Condensed Matter/Biophysics
Seminars
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Condensed
Matter/Biophysics Seminars are via Zoom until further notiice Contact: Yasu Takano or Dmitrii Maslov |
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August 31 |
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September 7 (No seminar – Labor Day) |
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September 14 |
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September 21 |
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September 28 |
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Samaresh Guchhait (Howard Univ.) |
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Turning material defects into a solution for quantum computation |
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Superconducting quantum circuits, composed of superconductor and amorphous dielectric, is a promising way to realize quantum computation. At very low-temperatures (<1 K), all amorphous solids contain low-energy excitations which is attributed to the motion of group of atoms between two potential wells separated by a tunnel barrier. These are called two-level system (TLS) defects. TLSs are known to cause performance limiting decoherence in qubits and noise in superconducting photon detectors. We have achieved a broadband TLS inversion in the GHz regime, as monitored by a quantum-regime superconducting resonator. The TLS inversion is caused and controlled by application of a strong pump field and a swept electric bias field. The resonator responds to the TLS inversion through changes in its resonance frequency and internal loss. For the lowest bias rates, inverted TLSs are confined in a narrow band near the pump frequency due to relaxation processes. In this regime the frequency and internal loss tangent are nearly unchanged. With increasing bias rates, a large fraction of TLSs are inverted and distributed in energy below the resonance frequency. This causes large change of resonance frequency and loss up to some maximum value. For even larger bias rates, the probability of TLS inversion is lower when interacting with the pump field due a larger probability of Landau-Zener tunneling. As a result, the shifts in resonance frequency and loss lower from their respective maximum values. Numerical simulations of (complex) permittivity change agree with experimental results. This is a promising way to improve relaxation and coherence times of superconducting qubit. |
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Mark Meisel |
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October 5 |
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Jiun-Haw Chu (Univ. of Washington) |
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Nematic quantum criticality in iron-based superconductors |
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Electronic nematicity refers to a self-organized electronic state that breaks rotational symmetry without long range translational order. In the iron-based superconductors, the nematic transition temperature can be continuously tuned by doping and pressure, which extrapolates to zero as the superconducting Tc is tuned to optimal. In this talk, I will present two striking phenomena associated with this putative nematic quantum critical point. First, we discovered that the superconductivity is extremely sensitive to the anisotropic strain near optimal doping – the Tc is reduced by five-fold under less than a percent anisotropic strain [1]. Second, using the combination of precision detwinning and elastoresistivity measurements, we found that the ratio between resistivity anisotropy and structural orthorhombicity within the nematic ordered phase enhances by fourfold as the doping approaches optimal, suggesting that the conduction electrons become increasingly sensitive to the lattice even as the nematic order is suppressed [2]. [1] P. Malinowski, Q. Jiang, J. Sanchez, Z.-Y. Liu, J. Mutch, P. Went, J. Liu, P. Ryan, J.-W. Kim, J.-H. Chu, “Drastic suppression of superconducting Tc by anisotropic strain near a nematic quantum critical point”, arXiv:1911.03390 (Nature Physics, 2020). [2] J.J. Sanchez, P. Malinowski, J. Mutch, J. Liu, J.-W. Kim, P. J. Ryan, J.-H. Chu, “The spontaneous elastoresistivity coefficient within the nematic ordered phase of iron pnictides”, arXiv:2006.09444v2. |
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James Hamlin |
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October 12 |
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Jeremy Levy (Univ. of Pittsburgh) |
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Correlated nanoelectronics |
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The study of strongly correlated electronic systems and the development of quantum transport in nanoelectronic devices have followed distinct, mostly non-overlapping paths. Electronic correlations of complex materials lead to emergent properties such as superconductivity, magnetism, and Mott insulator phases. Nanoelectronics generally starts with far simpler materials (e.g., carbon-based or semiconductors) and derives functionality from doping and spatial confinement to two or fewer spatial dimensions. In the last decade, these two fields have begun to overlap. The development of new growth techniques for complex oxides have enabled new families of heterostructures which can be electrostatically gated between insulating, ferromagnetic, conducting and superconducting phases. In my own research, we use a scanning probe to “write” and “erase” conducting nanostructures at the LaAlO3/SrTiO3 interface. The process is similar to that of an Etch-a-Sketch toy, but with a precision of two nanometers. A wide variety of nanoscale devices have already been demonstrated, including nanowires, nanoscale photodetectors, THz emitters and detectors, tunnel junctions, diodes, field-effect transistors, single-electron transistors, superconducting nanostructures and ballistic electron waveguides. These building blocks may form the basis for novel technologies, including a platform for complex-oxide-based quantum computation and quantum simulation. |
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Dominique Laroche |
