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Thursday, January 18th 2007

1:25 pm:

Although magnetism is fundamentally a quantum mechanical effect, it has clear macroscopic consequences. For example, the interactions occurring on the length scale of several Angstroms lead to materials with properties that allow one to attach messages to a refrigerator door, or produce 100 GB hard drives. The technological implications and diverse behavior have made magnetic materials a consistent venue of study for condensed matter physics. I will be presenting research on an organometallic antiferromagnet, piperazinium hexachlorodicuprate (PHCC). The frustrated interactions in this material lead to a magnetic ground state which only has very short range correlations such that PHCC is best described as a quantum spin liquid. The excitations associated with the quantum spin liquid ground state can easily be modified by the application of external magnetic fields. I will discuss the magnetic field versus temperature phase diagram of this system as determined by pulsed field magnetic susceptibility measurements, specific heat measurements and both elastic and inelastic neutron scattering experiments. In addition to two magnetic field-driven quantum critical points which can be described as a Bose-Einstein condensation of excitations, there also exists a reentrant phase transition between the disordered ground state and a long range ordered phase. Along with the ability to dramatically alter the excitations in PHCC using an applied magnetic field, I will also discuss a peculiar point in the zero-field excitation spectrum where higher energy excitations cause the lower energy excitations to abruptly decay.

Thursday, January 25th 2007

1:25 pm:

Transition metal dichalcogenides have long been known and explored. Due to their reduced dimensionality, such compounds sometimes display charge density wave (CDW) transitions or, upon doping with magnetic ions, often reveal dramatic changes of their physical properties. I will discuss the effects of transition metal intercalation on the properties of two layered

chalcogenide materials, TiSe2 and TaS2. Although TiSe2 is one of the first known CDW-bearing materials, the nature of its CDW transition remains controversial. Recently the interest in TiSe2 has been renewed by our discovery of the new superconducting state (SC) that emerges upon Cu doping. Thus CuxTiSe2 provides the first example of a system in which controlled chemical doping can be used to study the competition between the CDW and SC. I will also discuss experiments on FexTaS2 aimed at studying the sharp switching of the magnetization that we recently observed in this compound for x = 1/4. For this particular Fe content, FexTaS2 orders ferromagnetically below 160 K and displays very sharp hysteresis loops in the ordered state for H||c. The corresponding magnetoresistance is negative, and qualitatively reproduces the features observed in the M(H) data, by showing a sharp drop around the critical field for moment reversal.

Thursday, February 1st 2007

1:25 pm:

I will talk about magnetic insulators in which the symmetry of the spin interactions leads to strong fluctuations and qualitatively new ground states. Of particular interest are low-dimensional magnets in which the magnetic moments interact mainly in one or two directions, or frustrated magnets in which long-range magnetic order is impeded because of competing interactions. The proximity of such systems to quantum critical points can lead to emergent quantum coherence over macroscopic length scales and strong cross-coupling between magnetic order and the nuclear lattice. Case in point is a new class of multiferroic materials in which the magnetic and ferroelectric order parameters are directly coupled, and the application of a magnetic field can suppress or switch the electric polarization. Our neutron measurements reveal that ferroelectricity is induced by magnetic order and emerges only if the magnetic structure creates a polar axis. The spin dynamics and the field-temperature phase diagram of the ordered phases provide evidence that competing ground states are essential but not sufficient for ferroelectricity. The origin of the magneto-electric coupling is not understood at present, but it may arise from anisotropic exchange couplings such as Dzyahloshinskii-Moriya interactions.

Thursday, February 8th 2007

1:25 pm:

Ultrafast optical spectroscopy has long been used with great success to generate and probe non-equilibrium electronic excitations with femtosecond time resolution. The spatial resolution in these techniques, however, is limited to micron scales and structural dynamics can only be inferred indirectly. I will report direct measurements of structural dynamics with atomic scale spatial resolution by using ultrafast electron diffraction (UED). In UED, a femtosecond laser pulse is split into two, the first part is used to induce structural change and the second part

is used to generate ultrafast high energy electron packets via photoelectric effect. Recording the diffraction pattern of these electron packets at different times after the photo-excitation of the sample provides a movie of the laser induced structural change with sub-picosecond temporal and sub-Angstrom spatial resolution. I will discuss recent experiments where we used UED to observe lattice dynamics in cuprate superconductors in response to photo-excitation of the charge carriers. Above certain threshold laser intensity, we observe direct conversion between two structures with different c axis lattice constants indicating a non-equilibrium structural phase transition.

