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Thursday, January 22nd 2009

1:25 pm:

Solid hydrogen is a molecular quantum solid with unusual predicted properties. At very high pressures or densities it is predicted to become a monatomic metal and a possible high Tc superconductor. Atomic metallic hydrogen has never been experimentally observed, while theoretical predictions of the critical pressure for the transition are scattered over a large range. A recently predicted peak in the temperature versus pressure melt line has been considered as a precursor for metallization. We have used an innovative technique to extend the melt line to high temperature and pressures and observed a peak. In this talk I shall review some of the interesting properties of solid hydrogen, and discuss our recent results on its melt line.

Thursday, February 5th 2009

1:25 pm:

We develop an exact non-perturbative framework to compute the nonequilibrium steady state properties

of quantum impurities connected to leads subject to source-drain voltage. We show that in the open system limit the

non-equilibrium physics is captured by open system eigenstates de¯ned with boundary conditions set by the leads.

The eigenstates are current carrying and entropy producing, with the dissipation inherent in the limit.

We construct these eigenstates by means of a recently introduced Scattering (or Open) Bethe Ansatz approach, a

generalization to nonequilibrium of the Thermodynamic (or Closed) Bethe Ansatz. We compute the I(V) curve of the

Interacting Resonance Level and observe a Fermi Edge Singularity out of equilibrium as the impurity level approaches

resonance. We then apply the approach to the quantum dot (the nonequilibrium Anderson Impurity model) and

observe the formation of the Kondo peak as the gate voltage is decreased and the peak destruction as the source-drain

voltage is increased.

Thursday, February 12th 2009

1:25 pm:

Electronic properties in strongly correlated systems often reveal strikingly enhanced features compared with those in conventional materials. Examples are the high transition temperature in cuprate superconductors and the colossal magnetoresistance in manganites. The layered triangular lattice cobaltates Na_x CoO_2 exhibit several interesting electronic phases such as superconductivity, charge ordering and high thermopower, as the Na content is varied. In this talk I will describe our experimental progress in the high Na doping range characterized by even steeper enhancement of thermopower, which makes this material a strong candidate for low temperature thermoelectric applications. Tim permitting, I will present recent data showing dramatic changes in transport upon very small variations in Na content, and discuss these findings in relation to a recent discovery of periodic Na ordering.

Thursday, February 19th 2009

1:25 pm:

Thursday, February 26th 2009

1:25 pm:

A variety of condensed matter and cold atomic systems involving bosons on lattices exhibit the fascinating phenomenon of coexistent quantum states of matter. The presence of inhomogeneities in these systems can cause some regions to display superfluidity and others to display insulating behavior. This talk will discuss the basic physics underlying such coexistence, means of probing superfluid regions trapped between insulating regions, the collective properties of these trapped regions and the possibility of Josephson physics in these systems.

Thursday, March 5th 2009

1:25 pm:

I will describe the measurements of asymmetric conductance and the current shot noise through a carbon nanotube quantum dot with one ferromagnetic and one normal-metal lead. The observed asymmetry is spin-dependent, and stems from the interplay between the spin accumulation and the Coulomb blockade on the quantum dot. The results imply that the current is spin-polarized for one direction of the bias, and that the degree of spin polarization is fully and precisely tunable using the gate and bias voltages. As the operation of this spin diode does not require magnetic fields or optics, it could be used as a building block for electrically controlled spintronic devices.

Thursday, March 12th 2009

1:25 pm:

Thursday, March 19th 2009

1:25 pm:

Tuesday, March 24th 2009

12:20 pm:

Thursday, April 2nd 2009

1:25 pm:

Thursday, April 9th 2009

1:25 pm:

Thursday, April 16th 2009

1:25 pm:

Eu-rich EuO, of recent interest for its possible use in spintronics applications, exhibits properties similar to those of well-studied manganite systems like La2/3Ca1/3MnO3, including an insulator to metal transition associated with the onset of ferromagnetism and a large negative magnetoresistance response. Our work focuses on exploring comparisons between the EuO system and the manganites, using the binary EuO compound as a simplified laboratory with less structural complexity than the manganites. In this talk, I will describe our experimental work in the growth of Eu-rich EuO thin films and how the transport and magnetotransport properties of the films depend on the growth conditions, as well as discussing how future work can inform our understanding of the theoretical Kondo-lattice model that has been developed to describe the EuO system.

