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Condensed Matter Seminar

Prof. John Ketterson, Northwestern University
Speaker: Prof. John Ketterson, Northwestern University Title: Ferromagnetic Resonance and Spin Wave Studies in Permalloy Nanostructures and YIG Films Abstract: In recent years there has been a revival of interest in the phenomena of Ferro-magnetic Resonance (FMR) driven in part by the development of e-beam lithography as a tool to make magnetic nano structures but also due to the discovery of the spin Hall effect. Demagnetization effects dominate the behavior of FMR at microwave frequencies and we will review methods used to calculate the response of thin films as well as arbitrarily shaped objects and apply the latter to various patterned permalloy arrays. We will conclude with a discussion of some experiments that employ the spin Hall effect to study FMR. Host: W. Saslow

4 Sep 2015, 4:00PM | MIST M102
Hosted By: W. Saslow

V. Pokrovsky
Speaker: V. Pokrovsky Title: Superfluidity of Magnons Abstract: The magnon Bose-Einstein condensation in Yttirum Iron Garnet films at room temperature was discovered by the Münster experimental group (S.O. Demokritov) in 2006. In the work [1] we have shown that the magnon interaction leads to spontaneous violation of reflection symmetry and to the phase trapping. We predicted that both the symmetry of the condensate and the trapped phase change at varying magnetic field in thinner films. Since the magnon condensate is coherent the natural question is whether the condensate is superfluid.[2]. Though the normal magnon density exceeds the condensate density in about 100 times, the velocity of the superfluid part is by 5-7 decimal orders larger than that of the normal part at the same field gradients. Thus, the spin current is dominated by the condensate, i.e. superfluid. A deeper obstacle is that the phase trapping is inconsistent with the free motion whose phase linearly depends on coordinate. The superfluidity can start only after submission of a finite (threshold) energy to the condensate by an external source. At energy close to threshold, the phase on long intervals of length remains close to the trapped values and changes by 2? on a comparatively short intervals (phase solitons). The superfluid velocity remains almost zero between solitons and acquires finite value inside solitons. Thus, the current and number of magnons are not conserved locally transferring the spin momentum to the lattice, but they are conserved globally. All these phenomena are due to the dipolar forces. Host: V. Pokrovsky

2 Oct 2015, 4:00PM | MIST M102
Hosted By: V. Pokrovsky

Nader Mirabolfathi, TAMU
Speaker: Nader Mirabolfathi, TAMU Title: Toward Single e-h pair Resolution Large-Mass Phonon-Mediated Ionization Detectors Abstract: Experiments seeking to detect rare event interactions such as dark matter or Coherent Elastic Neutrino Nucleus Scattering (CENNS) are striving for large mass detectors with very low detection threshold. After a brief review of the dark matter and CENNS searches and the interest in low threshold and large mass detectors, I’ll discuss possible detector alternatives using a thermal phonon low temperature readout in semiconductor substrates. I’ll present latest results from one of our detectors that has led to world best resolution of <7 eV and our path toward single electron resolution detectors. Host: J. Ross

9 Oct 2015, 4:00PM | MIST M102
Hosted By: J. Ross

Meigan Aronson, TAMU
Speaker: Meigan Aronson, TAMU Title: Quantum Critical Fluctuations in Metallic Ferromagnets Abstract: Host: A. Finkelstein

16 Oct 2015, 4:00PM | MIST M102
Hosted By: A. Finkelstein

Konstantin Tikhonov, TAMU
Speaker: Konstantin Tikhonov, TAMU Title: Numerical study of spectrum and eigenfunction statistics on disordered random regular graphs. Abstract: We study the problem of single particle Anderson Localization on the Random Regular Graph (RRG) via exact diagonalization. Interest to this problem has recently revived largely in connection with the Many-Body Localization (MBL). MBL can be thought of as localization in the Fock space of Slater determinants, which play the role of lattice sites in a disordered tight binding model. In contrast to a d-dimensional lattice, the structure of Fock space is hierarchical and thus resembles a RRG. We study this problem numerically and show that contrary to recent claims, if shows regular localization transition with peculiar finite-size effects which obscure the existence of a sharp transition up to very large graph sizes. Host: A. Finkelstein

