parameter is not provided by octupoles, as usually assumed, but rather by the rank-5 triakontadipoles. Calculations of the coupled dynamics of spins, $\Gamma_5$ quadrupoles, and $\Gamma_5$ triakontadipoles in the ordered phase enable us to analyze the resulting structure of low-energy excitations. We show that the powder inelastic neutron scattering cross section should contain, in addition to the already-observed peak at 6.5 meV, a second weaker peak at about 14 meV. As a test, we have performed polarized inelastic neutron scattering experiments in the ordered phase of neptunium dioxide. The observation of magnetic scattering between 11 and 18 meV can indeed be attributed to a transition between two states with the same expectation value of the electric quadrupole, but opposite values of the magnetic triakontadipole. In contrast to resonant X-ray scattering, which detects the secondary order parameter (electric quadrupoles) associated with the 25 K phase transition, the results reported here are a direct manifestation of the primary (magnetic) order parameter.

Dr. Victor Yushankhai

Laboratory of Theoretical Physics, JINR, Dubna, Russia

Self-consistent renormalization theory of spin fluctuations in paramagnetic spinel LiV_2O_4

Abstract: The metallic spinel LiV2O4 , a rare 3d-electron heavy fermion system, is suggested to be near to a magnetic instability at T=0. The appearance of a rather peculiar paramagnetic ground state in LiV2O4 is likely related directly to the geometrical frustration of the magnetic V ion lattice. The observed in the inelastic neutron scattering and NMR measurements, the evolution of critical spin fluctuations with temperature and pressure is described in terms of properly parametrized self-consistent renormalization (SCR) theory.

Mr. Alexander Mai

Institute of Theoretical Physics, Dresden Technical University, Germany

Valence Transition and Superconductivity in the Two-Dimensional Extended Periodic Anderson Model

Abstract: The periodic Anderson model (PAM) describes the interplay of itinerant conduction and strongly correlated localized f electrons on a microscopic scale and can be used to investigate heavy fermion behavior and its breakdown. Using an extension of the Projector-based Renormalization Method (PRM) we study the valence transition as a function of the local f-energy $e_f$ and the orbital degeneracy $\nu_f$. It is claimed in the literature that in the presence of an additional local Coulomb repulsion $U_{fc}$ between f- and conduction electrons the system is susceptible to superconductivity in the valence transition regime. Therefore we extend the model in order to calculate superconducting order parameters originating from either an enhanced electron phonon mechanism or an additional $U_{fc}$ contribution.

Mr. Matthias Nyfeler

Institut for Theoretical Physics, Bern, Switzerland

From an Antiferromagnet to a Valence Bond Solid: Evidence for a First Order Phase Transition

Abstract: Using a loop-cluster algorithm we investigate the spin 1/2 Heisenberg antiferromagnet on a square lattice with exchange coupling J and an additional four-spin interaction of strength Q. We confirm the existence of a phase transition separating antiferromagnetism at J/Q > Jc/Q from a valence bond solid (VBS) state at J/Q < Jc/Q. Although our Monte Carlo data are consistent with those of previous studies, we do not confirm the existence of a deconfined quantum critical point. Instead, using a flowgram method on lattices as large as 802, we find evidence for a weak first order phase transition. We also present a detailed study of the antiferromagnetic phase. For J/Q > Jc/Q the staggered magnetization, the spin stiffness, and the spinwave velocity of the antiferromagnet are determined by fitting Monte Carlo data to analytic results from the systematic low-energy effective field theory for magnons. Finally, we also investigate the physics of the VBS state at J/Q < Jc/Q, and we show that long but finite antiferromagnetic correlations are still present.

Dr. Marina Poltavskaya

Dept. of Mathematical Modeling of Physical Processes, B. Verkin Institute for Low Temperature Physics and Engineering, Kharkov, Ukraine