Thursday, February 15th 2007

1:25 pm:

Recent angle-resolved photoemission spectroscopy (ARPES) results suggest a ubiquity and prominence of both quasiparticle and higher-energy correlated bands in cuprate systems. While ARPES is of great benefit to the understanding the electronic structure of correlated electron systems, the technique is limited in the sense that it probes only the occupied portion of the spectral function. In analogy to Raman spectroscopy, resonant inelastic X-ray scattering (RIXS) is a two-particle spectroscopic technique involving both particle removal and particle addition states, and is thereby capable of probing the unoccupied states of an interacting many-electron system. In contrast to Raman spectroscopy, RIXS provides the additional capability of exploring momentum dependence. I will give an introduction to the burgeoning technique of RIXS, using cuprates as an example, and present new results on the energy structure of certain model systems. I will also present results that suggest how scattering geometry may act as a sensitive probe of excitation symmetry, and suggest directions for the technique in future experiments.

Thursday, February 22nd 2007

1:25 pm:

Thursday, March 1st 2007

1:25 pm:

Thursday, March 15th 2007

1:25 pm:

Thursday, March 22nd 2007

1:25 pm:

Thursday, March 29th 2007

1:25 pm:

Thursday, April 5th 2007

1:25 pm:

Thursday, April 12th 2007

1:25 pm:

Strontium ruthenate Sr2RuO4 is an odd-parity superconductor, shown to exhibit p-wave pairing. Some of the possible p-wave states can further break time-reversal symmetry (TRS). However, unlike other known TRS breaking effects in solids, this case does not imply a "magnetic effect" since any such signal will be screened by the Meissner effect (except for surfaces, domain walls and imperfections). In the past two years, we have pursued a direct test of the broken time-reversal-symmetry in the bulk of Sr2RuO4 and other superconductors with potentially broken time reversal symmetry state, without relying on imperfections and defects by measuring the Polar Kerr effect (PKE). PKE is sensitive to TRS breaking since it measures the existence of an antisymmetric contribution to the real and imaginary parts of the frequency-dependent dielectric tensor, and such a contribution is necessarily absent if TRS is not broken in the material. In order to measure the very small static TRS-breaking effect in superconductors, we have developed a new PKE technique based on a fiber Sagnac interferometer with a zero-area Sagnac loop. Results on Sr2RuO4 as well as other unconventional superconductors will be shown.

1. Jing Xia, Maeno Yoshiteru, Peter T. Beyersdorf, M. M. Fejer,

Aharon Kapitulnik, Phys. Rev. Lett. 97 (2006),167002.

Thursday, April 19th 2007

1:25 pm:

Thursday, April 26th 2007

1:25 pm:

Thursday, May 3rd 2007

1:25 pm:

Thursday, May 10th 2007

1:25 pm:

Thursday, September 6th 2007

4:00 pm:

Thursday, September 13th 2007

1:25 pm:

Fluorescence fluctuation spectroscopy (FFS) extracts information about the transport and assembly of proteins from the signal fluctuations of fluorescently labeled biomolecules diffusing through a small optical observation volume. FFS was mainly developed for studies in aqueous solution, but has found an increasing number of applications measuring diffusion and protein association directly inside living cells. However, unlike in solution, proteins in cells are not only undergoing random diffusion, but also exist in immobile form when bound to large structures, such as the chromosome. The presence of immobile fluorophores in the observation volume presents new challenges for FFS that need to be addressed to allow a quantitative interpretation of cellular experiments. Two extreme cases of immobilization exist. The first is very sparse immobilization requiring the ability to detect single immobile molecules. The second is uniform distribution of immobilized molecules creating background fluorescence which biases the interpretation of fluctuation data. I present new approaches that extend FFS in order to quantify systems which contain these two types of immobilization. The first approach, based on periodic scanning of the laser beam, is also capable of characterizing hydrodynamic flow, which will be experimentally demonstrated. In addition, I will show that immobilization, at both extremes, can be quantitatively differentiated from mobile molecules by the modified FFS techniques. Finally, I will demonstrate the robustness of the methods by applying them to measure proteins in living cells.

Monday, September 17th 2007

7:45 pm:

Fueled by the ever increasing data density in magnetic storage and the need for a better understanding of the physical properties of magnetic nanostructures there exists a strong demand for high-resolution magnetically sensitive microscopy techniques. The technique with the highest available resolution is spin-polarized scanning tunneling microscopy (SP-STM) which combines the atomic-resolution capability of conventional STMs with spin-sensitivity. Beyond the investigation of ferromagnetic surfaces, thin films, and epitaxial nanostructures with unforeseen precision, it also allows the achievement of a long standing dream, i.e. the real space imaging of atomic spins in antiferromagnetic surfaces. The lecture addresses a wide variety of phenomena in surface magnetism which in most cases could not be imaged directly before the advent of SP-STM. After starting with a brief introduction to basics of the contrast mechanism, recent major achievements will be presented, like the direct obser-vation of the atomic spin structure of domain walls in antiferromagnets and the visualization of thermally driven switching events in superparamagnetic particles consisting of a few hundreds atoms only. To conclude the lecture, recently observed complex spin structures containing 15 or more atoms will be presented.