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October 19 |
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Miguel Gonzalez (Aramco Americas) |
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From macro to micro (to nano): mechanical resonators at all scales for rheology sensing in oilfield fluids |
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Real-time monitoring of oilfield fluid rheology is of crucial importance for oil recovery. During various operations such as drilling, production, stimulation, and intervention, the rheology of the fluids being produced or injected into the well need to be monitored to ensure safe operation, or to optimize the process being carried out. However, reliably measuring their physical properties can still pose a great challenge when the fluids are part of a multiphase flow or are complex inhomogeneous fluids whose properties can vary continuously with parameters such as temperature, pressure, flow rates, and composition. While specialized commercial instruments that make comparable measurements can be found in other industries, fewer systems tailored to the oil and gas industry exist or are too costly and complex to deploy in every well. In this presentation, I will discuss two examples of viscosity/density sensing systems based on mechanically resonating devices customized to different measurement scenarios in the field. In the first example, I demonstrate a ruggedized tuning fork device for in-line measurements of viscosity and density of non-Newtonian fluids. This instrument is a robust permanent tool for surface installation with applications in quality assessment of drilling mud or other wellbore fluids such as fracturing fluids. I will also discuss techniques currently being explored to extract rheological information from the vibrational damping measured on the resonator. In the second example, I will discuss miniaturized resonators fabricated using micro/nano-machining techniques. These devices can be integrated into small-scale measurement platforms, either for downhole deployment or for lab-on-a-chip platforms useful in portable field viscometry systems. |
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Yoonseok Lee |
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October 26 |
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November 2 |
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November 9 |
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Denis Bandurin (MIT) |
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Interaction-dominated transport in graphene: old mysteries and new regimes |
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Electron–electron (e–e) collisions can impact transport in a variety of surprising and sometimes counterintuitive ways. Despite long-time interest, experiments on the subject proved challenging because of the presence of momentum-relaxing scattering sources (e.g. phonons or impurities). Only recently, sufficiently clean electron systems in which transport dominated by momentum-conserving e–e collisions have become available, enabling the study of electron transport governed by interactions. This talk will begin by discussing the behaviour in monolayer graphene which by now is relatively well understood. I will show that at elevated temperatures, the behaviour of graphene’s electron fluid resembles that of classical liquids and gases with high viscosity [1,2]. I will discuss approaches that can be used to probe the transport governed by e–e interactions and talk about electron viscometry [3-4]. A very different behaviour is found for transport in twisted bilayer graphene (TBG). I will show that, unlike the case of monolayer graphene, e–e collisions in large-angle TBG can lead to the relaxation of electrical current and result in a quadratic temperature dependence of its resistivity. This surprising behaviour cannot be accounted for by existing scenarios (e.g. umklapp or multi-band scattering) and calls for alternative explanations. [1] D. A. Bandurin et al., Science 351, 1055 (2016). [2] D. A. Bandurin, A. Shytov et al., Nat. Comm. 9, 4533 (2018). [3] R. Krishna Kumar, D.A Bandurin et al., Nat. Phys. 13, 1182 (2017).[4] A.I. Berdyugin et al., Science 364, 6436, 162-165 (2019).. |
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Dmitrii Maslov |
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November 16 |
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Joon Sue Lee (Univ. of Tennessee) |
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Topological materials and their applications |
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Topology is a general concept that classifies objects based on their properties that remain under continuous deformation, which is quantified by topological invariants. A common example is the number of holes in geometrical objects, which distinguishes an orange (an object with no hole) from a donut (an object with a hole). In condensed matter physics, the geometrical object is replaced by the electron’s wavefunction. Topological classification of materials has extended our understanding of a wide variety of electronic, superconducting, and magnetic phases of matter, which can lead to new technologies. In this talk, I will discuss the properties of topological materials and their applications particularly in quantum computing and spintronics. Topological superconductors host Majorana bound states at their boundaries. In 1D topological superconductors, discrete states at zero energy (Majorana zero modes) appear at the ends of the 1D systems, which can be used to realize a topological quantum computer. Focus will be on Majorana zero modes in superconductor-semiconductor hybrid systems. In the later part, I will talk about applications in spintronics. Surface states of topological insulators are characterized by helical spin textures in momentum space. The inherent “spin-momentum locking” makes these materials promising for spintronic device applications. Experiments that reveal highly efficient spin-charge conversion will be presented. |
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Yoonseok Lee |
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November 23 |
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Chris Leighton (Univ. of Minnesota) |