23 Oct 2015, 4:00PM | MIST M102
Hosted By: A. Finkelstein

Arkady Shekhter, National high magnetic field laboratory
Speaker: Arkady Shekhter, National high magnetic field laboratory Title: Quantum oscillations near metallic quantum critical point. Abstract: Quantum criticality is a pervasive origin of new physics in metals. Tuning through the zero-temperature termination point of a line of classical phase transitions is thought to suppress the intrinsic energy scale, driving the system through a quantum critical point. The enhanced fluctuations in the vicinity of critical point in metallic high temperature superconductors are believed to be responsible for non-Fermi liquid transport behavior, such as linear-in-temperature resistivity over a broad temperature range.[1–3] Recent quantum oscillation measurements reveal a fast evolution of quasiparticle properties approaching critical doping in high temperature superconductors, in strong support of quantum critical origin of their phase diagram.[4, 5] We suggest that these quantum oscillation studies indicate doping evolution of quasiparticle scattering dynamics near the Fermi surface rather then divergence of quasiparticle mass. In particular, we observe that near quantum critical point the full temperature dependence of the amplitude of quantum oscillations, constrained by thermodynamic inequalities, cannot be accounted for with anomalous quasiparticle lifetime effects alone.


[1] J.A.N. Bruin, H. Sakai, R.S. Perry, & A.P. Mackenzie, Science 339, 804 (2013).
? [2] J.G. Analytis, H-H. Kuo, R.D. McDonald, M. Wartenbe, P.M.C. Rourke, N.E. Hussey & I.R. Fisher, Nature Physics 10, ?194 (2014).
? [3] R.A. Cooper, Y. Wang, B. Vignolle, O.J. Lipscombe, S.M. Hayden, Y. Tanabe, T. Adachi, Y. Koike, M. Nohara, H. Takagi, ?Cyril Proust, & N.E. Hussey, Science 323, 603 (2009). [4] B.J. Ramshaw, S.E. Sebastian, R.D. McDonald, J. Day, B. Tan, Z. Zhu, J.B. Betts, R. Liang, D.A. Bonn, W.N. Hardy, & ?N. Harrison, Science 348, 317 (2014).
? [5] P. Walmsley, C. Putzke, L. Malone, I. Guillamon, D. Vignolles, C. Proust, S. Badoux, A. I. Coldea, M. D. Watson, S. ?Kasahara, Y. Mizukami, T. Shibauchi, Y. Matsuda, & A. Carrington, Phys. Rev. Lett. 110, 257002 (2013). ? Host: A. Finkelstein

30 Oct 2015, 4:00PM | MIST M102
Hosted By: A. Finkelstein

S.V. Syzranov, University of Colorado, Boulder
Speaker: S.V. Syzranov, University of Colorado, Boulder Title: Unconventional localisation transition in high-dimensional semiconductors and Weyl semimetals Abstract: It is usually believed that increasing disorder strength in a d>2-dimensional system leads to the Anderson localisation transition with universal properties depending only on the space dimensionality. We demonstrate that systems with a power-law quasiparticle dispersion $xi_{f k}propto k^alpha$ in dimensions $d>2alpha$ exhibit another type of a disorder-driven quantum phase transition at the bottom of the band, that lies in a universality class distinct from the Anderson transition. In contrast to the conventional wisdom, it manifests itself in, e.g., the disorder-averaged density of states. For systems in symmetry classes that permit localisation, the striking signature of this transition is a non-analytic behaviour of the mobility edge that is pinned to the bottom of the band for subcritical disorder and grows for disorder exceeding a critical strength. Focussing on the conductivity and the density of states, we calculate the critical behaviour (exponents and scaling f unctions), using a renormalisation group, controlled by an $varepsilon=2alpha-d$ expansion. We also discuss how this novel transition can be observed in 3D Weyl semimetals and in recently realised 1D and 2D arrays of ultracold trapped ions with power-law interactions. Host: V. Pokrovsky