Thursday, April 30th 2009

1:25 pm:

The recent discovery of methods to isolate graphene has opened an arena for new physics and applications stemming from charge carriers governed by quantum-relativistic dynamics. Because of the 2-d nature of graphene the relativistic properties of its charge carriers are easily obscured by environmental disturbances such as potential fluctuations induced by insulating substrates. I will describe scanning tunneling spectroscopy[1,3] and transport[2,4] experiments on suspended graphene decoupled from substrate fluctuations. Our findings include direct observation of Landau levels, measurement of the Fermi velocity, and evidence for electron-phonon and electron-electron interactions. In addition we find that, in contrast to non-suspended samples, Quantum Hall plateaus associated with valley-splitting interactions appear in suspended graphene at very low fields.

1. G. Li , E.Y. Andrei - Nature Physics,3, 623 (2007)

2. X. Du, G. Li, A. Barker, E. Y. Andrei, Nature Nanotecnology 3, 491 (2008)

3. G. Li, A. Luican, E. Y. Andrei arXiv:0803.4016

4. X. Du, I. Skachko, E.Y. Andrei, PRB 77,184507 (2008)

Thursday, May 14th 2009

1:25 pm:

Wednesday, July 1st 2009

1:30 pm:

Abstract: Semiconductor quantum dots have a number of applications, and quantum coherent manipulation of the charge or spin of electrons is one really cool one. This talk will start with an introduction into quantum dots and the physics of both the Coulomb blockade and of quantum information, give a smattering of literature results on quantum coherent manipulation of electrons in Si and GaAs dots, and then finish with some of our recent data on electrostatically-gated Si devices including honeycomb stability diagrams.

Thursday, August 20th 2009

1:25 pm:

Quantum computing based on many different types of qubits has become an important field of research. A quantum computer with its entangled qubits should provide a much more efficient technique for specific tasks such as finding prime factors of large numbers and searching a non-structured data base. We are investigating the use of Josephson junctions in a SQUID configuration for these qubits. For success, the time for logic gate operations must be shorter than the coherence times. In this talk reviews of our experiments to make better junctions and to study and eliminate the sources of decoherence will be presented. Investigations of coherence times via Rabi oscillations and the Ramsey effect will be described and the reason for using a SQUID phase qubit will be discussed. A significant improvement in coherence times during this year has been obtained.

Thursday, September 17th 2009

1:25 pm:

I will present a systematic study of the microwave-induced oscillations in the magnetoresistance of a 2D electron gas for mixed disorder including both short-range and long-range components. I will discuss four distinct contributions to the photoconductivity tensor and show that the photoresponse depends crucially on the relative weight of the short-range component of disorder. Depending on the properties of disorder, the theory allows us to identify the temperature range within which the photoresponse is dominated by one of the mechanisms.

Thursday, September 24th 2009

1:25 pm:

The problem of inhibiting viral DNA ejection from bacteriophages by multivalent counterions, especially Mg^{+2}
counterions, is studied. Experimentally, it is known that MgSO_4
salt has a strong and non-monotonic effect on the amount of DNA ejected. There exists an optimal concentration at which the least DNA is ejected from the virus. At lower or higher concentrations, more DNA is ejected from the capsid. We propose that this phenomenon is the result of DNA overcharging by Mg^{+2}
multivalent counterions. As Mg^{+2}
concentration increases from zero, DNA net charge changes from negative to positive. The optimal inhibition corresponds to the Mg^{+2}
concentration where DNA is neutral. At lower/higher concentrations, DNA genome is charged. It prefers to be in solution to lower its electrostatic self-energy, which consequently leads to an increase in DNA ejection. Our theory fits experimental data well. The strength of DNA -
DNA short range attraction, mediated by Mg^{+2}
, is found to be -
0.003 *k*_{B}*T*-
per nucleotide base. Results from expanded ensemble Monte-Carlo simulation of hexagonal DNA bundles are discussed and are shown to be in good agreement with theoretical results.