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Electrolyte-gate-controlled magnetism |
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Recently, incorporation of electrolytes such as ionic liquids into field-effect transistors has been shown to enable electric double layer transistors (EDLTs), which can induce very large (1015 cm-2) charge carrier densities at surfaces. These densities corresponds to significant fractions of an electron or hole per unit cell in most materials, sufficient to electrically control electronic phase transitions. While this has stimulated great interest, challenges remain, including understanding mechanisms (electrostatic vs. electrochemical [1]), developing operando characterization methods, and assessing the full power and universality of the approach. Here, I review our work applying electrolyte gating using solid ion gels [1-6] to oxides and sulfides, focused on voltage control of magnetism. The latter is important, bearing potential for novel, low-power data storage and processing technologies. Our findings greatly clarify the issue of electrostatic vs. electrochemical mechanisms, showing that electrostatic gating vs. oxygen vacancy creation/annihilation can be understood based on bias polarity, and the enthalpy of formation and diffusivity of oxygen vacancies [1-6]. This understanding was developed via operando probes, such as synchrotron X-ray diffraction [3] and absorption/dichroism [6], as well as neutron reflectometry [3,5]. Electrical control of magnetism in La1-xSrxCoO3-d is then demonstrated in both electrochemical [3] and electrostatic [2,4,5] modes, reversibly modulating Curie temperatures over >200 K windows. Most recently, this has been advanced beyond electrical modulation of ferromagnets, or electrical induction of ferromagnetism from antiferro- or para-magnetic states, demonstrating reversible voltage-induced ferromagnetism in a diamagnet, using FeS2 (Fool’s Gold) as a model system [7]. [1] C. Leighton, Nat. Mater. 18, 13 (2019). [2] J. Walter, H. Wang, B. Luo and C. Leighton, ACS Nano 10, 7799 (2016). [3] J. Walter, G. Yu, B. Yu, A. Grutter, B. Kirby, J. Borchers, Z. Zhang, H. Zhou, T. Birol, M. Greven, and C. Leighton, Phys. Rev. Materials 1, 071403(R) (2017). [4] P.P. Orth, R.M. Fernandes, J. Walter, C. Leighton and B.I. Shklovskii, Phys. Rev. Lett. 118, 106801 (2017). [5] J. Walter, T. Charlton, H. Ambaye, M. Fitzsimmons, P.Orth, R. Fernandes, B. Shklovskii and C. Leighton, Phys. Rev. Materials 2, 111406(R) (2018). [6] B. Yu, G. Yu, J. Walter, V. Chaturvedi, J. Gotchnik, J. Freeland, C. Leighton and M. Greven, Appl. Phys. Lett. 116, 201905 (2020). [7] J. Walter, B. Voigt, E. Day-Roberts, T. Birol, R. Fernandes and C. Leighton, Sci. Adv. 6, eabb7721 (2020). |
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Dmitrii Maslov |
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November 30 |
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James Analytis (UC Berkeley) |
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Freezing of charge degrees of freedom across a critical point in CeCoIn5 |
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The presence of a quantum critical point separating two distinct zero-temperature phases is thought to underlie the strange metal state of many high-temperature superconductors. The nature of this quantum critical point, as well as a description of the resulting strange metal, are central open problems in condensed matter physics. In large part, controversy stems from the lack of a clear broken symmetry to characterize the critical phase transition, and this challenge is no clearer than in the example of the unconventional superconductor CeCoIn5. By direct Hall effect and Fermi surface measurements, in comparison to ab initio calculations, we observe a critical point that connects two Fermi surfaces with different volumes without a finite-temperature symmetry breaking phase transition. Rather, the transition involves an abrupt localization of one sector of the charge degrees of freedom. At low fields and temperatures, this transition causes a divergence in the Hall coefficient that is cut off on diluting either particles or holes. This remarkable behavior is unexpected in the conventional picture of metals, but does appear to be a non-trivial prediction of the theory of the fractionalized Fermi liquid. In this framework, the separation of spin and charge leads to a critical point connecting Fermi surfaces with different volumes. |
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Dmitrii Maslov |
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December 7 |
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Suchitra Sebastian (Univ. of Cambridge) |
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Yasu Takano |
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December 14 |
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Silke Buehler-Paschen (TU Wien) |
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Yasu Takano |
Condensed Matter/Biophysics
Seminars
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Condensed
Matter/Biophysics Seminars are via Zoom until further notice Contact: Yasu Takano or Dmitrii Maslov |
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January 11 |
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January 18 (No seminar – MLK Jr Day) |
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January 25 |
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February 1 |
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February 8 |
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February 15 |
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February 22 |
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March 1 |
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March 8 |
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March 15 |
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March 22 |
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March 29 |
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April 5 |
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April 12 |
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April 19 |
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