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Thursday, January 24th 2008

1:25 pm:

Thursday, January 31st 2008

1:25 pm:

Magnetic resonance imaging (MRI) is a powerful technique that typically

operates on the scale of millimeters to micrometers. Conventional MRI is

based on the manipulation of nuclear spins with radio-frequency fields and

the subsequent detection of spins with induction-based techniques. An

alternative approach, magnetic resonance force microscopy (MRFM), uses force

detection to overcome the sensitivity limitations of conventional MRI.

Recently we demonstrated that two-dimensional imaging of nuclear spins can

be extended to a spatial resolution better than 100 nm using MRFM [1]. We

have since made even greater strides in the process of imaging nanoscale

biological samples, achieving a three-dimensional resolution better than 10

nm. These results both demonstrate the feasibility of MRI on the nanoscale

and the applicability of MRFM to image biologically relevant samples.

If we are to further improve this resolution in order to eventually image on

the scale of single nuclear spins, the force sensitivity of the measurement

must be improved by roughly 2 orders of magnitude. In this talk I will

discuss some work done in pursuit of this goal including the application of

force detection techniques to do nanoscale MRI [1], the cooling of

mechanical oscillator modes to millikelvin temperatures using feedback [2],

and the design of nanoelectronic and nanomechanical devices to improve spin

detection sensitivity [3]. I will also discuss the manipulation of

statistically polarized spin ensembles [4] and our recent experiments using

MRFM to image single tobacco mosaic virus (TMV) particles.

[1] H. J. Mamin, M. Poggio, C. L. Degen, and D. Rugar, Nature Nanotech. 2,

301 (2007).

[2] M. Poggio, C. L. Degen, H. J. Mamin, and D. Rugar, Phys. Rev. Lett. 99,

017201 (2007).

[3] M. Poggio, C. L. Degen, C. T. Rettner, H. J. Mamin, and D. Rugar, Appl.

Phys. Lett. 90, 263111 (2007).

[4] C. L. Degen, M. Poggio, H. J. Mamin, and D. Rugar, Phys. Rev. Lett. 99,

250601 (2007).

Thursday, February 14th 2008

1:25 pm:

Abstract The advent of quantum mechanics last century was a tremendous step forward in our understanding of the natural world. The new understanding ushered in a wave of new technologies based on quantum mechanical principles, such as lasers, transistors, and magnetic resonance imaging devices. Researchers today are trying to develop novel technologies that use single quantum states as fundamental building blocks rather than statistical ensembles. In addition to the difficulty of manipulating quantum sized objects, an overarching problem in working with single quantum states is the short time scale over which their stored information is randomized due to interactions with the environment, a phenomena known as decoherence. In the first part of my talk I will present my experimental measurements of the decoherence times of electron states in mesoscopic quantum dots. My work aims to resolve some of the mysterious discrepancies seen in earlier measurements in this system. In the second part of my talk I will present my work of studying and manipulating of the spatial properties of the electron wave functions using the scanning tunneling microscope. This work shows the possibilities for creating novel quantum structures that can lend insight into quantum theories and might prove useful in future device applications.

Thursday, February 21st 2008

1:25 pm:

Electron spins in semiconductors are prime candidates for quantum information storage and processing because they exhibit relatively long coherence times. To realize this goal, electronic and optical properties of heterostructures are designed to efficiently generate, transport, manipulate, and detect electron spins1. A powerful tool for designing spin interactions in semiconductors involves magnetic doping, since magnetic ion and band electron spins may be strongly coupled. These exchange effects are often thousands of times larger than spin-orbit and hyperfine interactions in solely non-magnetic structures. Using nonequilibrium growth by molecular beam epitaxy, it is possible to realize metastable phases of magnetic ions alloyed with optoelectronic semiconductors. For example, Mn-doped GaAs remarkably combines semiconductivity with ferromagnetism, exhibiting a magnetic transition temperature (Tc) that is electrically tunable due to the carrier-mediated nature of the ferromagnetism. Although Tc has improved over the last ten years, defects inherent to nonequilibrium growth remain, limiting magnetic and optoelectronic quality. By exploring the phase diagram of GaMnAs over a broad range of magnetic doping, we have developed methods to largely remove these defects.