Two-Time Green Functions in the Theory of Low-Dimensional Spin-1/2 Magnets

Abstract: Analytical approaches based on the two-time Green function formalism proved to be very effective in calculating the thermodynamic properties of one- and two-dimensional quantum Heisenberg magnets. These methods give quantitative results in the wide temperature and magnetic field ranges and are appropriate for both ferromagnetic and antiferromagnetic exchanges. They are based on one or another decoupling scheme for higher Green functions, which result in a closed set of self-consistent equations for thermodynamic averages. Random phase approximation is the simplest variant of such scheme with decoupling at the first step. It usually gives satisfactory results for magnetization and spin susceptibilitiy at high magnetic fields. More complicated schemes are based on the decoupling of higher Green functions at the second step with introducing vertex parameters to be found. These schemes give quantitative results for both magnetic (magnetization, spin susceptibility) and thermodynamic (correlation functions, energy, heat capacity, correlation length) functions of low-dimensional quantum magnets in the external magnetic field and without it. The basic properties of two-time Green functions will be reviewed. The equations for the Green functions of low-dimensional spin-1/2 ferromagnet in the external magnetic field will be derived within the random phase approximation. For this system the scheme with the decoupling at the second step will be outlined. The characteristic features of the Green function theory in the case of antiferromagnetic exchange will be discussed.

Mr. Matous Ringel

Department of Condensed Matter Theory, Institute of Physics of the Academy of Science of CR, Prague, Czech Republic

Magnetic properties of impurities with strongly correlated electrons

Abstract: We study the single impurity Anderson model in an external magnetic field. There are no exact results for the spectral function in this situation. Using a resummation of the diagrammatic expansion we demonstrate that the strong coupling regime in a weak magnetic field is Kondo-like with a quasiparticle resonant peak split into two. We find two exponentially small Kondo scales (temperatures), one for transverse and one for longitudinal spin fluctuations. We show that the salient features of the spectral function in the Kondo regime can be seen already within an extended random phase approximation. To reveal the dependence of the Kondo scales on the bare electron interaction, however, one has to use a two-particle self-consistency with renormalized vertices. We use the parquet approach to derive the dependence of the Kondo scales on magnetic field.

Mr. Marc Warner

London Centre for Nanotechnology, University College London, England

Towards Organic Quantum Computation?

Abstract: Quantum computers show tremendous promise to revolutionise any field in which the modelling of a quantum mechanical system is currently performed using classical computation. This includes vast swathes of physics, chemistry, biology and medicine. We present the first steps towards the creation a phthalocyanine organic quantum computer. Thin films of spin diluted copper phthalocyanine (CuPc) were prepared on a kapton substrate by organic molecular beam deposition. The spin of the electron on the copper atom serves as the quantum bit (qubit). The CuPc can be templated, where the molecular plane of the CuPc molecules are forced to lie parallel to the substrate, by coating the substrate with a layer of PTCDA. Alternatively the substrate can be left uncoated, under which circumstances the plane of the molecule lies perpendicular to the substrate. The work presented consists of the characterization of these samples using continuous wave (CW) EPR, measurements of the coherence times and the demonstration of Rabi oscillations with pulsed EPR. The results from the CW rotation patterns of the templated spin diluted CuPc samples showed clearly that the CuPc and metal free phthalocyanine form a good approximation of a single crystal. The varying spin dilution shows the effects of couplings between phthalocyanines broadening the linewidths of the spectra. Finally the dephasing time (T2) measurements of order 1μs show that quantum information processing is a possibility in these systems.

Mr. Krzysztof Wohlfeld

Condensed Matter Theory, Jagellonian University, Cracow, Poland

Novel Spin-Orbital Polarons in Cubic Vanadates

Abstract: When a single hole is introduced into the insulating plane of undoped cubic vanadates (such as e.g. LaV0$_3$) it cannot move freely as it can disturb the alternating orbital (AO) and antiferromagnetic (AF) order present in the system. Instead, its motion is strongly renormalized due to the polaron-like coupling between the hole and the collective excitations of the AO state (orbitons) and the AF state (magnons). Here, we investigate this phenomenon using self-consistent Born approximation and show how these novel spin-orbital polarons differ from the well-known spin polarons in the high-Tc cuprates or from the orbital polarons in the manganites or fluorides.

Mr. Yao Yao

Department of Physics, Fudan University, Shanghai, China

Controllable spin-current blockade in a Hubbard chain

Abstract: We investigate the spin/charge transport in a one-dimensional strongly correlated system by using the adaptive time-dependent density-matrix renormalization group method. The model we consider is a non-half-filled Hubbard chain with a bond of controllable spin-dependent electron hoppings, which is found to cause a blockade of spin current with little influence on charge current. We have considered (1) the spread of a wave packet of both spin and charge in the Hubbard chain and (2) the spin and charge currents induced by a spin-dependent voltage bias that is applied to the ideal leads attached at the ends of this Hubbard chain. It is found that the spin-charge separation plays a crucial role in the spin-current blockade, and one may utilize this phenomenon to observe the spin-charge separation directly.