Thursday, October 1st 2009

1:25 pm:

Metamaterials are artificially engineered structures that have properties, such as negative refractive index, n, nonexistent in natural materials. The recent development of metamaterials with negative n confirms that structures can be fabricated and interpreted as having a negative permittivity, ε, and a negative permeability, μ, simultaneously. Since the original microwave experiments for the demonstration of negative index behavior in split ring resonators (SRRs) and wire structures, new designs have been introduced, such as fishnet, that have pushed the existence of the negative refraction at optical wavelengths [1, 2]. Most of the experiments with the fishnet structure measure transmission, T, and reflection, R, and use the retrieval procedure [3,4] to obtain the effective parameters, ε, μ, and n. Although, stacking of three [5], four [6], five [7] functional layers and recently fabricated [8] ten-functional layer fishnets (21 layers of silver and MgF2) have been realized, they do not constitute a bulk metamaterial. Even the thickest fabricated [8] fishnet structure only has a total thickness, 830 nm, half of the wavelength (λ=1700 nm). However, it is very important to study how the optical properties change as one increases the number of layers. How many layers are needed to achieve convergence of the optical properties? How do optical properties behave as one change the distance between two neighboring fishnets? If the distance is small, we have a strong coupling case. The convergence of optical properties is slow, and more importantly, it does not convergence to the isolated fishnet case. What is the mechanism for negative n in the strong coupling limit? Here, we report a detailed study of the weakly and strongly coupled fishnets to understand the origin of negative n, as well the mechanism of low losses (that is, high figure of merit (FOM)) for the strongly coupled fishnets. We also study the convergence of the retrieval parameter (ε, μ, and n) as the number of unit cells (layers) increases. For the weakly coupled structures, the convergence results for n and FOM are close to the single unit cell. As expected, for the strongly

coupled structures, hybridization is observed and the retrieval results for n and FOM are completely different from the single unit cell. We demonstrate that the high value of FOM for the strongly coupled structure is due to periodicity

effects.

Thursday, October 8th 2009

1:25 pm:

Thursday, October 15th 2009

1:25 pm:

We use simple coexistence rules for the phase diagram of the iron arsenides to deduce the pairing state in these new superconductors. Recent experimental evidence in the iron arsenite superconductor Ba(FeCo)2As2 demonstrates that antiferromagnetic long range order coexists with bulk superconductivity. We show that static antiferromagnetism can be used to probe the Cooper pair wave function in this coexistence region. Magnetism leads to a Josephson like coupling between Cooper pairs in different Fermi surface sheets. The relative phase of the gap function then determines whether superconductivity and antiferromagnetism are merely competing or are mutually exclusive. While conventional superconductivity yields a phase diagram with a first order transition, terminating at a bicritical poing, a tetracritical point is only possible for unconventional pairing states, where the order parameter has different sign on different pieces of the Fermi surface. We demonstrate that the observed phase coexistence is not compatible with conventionalsuperconductivity and find strong evidence for superconductivity where the order parameter on different Fermi surface sheets is out of phase. The robustness of our conclusion is caused by the fact that the system is in the vicinity of an SO(6)-symetric fixed poing.

Thursday, October 22nd 2009

1:25 pm:

The superconductor-insulator transition in the presence of strong compensation was recently realized in La doped YBCO. Compensation of acceptors by donors makes it possible to change independently the concentration of holes n and the total concentration of charged impurities N. We propose a theory of the superconductor-insulator phase diagram in the (N, n) plane, which exhibits interesting new features in the case of strong coupling superconductivity, where Cooper pairs are compact, non-overlapping bosons. For compact Cooper pairs the transition occurs at a significantly higher density than in the case of spatially overlapping pairs. We establish the superconductor insulator phase diagram by studying how the potential of randomly positioned charged impurities is screened by by strongly bound Cooper pairs. The pairs are either delocalized or localized in the resulting self-consistent potential.

Thursday, October 29th 2009

1:25 pm:

Complex oxides such as perovskites have emerged as one of the most important platforms for the discovery of new phenomena in condensed matter physics. Colossal magnetoresistance in the maganites, and high temperature superconductivity in the cuprates are classic examples. The extraordinarily diverse physical phenomena displayed by these materials is due in large part to the strong competition between the various active degrees of freedom, which leads to a subtle energy balance between the multitude of available ground states. It is perhaps unsurprising that under these conditions, particularly in randomly doped systems, nanoscopic electronic inhomogeneity is ubiquitous. “Magneto-electronic phase separation”, where multiple electronic and magnetic phases coexist spatially, even in the absence of chemical segregation, has been observed in a wide range of materials and is widely believed to be due to electronically-driven phase separation. In this talk I will elaborate on the features of the doped perovskite cobaltites (e.g. La1-xSrxCoO3) which make them model systems for the study of this nanoscopic phase separation. We have used single crystals of these materials to study the phenomenology, consequences, and origins, of the magnetically phase-separated state, mostly by neutron scattering, transport, and heat capacity. Our primary conclusions are that (i) the spontaneous magnetic nanostructuring has many interesting consequences including the existence of “GMR-type” effects in a bulk solid, and (ii) the magnetic phase separation is driven purely by the local doping fluctuations that are inevitable at these nanoscopic length scales. In essence the nanoscale inhomogeneity is doping fluctuation-driven rather than electronically-driven, challenging, at least in these materials, the commonly accepted electronic phase separation scenario

Thursday, November 5th 2009

1:25 pm:

In certain very thin regimes of ferromagnetic materials, it is energetically favorable for vortices to condense. I will discuss how such vortices move when driven by the Landau-Lifschitz-Gilbert equations. I will show how such a result can be proven by understanding some intrinsic quantities and their conservation laws.

Thursday, November 12th 2009

1:25 pm:

Magnetic correlations might cause the superconductivity in the cuprates, and they are generally believed to be antiferromagnetic and to arise from the underlying copper-oxygen planes. Using neutron scattering on the model compound HgBa2CuO4+δ, we recently discovered the existence of a prominent magnetic excitation with unusual characteristics: (i) the excitation appears to involve active degrees of freedom on both planar and apical oxygen; (ii) it is present throughout the entire Brillouin zone; (iii) it exhibits a weak doping dependence and dispersion, and (iv) a maximum energy of 56 meV at the antiferromagnetic point, where it meets the magnetic resonance, a well-known spin-one excitation that appears in the superconducting state; (v) furthermore, unlike the resonance, the new exciation maintains its integrity in the normal state up to the pseudogap temperature (T*), and thus appears to be associated with the novel magnetic order recently identified in the pseudogap phase. I will also discuss our recent finding of a universal relationship between the magnetic resonance energy (Er) and the superconducting pairing gap (&Delta) that is valid for the three different classes of unconventional superconductors, ranging from being close to the Mott-insulating limit to being on the border of itinerant magnetism.

Thursday, November 19th 2009

1:25 pm:

The discovery of a high-mobility two-dimensional electron gas at the interface between a polar and non-polar insulating oxide has motivated transport experiments aiming at eliciting various quantum effects. At room temperature, an electric field-tunable hysteretic metal-insulator transition was discovered. At low temperatures (below 1 K), interfacial superconductivity and magnetism were reported. Here, I describe low-temperature magnetotransport experiments in a nanowire formed at the interface between LaAlO3 and SrTiO3. Distinct plateaus are observed and associated with quantized magnetoresistance at integer and fractional Landau level filling factors ν=2,3,...,9, and the fractional filling factors ν=7/3 and 11/5. The quasi-one-dimensional nature of the conducting channel, combined with the large electric field-tunable dielectric permittivity of SrTiO3, is believed to contribute to the stability of the integer and fractional quantum Hall states.

Thursday, November 26th 2009

1:25 pm:

Thursday, December 3rd 2009

1:25 pm:

Thursday, December 10th 2009

1:25 pm:

In disordered insulators, the interplay of the disorder and the long-range electron-electron interaction gives rise to a correlated regime at low temperature, the Coulomb glass or electron glass. The single-particle density of states exhibits a soft "Coulomb" gap, the shape of which is still debated and is reflected in the hopping conductivity. It was also predicted long ago that disordered insulators exhibit a "glassy" dynamics, which has now received ample experimental confirmation. In recent mean-field theoretical work, it was proposed that the glassy phenomenology reflects a transition to an equilibrium glass phase, similar to the much-debated Almeida-Thouless transition in spin glass. I will discuss the results of large-scale Monte Carlo simulations of the Coulomb glass addressing these questions. Our main results are a strong evidence against the existence of an equilibrium glass phase in 3D, and a confirmation of the Efros-Shklovskii prediction of a parabolic shape of the density of states at the Coulomb gap. I will also discuss results for the 3D Random Field Ising model that highlight the role of the interaction range, and for the 2D Coulomb glass.

Thursday, December 17th 2009

1:25 pm:

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