In the ferromagnetic regime, we utilize a combinatorial growth method to systematically reduce nonstoichiometric defects and synthesize material at the Mn-doping limits of ferromagnetism2. At the dilute doping limit, we find growth conditions for producing GaMnAs with optoelectronic quality and spin lifetimes on par with non-magnetic heterostructures3. Using this system we optically address and detect the spins of extremely small numbers of Mn ions. Surprisingly, we identify a new method for manipulating magnetic ions without magnetic fields4. A dynamic exchange mechanism polarizes a few hundred Mn ions within GaAs quantum wells, a magnetization that can be optically oriented. We observe Mn ion spin coherence times exceeding 10 ns, suggesting that they may be useful systems for information processing. These studies have led to experiments currently probing single magnetic ions.

1. D. D. Awschalom and M. E. Flatté, Nature Physics 3, 153 (2007).

2. R. C. Myers et al., Phys. Rev. B 74, 155203 (2006); S. Mack, R. C. Myers et al., (in preparation).

3. R. C. Myers et al., Phys. Rev. Lett. 95, 017204 (2005).

4. R. C. Myers et al., Nature Materials (in press, 2008).

Thursday, February 28th 2008

1:25 pm:

Understanding the interactions of electron spins and photons in semiconductors may enable the development of new devices with enhanced functionality and performance, such as spin-based devices that combine logic and storage and fast optical switches for information processing. In the first half of this talk, we describe time- and spatially-resolved optical measurements of electron spin polarization in semiconductor heterostructures. We investigate the effects of strain and quantum confinement on electron spins in semiconductors and explore methods of electrically generating spin polarization in non-magnetic materials through spin-orbit coupling and the spin Hall effect. In the second half of this talk, we focus on recent developments in photonics for applications such as on-chip light sources and optical devices. The high index contrast of silicon waveguides offers small optical modal areas so that high optical power densities can be achieved for observing nonlinear effects in compact devices, and silicon photonic devices, such as Raman lasers, amplifiers, modulators and wavelength converters, have recently been demonstrated. Finally, we discuss planar photonic crystal cavities for realizing strong localization of light with small mode volumes for achieving substantial reductions in lasing threshold.

Thursday, March 13th 2008

1:25 pm:

Thursday, March 20th 2008

1:25 pm:

Thursday, March 27th 2008

1:25 pm:

I will describe two sets of experiments using ultra cold atoms in optical lattices. In one, we accurately measure the condensate fraction in a 2D Bose-Hubbard system as a function of the lattice depth and compare to theory. In the other, we demonstrate a method of adiabatically dressing spin-dependent lattices, a technique that can in principle generate non-trivial sub-wavelength structure. Non-adiabatic spin-flip loss is one of the limiting processes in this lattice, and we measure spin-flip loss under a variety of conditions.

Wednesday, April 2nd 2008

12:20 pm:

A complete formulation of quantum information theory must simultaneously take into account at least three important concepts or principles, namely: (a) quantum entanglement, (b) quantum coherence versus decoherence (i.e., in the presence of dissipation), and (c) the quantum-classical limit (or quantum-classical interface). We discuss how the mixed coherent states that we have introduced can provide a valuable tool in implementing these ideas.

The (over-)complete set of pure Glauber (harmonic oscillator) coherent states is a very useful basis for many purposes. The P-representation then provides a diagonal expansion of an arbitrary operator in ? in terms of projection operators onto the coherent states. We discuss the extension of these results to the analogous mixed states introduced by us, which describe comparable displaced harmonic oscillator systems in thermodynamic equilibrium at nonzero temperatures T. These thermal coherent states provide a very useful "random" (or "thermal" or "noisy") basis in H, since the corresponding statistical density operator provides a probability measure on H. We prove a resolution of the identity for these states and use it to generalise the usual pure (T = 0) coherent state formalism to the mixed (T ne 0) case. An important and unexpected result is that our temperature-dependent P- and Q-representations are the analytic continuations to negative temperatures of each other.

