Recent developments in the epitaxial growth of carbon-doped, two-dimensional hole systems (2DHS) in GaAs have resulted in samples of unprecedented quality.  These new samples allow for the exploration of correlations in two dimensions in a regime where strong spin-orbit coupling plays an important role.  In this talk I will describe the growth of such structures and discuss some of our recent observations of anisotropic charge transport in a perpendicular magnetic field at filling factors v=7/2, 11/2 and 13/2 at low temperatures.  In start contrast with 2d electron systems, the transport at v=9/2 is isotropic for all temperatures.  Isotropic hole transport at v=7/2 is restored for sufficiently low 2D densities or an asymmetric confining potential.  The density and symmetry dependences of the observed anisotropies suggest that strong spin-orbit coupling in the hole system contributes to the unusual transport behavior.

Hole-mediated ferromagnetism in III-V ferromagnetic semiconductors gives us the opportunity of studying direct manipulation of local spins through carrier spins. Studying the influence of optical excitation on magnetic moments in the time regime of nano-second or less is especially interesting, since it gives insight about the interaction among electron, lattice, and spin systems. The interaction between electron and spin systems via the lattice system has been studied extensively, and can be understood in terms of thermalization of spins. On the other hand, understanding the direct pathway of energy and momentum transfer between the electron and spin systems has just begun with novel materials and advanced experimental/theoretical approaches [1-3].
In this presentation, we are concerned with the effect of optical excitation on ferromagnetism in III-V ferromagnetic semiconductors (III-V FMS). I will first describe materials aspect of III-V FMS and physics of carrier-mediated ferromagnetism. This is followed by the discussion of three different light-induced phenomena: light-induced precession of ferromagnetically-coupled Mn spins (magnetization) [4], dynamic perpendicular magnetization [5], and ultrafast demagnetization [3]. The first two phenomena have been studied by collecting time-resolved magneto-optical (MO) signals from ferromagnetic (Ga,Mn)As through one-color, pump-and-probe technique, whereas the third phenomenon by time-resolved polar Kerr signals from ferromagnetic, narrow-gap (In,Mn)As through two-color, pump-and-probe technique. As to the light-induced precession (5 GHz), experimental results indicate that optical excitation can result in the variation of magnetic anisotropy driven at least in part by some non-thermal process. As for the dynamic perpendicular magnetization, experimental data suggest that excitation with circular polarization causes ultrafast inducement of magnetization along optical orientation followed by a single exponential decay (50 psec). The experimental data showing the ultrafast demagnetization under intense light pulses (100fsec) suggest the direct heat exchange between electron- and spin-subsystems, presumably through dynamic polarization of hot holes followed by ultrafast spin relaxation. These experimental data would lead us to establish fundamental concepts of ultrafast manipulation of collective spins and magnetization.
References
[1] E. Beaurepaire, J.-C. Merle, A. Daunois, J.-Y. Bigot, Phys. Rev. Lett., 76, 4250 (1996).
[2] A.V. Kimel, A. Kirilyuk, A. Tsvetkov, R.V. Pisarev, and Th Rasing, Nature 429, 850 (2004).
[3] J. Wang, et al., Phys. Rev. Lett. 95, 167401 (2005).
[4] A. Oiwa, et al., J. Supercond. Novel Mag. 18, 9 (2005); Y. Hashimoto and H. Munekata, arXiv: 0707.4055v1.
[5] Y. Mitsumori, et al., Phys. Rev. B 69, 033203 (2004); A. Oiwa, et al., Phys. Rev. Lett. 88, 137202 (2002).

Metallic magnets on geometrically frustrated lattices have attracted interest because of their novel low temperature phenomena such as the d-electron heavy fermion behavior of LiV2O4, YMn2, and the anomalous Hall effect (AHE) of Nd2Mo2O7. Among them, the pyrochlore magnet Pr2Ir2O7 is quite unique for its metallic spin liquid behavior, and for novel transport phenomena due to the strong geometrical frustration. We discuss the origin of pronounced non-Fermi-liquid behavior and unconventional AHE in terms of spin-chirality of Ising-like Pr moments..

Carbon-nanotube provides a unique platform for fabricating nanostructures beyond the reach of standard lithography techniques. Such nanostructures can be explored in broad-scope mechanical or electronic applications. I will give a few examples including electromechanical resonator, field-effect transistor and tunneling gap. Apart from their potential applications in sensors or actuators, these nanostructures reveal interesting physical phenomena such as non-linear mixing effect and random telegraph signals. In the long run, they may provide promising systems for probing the imprint of quantum interactions in the macroscopic level.

