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Friday, January 20th 2017

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Friday, January 27th 2017

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Friday, February 3rd 2017

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Friday, February 10th 2017

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Friday, February 17th 2017

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Friday, February 24th 2017

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Friday, March 3rd 2017

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Friday, March 10th 2017

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Friday, March 17th 2017

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Friday, March 24th 2017

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Friday, March 31st 2017

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Friday, April 7th 2017

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Friday, April 14th 2017

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Friday, April 21st 2017

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Friday, April 28th 2017

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Friday, May 5th 2017

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Friday, September 8th 2017

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Friday, September 15th 2017

12:20 pm:

Due to the nonlinear dielectric response within SrTiO3 (STO),

an accumulation layer created by positive charges at the surface of the STO sample (x = 0) has an electron density profile n(x) that slowly decays as 1=x^{12/7}. Previous works have shown that the slow decay of the tail leads to a divergence of kinetic coefficients when the scattering is primarily at the surface. Here we extend that work to the thermodynamic properties of the layer, and show that a similar divergence occurs in the magnetization and the specific heat when averaged over the accumulation layer. I will discuss several ways in

which the averaging may be truncated. The second half of the talk will focus on the capacitance as a function of the back gate voltage and explain how it can be used as an experimental tool for studying the tail of the distribution.

Friday, September 22nd 2017

12:20 pm:

NbSe_2 monolayers have been observed to superconduct in presence of relatively large (~35T) in-plane magnetic fields (as compared to the naïve Pauli limit estimate), an effect attributed to large out-of-plane spin orbit coupling (referred to in the literature as Ising SOC) in the niobium atom that pins electron spins perpendicular to the NbSe_2 plane. This lead some theorists to speculate about the effect of such magnetic fields on the nature of the superconducting gap function, which due to the SOC may be topologically non-trivial. In particular, it has been suggested that for fields above the Pauli limit, NbSe_2 becomes a nodal topological superconductor. In order to investigate this claim, I perform an RG analysis (without SOC) and solve the linearized gap equation using a simplified single band model that incorporates SOC. It turns out that while there exists a self-consistent solution to the gap equation in which an in-plane magnetic field induces a phase transition from a fully gapped phase to a nodal one, such a solution is neither favored by RG nor likely to occur in a real system due to in-plane (Rashba) SOC caused by ripples in the monolayer, which leads instead to a topologically trivial gap.

Friday, September 29th 2017

12:20 pm:

The spin-orbit torque (SOT) arising from materials with large spin-orbit coupling promises a path for ultra-low power and fast magnetic-based storage and computational devices. We investigated the SOT from magnetron-sputtered BixSe(1-x) thin films in BixSe(1-x)/CoFeB heterostructures by using dc planar Hall and spin-torque ferromagnetic resonance (ST-FMR) methods. Remarkably, the spin Hall angle (SHA) was determined to be as large as 18.62 ± 0.13 and 8.67 ± 1.08, using the dc planar Hall and ST-FMR methods, respectively. Moreover, switching of perpendicular CoFeB multilayers using SOT from the BixSe(1-x)has been observed at room temperature (RT) with critical switching current density 4.3 × 105 A/cm2, which is the lowest-ever critical switching current density reported in a bilayer system. The transmission electron micrographs (TEM) show that the sputtered BixSe(1-x) films have a polycrystalline structure, whose grain size decreases with the film thickness. Quantum transport simulations using realistic sp3 tight binding model suggests that the giant SOT in sputtered BixSe(1-x)is due to a quantum confinement effect, whose spin-accumulation enhances with reduced size and dimensionality. The demonstrated giant SHA, ease of growth of the films on a silicon substrate, and successful growth and switching of perpendicular CoFeB multilayers on BixSe(1-x) film provide an avenue for the use of the topological insulator (TI) BixSe(1-x) as a spin-current generator in SOT-based memory and logic devices.

Friday, October 6th 2017

12:20 pm:

Spin relaxation in metals is often discussed within the framework of Elliott-Yafet (EY) relaxation, where the spin relaxation and elastic scattering rates are related via a proportionality constant referred to as the EY parameter. Although this parameter is known for some scattering sources (e.g., phonons) in certain elemental metals, experimental knowledge and general understanding remains limited. Here, we significantly expand the experimental knowledge base by determining EY phonon and defect parameters in Al and Cu non-local spin valves with channel thicknesses from 400 to 20 nm. For Al we find a surprisingly broad range for the phonon parameter which contrasts with expectations of a constant value for a specified material. In particular, we find an unexpected size dependence that is discussed in terms of finite size effects and the Debye temperature.These results identify factors limiting metallic spin diffusion lengths and hence the performance of nanoscopic spin transport devices.