The above formalism for thermal coherent states is further generalised to a broader class of so-called negative-binomial mixed states, in terms of which a resolution of the identity operator in H is again constructed. The thermal coherent states are just the limiting case k = ½ of this larger class, characterised by a new parameter k. The negative-binomial distribution is itself intimately related to the discrete series of SU(1,1) representations. Indeed, the Hilbert space of a two-mode harmonic oscillator can be expressed as a direct sum of an infinite number of subspaces each of which is related to a particular representation in the discrete series of SU(1,1) representations. We have shown previously how such states can be useful for processes involving general fluctuation-dissipation phenomena. We consider the pure SU(1,1) coherent states discussed by Perelomov in the two-mode harmonic oscillator Hilbert space, and show that the partial trace with respect to one of the two modes leads, rather miraculously, to our negative-binomial mixed states. This observation is then used to show how the formalism of thermofield dynamics may be generalised to a correspondingly much broader negative-binomial field dynamics, which we expect to have many uses for open systems.

Thursday, April 3rd 2008

1:25 pm:

To date, the transport properties of spin-split one-dimensional (1D) subbands in coupled quantum wire systems with wave function hybridization (mode coupling) are widely unkown. We explore the magnetic field control and the influence of spin effects on mode coupling in short quantum wires with large 1D subband spacings (>10meV). Recently, we demonstrated the direct high-resolution energy spectroscopy of mode coupling [1] in such systems which allow a single-mode control. The access to spin-split transport channels and mode-coupled spin subbands may be of interest for spin-dependent quantum transport devices.

Here, we investigate 1D subbands, in particular for coupled modes, by tuning electric and magnetic fields. We observe, magnetooscillations of the energy splittings in tunnel-coupled 1DES with nearly identical 1D confinement in symmetric configuration in longitudinal magnetic fields [2], which are reproduced by an analytic expression [3]. In perpendicular magnetic fields we observe spin-split 1D subbands and strong indications of mode-coupling between spin split subbands in various asymmetric quantum wire configurations.

[1] S.F. Fischer et al., Nature Physics 2, 91 (2006).

[2] S.F. Fischer et al., Phys. Rev. B 74, 115324 (2006).

[3] L.G. Mourokh, Appl. Phys. Lett., 90, 132108 (2007).

Thursday, April 10th 2008

1:25 pm:

Thursday, April 17th 2008

1:25 pm:

I will discuss a recent experiment where the flux threading a dc SQUID was measured to change with a 1/T temperature dependence, characteristic of the paramagnetic response of electron spins. With the magnitude of the effect being proportional to the density of trapped vortices, this data is compatible with the thermal polarization of surface spins in the trapped fields of superconducting vortices. In the absence of trapped flux, the surface spins also show evidence of 2-D spin-glass freezing at a temperature below about 100 mK. These results suggest that surface spins are present on metals with a density of about 1/nm^2; however, the microscopic origin of the spin states are currently unknown. It also suggests an explanation for the "universal" 1/f flux noise observed in SQUIDs that has not been understood for 20 years.

This experiment was performed at the University of Wisconsin in

collaboration with Robert McDermott.

Thursday, April 24th 2008

1:25 pm:

Thursday, May 1st 2008

1:25 pm:

Thursday, May 8th 2008

1:25 pm:

Thursday, May 15th 2008

1:25 pm:

Thursday, May 22nd 2008

1:25 pm:

Thursday, September 11th 2008

1:25 pm:

Abstract: In 1989, the smectic-C_alpha* phase was first identified between the smectic-A and smectic-C* phases by thermal studies on one antiferroelectric liquid crystal compound. Because of its incommensurate nano-scale helical pitch (INHP) structure, the molecular arrangements in the smectic-C_alpha* phase was first identified about 10 years later by our resonant x-ray diffraction studies. Subsequently, among various liquid crystals, different temperature dependences of INHP's along with the magnitude of INHP yield many interesting physical phenomena. A simple free-energy expansion with five expansion terms proposed by our research group provides us with important guides through this research project.