I will present a recently developed continuous-time QMC method for impurity models [1,2] which is based on a diagrammatic expansion of the partition function in the impurity-bath hybridization. This algorithm enables the efficient DMFT-simulation of strongly correlated systems and I will illustrate its power and flexibility with applications to Hubbard, Holstein-Hubbard, Kondo-lattice and multi-orbital models.
[1] PRL 97, 076405 (2006)
[2] PRB 74, 155107 (2006)

Topological quantum computation is a new, emerging paradigm for fault-tolerant quantum computation. The proposed topological quantum computer relies on the existence of non-Abelian anyons, which are thought to occur in certain fractional quantum Hall states as elementary excitations. In this talk, I will talk about (a) the motivation for studying topological quantum computation, (b) how Fibonacci anyons may be used for topological quantum computation, and (c) our recent effort on interferometry experiments to study the postulated non-Abelian behavior in the fractional quantum Hall effect.

The present focus on quantum critical phenomena in correlated matter is driven by the puzzling enigma concerning the origin of the superconductivity (SC) in these materials. The prototypical heavy-fermion superconductor CeCu2Si2 yields a possible clue to this question: For this material, itinerant antiferromagnetic (AF) or spin-density-wave (SDW) order was found to be smoothly suppressed by applying a critical pressure, pc, of a few kbar only. Here, pc denotes a SDW quantum critical point (QCP) which involves three-dimensional (3D) critical fluctuations. Since the integrity of the ("composite") heavy charge carriers is guaranteed at a 3D-SDW QCP, it is often called a conventional QCP.
In the first part of this lecture, recent results of inelastic neutron-scattering experiments are presented which suggest that in CeCu2Si2 quantum critical SDW fluctuations may mediate the formation of Cooper pairs. In the second part of the lecture, the isostructural compound YbRh2Si2 which shows weak AF order below a N�el temperature TN  70 mK will beintroduced. TN was found to smoothly vanish upon applying a very low magnetic field ( 60 mT). As demonstrated by a number of different experimental probes, the heavy quasiparticles seem to disintegrate on the approach of this field-induced QCP.
In conclusion, while the conventional QCP in CeCu2Si2 appears to favor unconventional SC, in the vicinity of the unconventional QCP in YbRh2Si2 no SC was observed, at least down to T  10 mK.
This work was done in collaboration with J. Custers, P. Gegenwart, C. Geibel, H. S. Jeevan, R. K�chler, S. Paschen, J. Sichelschmidt, O. Stockert and S. Wirth (MPI CPfS) as well as E. Abrahams and P. Coleman (Rutgers University), C. P�pin (CEA Saclay) and Q. Si (Rice University).

We will discuss the properties of classical and quantum Heisenberg models on the lattices with corner sharing triangles; namely two dimensional Kagome and three-dimensional Hyper-Kagome lattices. The roles of thermal and quantum fluctuations, and the resulting ground states will be the focus of the discussion. Connection to recent experiments on Volborthite, Na4Ir3O8, and Zn-paratacamite will also be discussed.

The interference of multiply scattered quantum mechanical matter waves causes small but noticeable corrections to the conductance of a metal at low temperatures. Historically, one separates the interference correction to the electrical conductance into "weak localization", a small negative correction to the conductance averaged over an ensemble of conductors with different impurity configurations, and the "conductance fluctuations", the sample-to-sample fluctuations measured with respect to the ensemble average.
What is the fate of quantum interference corrections in the limit that the wavelength of the electrons becomes small in comparison to all other relevant length scales? This limit is a "classical limit" similar to the transition from wave optics to ray optics that occurs when the typical size of optical elements becomes much larger than the wavelength of light. I'll show that, whereas the interference correction to the ensemble-averaged conductance (weak localization) disappears in this classical limit, the quantum interference contribution to the sample-specific conductance fluctuations remains surprisingly unaffected.

The variation in the quasiparticle weight Z on moving around the fermi surface in correlated metals is studied theoretically. Our primary example is a heavy Fermi liquid treated within the standard hybridization mean field theory. The most dramatic variation in the quasiparticle weight happens in situations where the hybridization vanishes along certain directions in momentum space. Such a "hybridization node" is demonstrated for a simplified model of a Cerium-based cubic heavy electron metal. We show that the quasiparticle weight varies from almost unity in some directions, to values approaching zero in others. This is accompanied by a similar variation in the quasiparticle effective mass. Some consequences of such hybridization nodes and the associated angle dependence are explored. Comparisons to somewhat similar phenomena in the normal metallic state of the cuprate materials are discussed. A phenomenological picture of the pseudogap state in the cuprates with a large Fermi surface with a severely anisotropic spectral weight is explored. In certain cases, the quasiparticle residue goes to zero at isolated points (in two dimensions) on the Fermi surface. We also point out the possibility of quantum Hall phenomena in two dimensional Kondo insulators, if the Kondo singlet has complex internal angular momentum.