Thursday, September 20th 2007

1:25 pm:

Thursday, September 27th 2007

1:25 pm:

In this talk I will discuss how physics concepts can be useful for understanding issues arising in the field of computational complexity, the study of the amount of computational resources needed to solve different problems. In particular, I will show how a renormalization group construction similar to those used to provide insight into phase transitions in physical systems can provide new insight into how to distinguish computational problems that can and cannot be solved efficiently.

Thursday, October 4th 2007

1:25 pm:

Thursday, October 11th 2007

1:25 pm:

Hubbard U-corrected LDA or GGA have proven very effective in describing several systems characterized by strongly localized electronic states for which standard (approximate) DFT functionals fail. In this talk I will present our scheme to evaluate the effective electronic interaction of the "+U" functional in a fully consistent way using linear-response theory. The successful application of this approach to the study of several transition-metal compounds will be discussed presenting the improvements in the description of their structural, electronic, chemical and electro-chemical properties. Examples will include minerals in the Earth's interior [1], cathode candidate materials for lithium-ion batteries [2] and catalytic reactions on molecules [3,4]. [1] M. Cococcioni and S. de Gironcoli, PRB (2005). [2] F. Zhou, M. Cococcioni, A. C. Marianetti, D. Morgan and G. Ceder, PRB (2004).

Thursday, October 18th 2007

1:25 pm:

An assembly of inelastically colliding hard spheres - the granular gas - is a simple model of °ow of granular materials. It provides a fascinating example of a complex system far from equilibrium. Granular gases exhibit a spontaneous clustering instability: development of clusters of particles and voids between them. A similar instability appears in gases and plasmas that cool by their own radiation. Nonlinear theory of the clustering instability has been a major unresolved problem of granular dynamics. We simplified this problem by considering a channel geometry, so that the coarse -grained °ow is one-dimensional. We found that, in the framework of idealized hydrodynamic equations, the gas exhibits a ¯nite-time density blowup. The \attempted" singularities are usually arrested only when the close packing density of hard spheres is reached. Molecular dynamics simulations support the hydrodynamic predictions until close to the time of attempted density blowup. In a certain limit of the instability, the dynamics is describable by a zero-viscosity Burgers equation which makes this system a distant cousin of a structure forming expanding Universe.

Thursday, October 25th 2007

1:25 pm:

We poorly understand the microscopic properties of amorphous solids, such as transport, force propagation or even the nature of their mechanical stability. These questions are related to the presence of soft modes in their vibrational spectrum. We explain the nature of these modes in repulsive, short-range systems. This enables to derive a microscopic criterion of rigidity which extends a previous result of Maxwell. This implies that rigidity is not a local property, but is characterized by a length which depends on the packing geometry, and which can be large and even diverge, e.g. near the random close packing. We argue that this description applies to granular media, silica and colloidal glasses. We propose a description of the glass transition in hard sphere systems in terms of these soft modes.

This leads to several predictions, in particular a non-trivial power law scaling characterizing the packing geometry in the glass phase, that we check numerically.

Thursday, November 1st 2007

1:25 pm:

The coupled cluster method (CCM) has become one of the most pervasive and most powerful of all ab initio formulations of quantum many-body theory. It has yielded numerical results which are among the most accurate available for a wide range of both finite and extended physical systems defined on a spatial continuum. This widespread success has spurred recent applications to similar quantum-mechanical systems defined on an extended regular spatial lattice. In particular, we have shown how the systematic inclusion of multispin correlations for a wide variety of quantum spin-lattice problems can be very efficiently implemented with the CCM. The method is not restricted to bipartite lattices, to spin-half systems, or to non-frustrated systems, and can thus deal with problems where, for example, the quantum Monte Carlo (QMC) techniques would be faced with the infamous "minus-sign problem." In this talk I briefly review the CCM itself and then discuss an illustrative example from among many applications made to quantum spin-lattice systems, for a model with two types of interactions which exhibits competition between magnetic order and dimerization. As in all other cases the CCM may readily be implemented to high (LSUBm) orders using computer-algebraic techniques. Values for ground- and excited-state properties are obtained which are fully competitive with those from other state-of-the-art methods, including the much more computationally intensive QMC techniques, in the special cases where the latter can be applied. The raw LSUBm results are themselves generally excellent. They converge rapidly and can be extrapolated in the truncation index, m. The CCM can also provide valuable information on the quantum phase transitions, quantum order, and quantum criticality, as we show in our example. For such strongly correlated models of magnetism with competing interactions in two (or higher) dimensions, the CCM probably now represents the most powerful general method available, as I hope to show.