Thursday, September 18th 2008

1:25 pm:

Growing interfaces appear commonly in the natural world. Some of them are as important as the technologically motivated thin film deposition, medically relevant as bacterial colonies development, and physically interesting as fluid flow in porous media. Despite their diverse origin, all these phenomena have been studied within the common formalism of scaling analysis. This methodology has been successfully applied to a broad range of situations, but however, there are a number of restrictions that limit scaling analysis as we know it. One of them is the assumption that the interface must be planar and the substrate size constant for all times. Despite the usefulness of this representation in some cases, there are many growth profiles that can not be described according to it. Physical settings such as adatom and vacancy islands on crystal surfaces present interfaces that violate the hypothesis of the Euclidean representation. Biological systems are also characterized by an approximate spherical symmetry: bacterial colonies, plant calli, and tumors develop rough surfaces which are not describable from a planar reference frame. A particularly important example of spherical growth is tumor development. Standard scaling analysis applied to this case suggested a behavior compatible with molecular beam epitaxy surface dynamics, and it served as the basis of a therapeutic strategy targeted in stopping tumor growth. However, the results obtained in these works enter in contradiction with others present in the medical literature, raising the problem of understanding the dynamics of curved interfaces to one important open question in scaling analysis. Due to the disparity of experimental and numerical results obtained so far, it seems necessary to build a theory based on analytical progress in order to clarify curved interface dynamics. Such a theoretical framework started with the introduction of the radial Kardar-Parisi-Zhang and Mullins-Herring equations. The analysis of these equations revealed that the properties of spherical interfaces are totally different from their planar counterparts. We will comment the analysis of growth phenomena on curved interfaces performed so far, pointing out the profound differences in morphological and dynamical properties that they present. Surfaces of a two or higher dimensional nature become flat in the long time limit, due to noise irrelevance in such cases. However, some residual roughness is developed in the first stages of growth, and it can be the source of spurious results in numerical simulations initialized with small cluster sizes. The one dimensional situation is different, as fluctuations, despite being marginal and only of a logarithmic amplitude, are not irrelevant. In this case, flat interfaces correspond to models showing a super-ballistic diffusivity, which is able to erase the effect of early fluctuations asymptotically in time. Sub-ballistic diffusivity plays actually no role in the large scale dynamics and this type of equations reduce to the radial random deposition model, characterized by an uncorrelated interface of marginal logarithmic amplitude, in the long time limit. The critical situation, in which correlations propagate ballistically, has again a logarithmic width interface, due to the marginal intensity of the noise term, but the correlation function is substantially different. The analysis of this function for long times and short spatial scales reveals the appearance of the new local dynamical exponent, which is nonuniversal. These facts have a strong impact on the analysis of growing interfaces, and imply the necessity of reconsidering some of the experimental results obtained so far.

Thursday, September 25th 2008

1:25 pm:

Thursday, October 2nd 2008

1:25 pm:

Being a truly two-dimensional crystal, graphene is rippled, due to both intrinsic thermal instability and stresses created by a substrate. These ripples result in a pseudomagnetic gauge field which effects essentially on the dynamics of charge carriers. In particular, they can lead to appearance of topologically protected zero-energy pseudo-Landau levels and to charge inhomogeneity. Some recent experimental data seem to support a hypothesis that the pseudomagnetic field in corrugated graphene is the main scattering factor limiting the electron mobility.