We provide a heuristic argument for disorder rounding of a first order quantum phase transition into a continuous quantum phase transition. This is illustrated by renormalization group analysis of the N-color Ashkin-Teller chain, the q- state Potts chain, the biquadratic spin one anti-ferromagnetic chain and the O(N) vector model with cubic anisotropy. In most of the cases, the results of renormalization group analysis are in excellent agreement with our heuristic argument. In a certain range of parameter space of the Ashkin-Teller model renormalization group calculations break down and indicate lack of universality. This may imply the persistence of the first order quantum phase transition and a possible modification of Aizenman-Wehr theorem for the quantum phase transitions.

When two layers of two-dimensional carriers are brought into close proximity, new phenomena resulting form the carrier-carrier interaction in opposite layers occur. Here, we explore a few of these phenomena observed in interacting GaAs hole bilayers, at low temperatures and in high magnetic fields. One key experimental observation in these systems is a charge neutral superfluid at total Landau level filling (nu) factor one when equal and opposite currents are passed in the two layers (counterflow). At the lowest temperatures (T) both Hall and longitudinal counterflow resistivities (rho_xx and rho_xy) vanish at nu=1, a finding which demonstrates the existence of a counterflow superfluid in the limit of T=0 at this filling factor. This phenomenon can be explained by the formation of electron-hole pairs (excitons) in the two, half-filled layers, and the resulting excitonic condensation at the lowest temperatures. The counterflow rho_xx and rho_xy dependence on temperature and bilayer density suggests that the counterflow dissipation is caused by existence of mobile vortices which move across the superfluid current.

The study of critical phenomena in equilibrium many-body systems is  by now a well-established subfield of condensed matter physics. The central issue in equilibrium critical phenomena is the emergence of universal behaviour among various systems with different microscopic properties when they are in the  vicinity of a critical point. Whether or not this universality persists in  systems driven far from equilibrium is an interesting but largely unexplored  problem. In this talk, I will address this issue by considering a thin  itinerant electron magnet driven into a nonequilibrium steady state by current  flow across the system. The theory is formulated using the Keldysh functional  integral formalism, and the renormalization group scheme is generalized to the  nonequilibrium case to distinguish different universality classes of  nonequilibrium perturbations. We show the relevance of departures from  equilibrium at an equilibrium critical point and present the consequent  decoupling of statics and dynamics by the nonequilibrium drive. By mapping our  problem onto an effective classical theory we find that the leading effect of  the nonequilibrium drive is to generate an effective temperature.

A supersolid state of matter, i.e. a solid with crystalline structure that exhibits frictionless superflow, was theoretically proposed to occur in solid 4He almost 40 years ago. The first experimental indication for the existence of supersolidity in helium was discovered in 2003 when Kim and Chan employed the torsional oscillator technique to observe nonclassical rotational inertia (NCRI). In order to determine if supersolidity is an intrinsic property of helium we have studied the effect of disorder on the NCRI. We find that the supersolid signal can be substantially reduced or sometimes eliminated by annealing of the
helium crystal. Also, we have been able to increase the supersolid signals by three orders of magnitude by geometrically confining the sample space. We believe that the supersolid signal is strongly modified by varying the amount of crystalline disorder.

Usually magnetic hysteresis is caused by the imperfections and defects in the crystal structure resulting in pinning of magnetic flux in superconductors or domain walls in ferromagnets. The hysteresis may also be caused by shape and surface - related energy barriers (e.g., geometric and Bean-Livingston) that result in a spatially nonuniform free energy and corresponding metastable states of the system.
I will describe a different type of magnetic hysteresis that is only observed in clean, pinning-free samples where structure of the mixed phases patterns becomes a variable in finding the ground state. This hysteresis cannot be annealed or removed by any sample improvement. We call this phenomenon a “topological hysteresis“ to indicate that the difference in the topologies of the intermediate state in type-I superconductors or ferromagnetic domains in soft ferromagnets can lead to a measurable hysteretic response of magnetization. Furthermore, structure of the intermediate state in type-I superconductors maps onto a 2D froth. However, unlike usual soap froth where time – driven coarsening is irreversible (drainage and drying), coarsening of the suprafroth is driven by easily manageable parameters – temperature and magnetic field. Similarities and differences between suprafroth and conventional froths will be discussed.
1. R. Prozorov, R. W. Giannetta, A. A. Polyanskii and G. K. Perkins, "Topological hysteresis in the intermediate state of type-I superconductors",  Phys. Rev.B 72, 212508 (2005).
2. R. Prozorov, "Equilibrium topology of the intermediate state in type-I superconductors of different shapes",  Phys. Rev. Lett. 98, 257001 (2007).