Thursday, November 8th 2007

1:25 pm:

A number of recent experiments have achieved paired superfluidity of trapped fermionic atomic gases. Such pairing, occurring between two atomic hyperfine-state species (forming a pseudo-spin-1/2 system), is possible due to the strong attractive interactions provided by a magnetic field tuned Feshbach resonance (FR). At equal populations, the superfluidity of resonantly interacting Fermi gases undergoes the well-studied crossover between Bardeen-Cooper-Schrieffer (BCS) pairing and Bose-Einstein condensation (BEC) as a function of FR detuning (or

interaction strength). I will discuss recent work aimed at

understanding the case of unequal populations (i.e., imposed

spin polarization), an easily controllable experimental knob that is predicted to interrupt the continuous equal-population BCS-BEC

crossover, yielding a variety of distinct phenomena including regions of singlet paired superfluid, unpaired polarized normal Fermi liquid, polarized Fulde-Ferrell-Larkin-Ovchinnikov superfluid, polarized magnetic superfluid, and phase-separated mixtures of these uniform states.

Thursday, November 15th 2007

1:25 pm:

Large Hermitian eigenvalue problems appear frequently in electronic structure theory. The underlying quantum many-body problem is often referred to as "incomputable" since it involves Hilbert spaces whose dimensionality grows exponentially with the number of particles. This complexity is, in fact, key to the idea of quantum computer, since a moderately large quantum many-body system possesses an amount of information greater than any conventional computer can handle. Yet, exact diagonalization of finite clusters using conventional computers is a valuable tool that helps us understand many physical phenomena.

In this presentation I will review the Lanczos recursion and related Krylov subspace methods that allow us to solve ultra-large Hermitian eigenvalue problems. I will also discuss a complementary classical problem that leads to Hamiltonian (non-Hermitian) eigenvalue equation, and an intricate relation that exists between its eigenvalues and the eigenvalue differences, or excitation energies, of the quantum system.

I will present a generalization of Rayleigh-Ritz minimum principle and of Lanczos recursion to this class of problems and discuss implications to time-dependent (TD) quantum ansatz methods, such as TD density functional theory (TD-DFT), TD Hartree Fock (TDHF), etc.

Thursday, November 29th 2007

1:25 pm:

The essence of the emerging field of spin-electronics or “spintronics” is to utilize electron spin to create new microelectronic devices or increase the functionality of existing ones. One thing common to all such devices is a source of spin-polarized electrical current, most commonly a ferromagnetic material. It has therefore become a priority to develop materials that have high spin polarization at the Fermi level, P. The ideal case would be a 100 % polarized material, known as a “half-metallic ferromagnet”. Although numerous materials have been predicted to be half-metallic, an extensive experimental search has yielded few viable candidates. In this talk I will present a different approach. The idea is to “engineer” a highly polarized ferromagnet by alloy control over the electronic band structure, rather than simply searching for one based on the predictions of band structure calculations. Using a model system, Co1-xFexS2, we have demonstrated the feasibility of this concept and have achieved tunable spin polarization in the range –56 % < P < +85 %. The system has allowed us to probe the electronic, magnetic and thermodynamic properties as a controlled function of the spin polarization. I will end with a summary of our efforts to improve on this P value by increases in material quality (in stoichiometric single crystals) as well as growth of epitaxial thin films for inclusion in heterostructured devices such as GaAs-based spin injection structures.

Thursday, December 6th 2007

1:25 pm:

Organic materials have received considerable attention in the context of future sources of inexpensive, renewable energy. Photovoltaic devices constructed from organic thin films are amenable to high throughput processing techniques using lightweight and flexible substrates, potentially enabling the low-cost fabrication of large area devices. While qualitatively similar to their inorganic semiconductor counterparts, the van der Waals bonding of organic solids leads to the formation of excitonic bound states upon optical excitation. As such, architectures for photoconversion must dissociate the exciton into its constituent charge carriers. This additional requirement for efficient operation is an important consideration in the design of candidate active materials and device architectures. This talk will examine recent progress in the development of organic photovoltaic cells, as well as the potential for further improvement in device performance and an improved understanding of device physics.

Thursday, December 13th 2007

1:25 pm:

Thursday, December 20th 2007

1:25 pm:

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