Thursday, October 9th 2008

1:25 pm:

Recently a new single layer material -- graphene has been discovered. This is a material where Dirac points in the fermionic spectrum lead to a very unusual properties, such as transport properties and impurity states. We will argue that these properties are not unique to graphene and in fact are a direct consequence of Dirac spectrum in fermionic excitation sector. Strong similarities with d-wave superconductors, superfluid 3He, p-wave superconductors and with other materials exhibiting Dirac electronic spectrum are suggestive and offer a unifying perspective. We will argue that this discovery signifies the emergence of a new class of materials, that can be called ***Dirac Materials***, the class where nontrivial properties emerge as a direct consequence of Dirac spectrum of excitations. We will address the local electronic properties

of graphene such as impurity states, electronic inhomogeneity and discuss broad similarities with dirac physics seen in other

materials. We will also discuss the inelastic electron tunneling

spectroscopy (IETS) and role of phonons in Dirac Materials.

Thursday, October 16th 2008

1:25 pm:

We study the role of electron-electron Coulomb interaction in clean graphene and determine the d.c. conductivity, the shear viscosity, the diamagnetic response and electronic compressibility of this unique material. Key for an understanding of the Coulomb interaction is the fact that clean, undoped graphene is quantum critical with a marginally irrelevant 'fine structure constant'. Using standard crossover arguments combined with a quantum kinetic theory, we derive scaling laws, valid near this quantum critical point, that dictate the nontrivial magnetic and charge response of interacting graphene. The most dramatic consequence of this analysis is the anomalous collision-dominated, hydrodynamic transport: the d.c. conductivity of clean graphene is shown to diverge for decreasing temperature (as the square of the logarithm of T), making this material a quantum critical metal. The shear viscosity vanishes as the temperature goes to zero, making it an almost perfect fluid.

Thursday, October 23rd 2008

1:25 pm:

Thursday, October 30th 2008

1:25 pm:

Thursday, November 6th 2008

1:25 pm:

The recent discovery of the extraordinary optical transmission (EOT) effect in periodic nanohole arrays in a metal film shows promise for building a new generation of biosensors utilizing this unique phenomenon in nano-structured metallic substrates. These sub-wavelength holes act as miniaturized optical antenna to capture incident photons and convert them to energetic surface plasmon waves, which can be used as a sensitive probe for biomolecular interactions on the surface or as new electrodes for enhancing the power conversion efficiency of photovoltaic cells. In this presentation, the application of the EOT effect for real-time label-free SPR biosensing will be demonstrated, followed by surface enhanced Raman scattering (SERS) applications and plasmonic photovoltaic cells.

Thursday, November 13th 2008

1:25 pm:

Thursday, November 20th 2008

1:25 pm:

I will discuss the resistance of a long non-uniform quantum wire, in which the strength of the electron-electron interactions varies smoothly at large length scales. Contrary to the expectations based on the Luttinger-liquid description of the interacting electron systems, I will show that these inhomogeneities lead to a finite resistivity of the wire. This effect is a consequence of the lack of momentum conservation in an inhomogeneous wire as well as of the non-trivial processes of electron equilibration in one dimension. Estimating the rate of change of momentum associated with non-momentum-conserving scattering processes, I will derive the expression for the resistivity of the wire in the regime of weakly interacting electrons and find a contribution linear in temperature for a broad range of temperatures below the Fermi energy.

Thursday, November 27th 2008

1:25 pm:

Thursday, December 4th 2008

1:25 pm:

What does a fractional quantum Hall liquid and Kitaev’s proposals for topological quan¬tum computation have in common? It turns out that they are physical systems that exhibit degenerate ground states with properties seemingly different than ordinary (Landau-type) phases of matter, such as ferromagnets. For example, those (topologically quantum ordered) states cannot be characterized by (local) order parameters such as magnetization. How does one characterize this new order? I will present a unifying framework which will allow us to engineer physical systems displaying topological quantum order. What are the physical properties of these new orders? How robust are they to temperature eﬀects? What are they useful for? Topologically quantum ordered states of matter seem to be ideal physical systems to store and manipulate quantum information since they are believed to be robust against decoherence with an environment, and thus appropriate for building a quantum computer and quantum memories. I will discuss the role of temperature in the protection of quantum information. Have we ﬁnally found a new technological application for quantum Hall liquids?

Thursday, December 11th 2008

1:25 pm:

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