We study photon localization in a gas of cold atoms, using  a Dicke Hamiltonian that accounts for photon mediated atomic dipolar  interactions. The photon escape rates are obtained from a new class of  random matrices. A scaling behavior is observed for photons escape rates as a function of disorder and system size. Photon localization  is described using statistical properties of random networks which  display a "small world" cross-over. Those results  are compared to the Anderson photon localization transition.

The remarkable control on the parameters reached in cold atoms loaded in optical lattices setups has led to the employ of out of equilibrium processes to increase the ability to probe the properties of these systems. We study the response of one dimensional 1D ultracold atoms in optical lattice to external time-dependent perturbations like the periodic modulation of the lattice height. We find that for bosons, sharp energy absorption peaks are not unique to the Mott insulating phase at commensurate filling but also exist for superfluids at incommensurate filling. For strong interactions, the peak structure provides an experimental measure of the interaction strength. For fermions, the spectrum of the induced double occupancy allows to determine the pairing gap in a superfluid state and the interaction energy in a Mott-insulating state.

One of the most important current goals of condensed matter physics research involves elucidating the competition between diverse quantum phases - such as between antiferromagnetic or charge-density wave (CDW) and superconductor (SC) phases - in strongly correlated materials. By studying the low temperature inelastic light scattering spectra of correlated materials while pressure- and/or magnetic-field-tuning their phase behavior, we have been able to investigate the evolution of the low frequency excitation spectra through various quantum phase transitions in correlated materials. In this talk, I'll focus on the remarkable competition among novel phases observed as a function of both pressure and Cu-intercalation in 1T-TiSe2, which is a semimetal or small-gap semiconductor that develops a commensurate CDW with a 2a×2a×2c superlattice structure at temperatures below a second-order phase transition at TCDW ~ 200K. Our low temperature pressure-dependent studies of 1T-TiSe2 revealed that lattice compression leads to quantum-mechanical (T ~ 0) melting of the CDW phase through a novel incommensurate phase that may have hexatic order. More recently, Morosan et al. [1] discovered that lattice expansion of 1T-TiSe2 via copper intercalation (i.e., CuxTiSe2) leads to the suppression of the CDW transition temperature, and the emergence near x=0.04 of a SC phase having a maximum Tc of 4.15 K at x=0.08. Among other interesting results, we find evidence for x-dependent ‘quantum’ (T~0) soft-mode behavior in CuxTiSe2 that is consistent with a quantum critical point hidden in the superconductor phase of CuxTiSe2, suggesting a possible connection between quantum criticality and superconductivity. [1]. E. Morosan et al., Nature Phys. 2, 544 (2006).

We review the contradictory experimental results obtained by the MIT and Rice groups, lead respectively by Wolfgang Ketterle and Randy Hulet, on resonantly-interacting polarized Fermi gases.  In particular, the former group appears to see a quantum phase transition between a paired superfluid state and a polarized normal state, whereas the latter group does not see such a transition. We discuss the progress that has been made in the last year to resolve this discrepancy and present the open problems in the field.

In the world of spintronics the revolution of the strongly correlated systems dubbed as diluted magnetic semiconductors has seen a resurgence over the past five years in attempting to achieve ever higher Curie temperatures. This has been fueled in part from our clearer understanding of the paradigm of these materials, (Ga,Mn)As, which is at present one of the best understood ferromagnetic systems in its most metallic form. Through a diversity of theoretical tools and in constant comparison with experiments, the understanding of what it takes to achieve a room temperature DMS has gained a strong foothold and new attempts to find the ultimate combination which optimizes the diverse parameters which determines the Curie temperature is underway. I will review part of the current understanding of (Ga,Mn)As gained through phenomenological effective Hamiltonian models and other more microscopic calculations and what it makes us believe its understanding is quite good, and what new materials are being explored to reach a room temperature ferromagnetic semiconductor.

Quantum dots offer unique possibilities for the controlled study of many-body correlations. This talk presents theoretical predictions for a double-quantum-dot device consisting of one dot in its Kondo regime and a larger, effectively noninteracting dot, both coupled in parallel to a pair of external leads. This system maps onto a single-impurity Anderson model with a structured (nonconstant) density of states. By tuning a plunger gate voltage on the noninteracting dot, different features of this effective density of states can be brought to the Fermi energy. In one regime, band filtering through the noninteracting dot splits the Kondo resonance, even though the singlet ground state remains robust. The system can also be continuously tuned to create a power-law vanishing of the effective density of states and to access a pair of quantum phase transitions separating Kondo-screened many-body ground states from a non-Kondo phase. These zero-temperature transitions have characteristic signatures in the zero-bias conductance at temperatures T > 0. The prospects for experimental detection of the transitions are enhanced by the fact that the conductance features increase in prominence with increasing T up to quite high temperatures. If time permits, the talk will end with a discussion of the effect on the aforementioned results of having weak-but-nonvanishing Coulomb interactions in the larger dot.