6 Nov 2015, 4:00PM | MIST M102
Hosted By: V. Pokrovsky

Sarbajit Banerjee, TAMU, Chemistry
Speaker: Sarbajit Banerjee, TAMU, Chemistry Title: The Electronic and Magnetic Phase Transitions of Ternary Vanadium Oxides: Building a new Sandbox for Modulating Electron Correlation Abstract: Oxides exhibit a remarkable 22 orders of magnitude variation in their electrical conductivity at room temperature. A most remarkable phenomenon manifested in a still rather sparsely populated set of compounds is a metal—insulator transition wherein the electrical conductivity of a single material can be abruptly switched (often at timescales as fast as femtoseconds) across several orders of magnitude and from insulating to metallic behavior in response to temperature, pressure, application of a voltage, and/or photo-excitation The abruptly discontinuous change in electrical conductivity arises from the transformation of localized valence electrons to itinerant electrons in the material, and is often accompanied by similarly abrupt changes in optical transmittance and magnetic susceptibility. The emergent complexity at the cusp of these transitions is underpinned primarily by electron—electron interactions and as such are greatly relevant to the design of new functionality.
In this talk, I will focus on our recent results on the influence of finite size and doping on the metal-insulator phase transitions of the binary vanadium oxide VO2. We have achieved substantial tunability of the critical transition temperature between -20 and 70°C through control of dimensionality, morphology, and dopant concentration in hydrothermally prepared single-crystalline VO2 nanostructures. The tunability of the phase diagram portends applications of these materials as dynamically switchable glazings for energy efficient windows (smart windows!). I will further discuss colossal metal—insulator switching recently discovered in MxV2O5 bronze phases. These structures provide a versatile platform for systematically tuning geometric and electronic structure and thus the extent of electron correlation. I will conclude by discussing the implications of electron correlation for the design of cathodes and photocatalysts. Host: J. Ross

13 Nov 2015, 4:00PM | MIST M102
Hosted By: J. Ross

Joseph P. Heremans, Ohio State University
Speaker: Joseph P. Heremans, Ohio State University Title: Magnon drag Nernst effect and thermal magnon transport in antiferromagnets. Abstract: The flux of heat carried by magnons in the presence of a temperature gradient corresponds to a flux of magnetization, which in turn can interact with conduction electrons. In the spin-Seebeck effect (SSE), the interaction takes place across an interface between a ferromagnet (typically YIG) and a normal metal (typically Pt), and is parametrized by the spin mixing conductance. The free electrons in the Pt are then spin-polarized over a spin diffusion length, and give rise to a transverse voltage via the inverse spin-Hall effect in the Pt. The loss mechanisms are primarily the inefficiency of the inverse spin Hall conversion of a magnon to a charge flux, and the limitations imposed by the magnon thermal length near the Pt/YIG interface. In 2011, Lucassen [1] suggested that magnon-electron drag is another form of advective thermal spin/charge transport akin to the SSE, but in a uniform material, eliminating the need for an interface. In the colloquium we provided proof of this in the thermopower, but not for the Nernst coefficient. Indeed, it is known that phonon-drag cannot per se generate a skew force and thus a Nernst effect [2], and therefore this is likely the case for magnon-drag as well. In contrast, here we present a new model based on ambipolar transport. Spin-up and spin-down electrons in Fe are considered as charge carriers with separate magnon-drag Seebeck coefficients. The difference between these partial Seebeck coefficients leads to a large magnon-drag Nernst coefficient even in the absence of a skew force. Here we will provide experimental data and a new tentative model for magnon-drag Nernst coefficients. We then explore thermal magnon transport in a uniaxial antiferromagnet KNiF3 that promises to become an experimental platform for thermally induced spin transport and possibly dynamically-induced spin superfluidity [3]. The existence of a large number of insulating and semiconducting antiferromagnets in which advective thermal spin/charge transport is possible broadens the range of possible materials for this research.

[1] Lucassen et al., Appl. Phys. Lett. 99, 262506 (2011); [2] Korenblit, L. Y., Sov. Phys. Semiconductors2, 1192 (1962) ; [3] Takei et al, Phys. Rev. B 90, 094408 (2014) Host: A. Finkelstein

20 Nov 2015, 4:00PM | MIST M102
Hosted By: A. Finkelstein

Alex Kamenev, the University of Minnesota
Speaker: Alex Kamenev, the University of Minnesota Title: Topological Anderson Insulators Abstract: I will discuss the role of disorder and localization in topological insulators. These systems may be described in terms of two-parameter scaling theory, similar to the one conjectured long ago for Integer Quantum Hall effect. Certain critical values of the topological number define quantum phase transition boundaries between distinct topological sectors. At such phase transitions topological Anderson insulators become strange metals, characterized by an ultra-slow Sinai diffusion. Possible experimental consequences of these results will be discussed. Host: V. Pokrovsky

4 Dec 2015, 4:00PM | MIST M102
Hosted By: V. Pokrovsky

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