Friday, October 13th 2017

12:20 pm:

We have measured the 1/f noise in the spin glass state of CuMn and AgMn thin films. Our results are consistent with previous measurements that have demonstrated low-frequency resistance fluctuations abruptly increase in magnitude as a spin glass is cooled. It is believed that the noise originates from fluctuations in the orientation of the local magnetic moments (``spins'') within the spin glass. It was originally suggested that the abrupt increase in the noise magnitude corresponded to the glass temperature of the spin glass system. However, we have found that the noise increases at a somewhat lower temperature, indicative of a second transition. Some theoretical work has predicted transitions below the glass temperature, though none has been previously observed. We are working to understand the nature of this transition presently.

Friday, October 20th 2017

12:20 pm:

Ferromagnetic resonance (FMR) linewidth measurements are used to study the magnetization damping constant of Heusler compound Co(2)FeAl. The FMR linewidth is found to be highly anisotropic and exhibit a nonlinear dependence on FMR frequency, contrary to simple expectations . A magnon-magnon scattering theory is used to explain the linewidth behavior, which reveals a ultra-low intrinsic damping constant of less than 10^-3. Prospects for further exploration of intrinsic damping processes based on these results are discussed.

Friday, October 27th 2017

12:20 pm:

Superconductivity in SrTiO3(STO) has remained an open question for more than half a century. In this talk I will present experimental results showing superconductivity in Nd doped STO thin flims (50-250 nm), where Nd is expected to carry a net magnetic moment. All the samples we studied show an anisotropic resistance along the two in-pane orthogonal directions, where the normal state resistances along the two directions differ by more than several factors. Although the normal state resistances between the channels seem to be qualitatively similar, the resistive superconducting transition is drastically different. Along the channel with larger normal state resistance we observe a smooth transition from metallic to superconducting state with decreasing temperature (“normal channel”), but along the channel with smaller normal state resistance we observe an insulating state (resistance rising by up to an order of magnitude in a narrow temperature range) between the metallic and superconducting states. The onset of the insulating state is closely related to superconductivity and that its behavior is qualitatively different from known effects of granularity or superconducting fluctuations in an insulating matrix. For the “normal channel” the out of plane upper critical field as a function of temperature shows opposite curvature from what would be expected based on WHH theory and that the critical field doesn’t seem to saturate with decreasing temperature. I will present possible explanations for such anomalous behavior, including effects related to magnetic impurities and two-band superconductivity.

Friday, November 3rd 2017

12:20 pm:

The recent discovery of CDW order in hole- and electron-doped cuprates has attracted much attention, as there appears to be a close connection between superconductivity and charge-order, and possibly also the mysterious pseudogap phenomenon. A crucial question regarding the nature of the CDW phenomenon in the hole-doped cuprates is whether the observed short-range order is static or associated with dynamic spectral features. We studied the model high-Tc superconductor HgBa2CuO4+δ (Hg1201) via resonant inelastic X-ray scattering (RIXS) at the L3-edge of copper. We measured a single crystal of Hg1201 with Tc = 69K (hole concentration p = 0.083) and constructed energy-momentum RIXS intensity maps, with focus on the CDW momentum transfer, qCDW. The connection between CDW and other low energy excitations (magnons, phonons) will also be discussed.

Friday, November 10th 2017

12:20 pm:

Z2 vortex crystals have been recently shown to be

realized in triangular Kitaev-Heisenberg systems. These Z2 vortices are

stabilized by spin-orbit coupling introduced via the Kitaev term in the

Hamiltonian. Here, we analyze the magnetic excitations of these Z2 vortex

crystals and investigate the spin wave spectrum, energy gap and

dynamical spin structure factor in terms of the interplay between the

Kitaev and isotropic Heisenberg interaction.

Friday, November 17th 2017

12:20 pm:

The Mott-insulating rare-earth (R) titanates (RTiO3) are known to exhibit a ferromagnetic-antiferromagnetic transition as the Ti-O-Ti bond angle increases in the GdFeO3-type distorted perovskite structure. Evidence for a similar transition at a critical doping xc~ 0.3 has been found in the solid-solution system Y1-xLaxTiO3. Although neutron scattering has been used to study the magnetic response of single crystals of the parent compounds YTiO3 and LaTiO3, such studies have not been reported for the solid solution. Here we report on our neutron scattering work on the magnetic order and excitations in doped system Y1-xLaxTiO3.

Friday, November 24th 2017

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Friday, December 1st 2017

12:20 pm:

Heavy metal/ferromagnet (HM/FM) bilayers have garnered a lot of attention in recent years, owing to the rise of spintronics. These systems are central to the study of spin-orbit torque, originating from the interaction between spin polarized electrons from HM and magnetization of FM. This interaction can produce change in resistance of the device, which is strongly dependent on the relative orientation between electron spin and the magnetization, called spin Hall magnetoresistance (SMR). The relative change in MR is theoretically predicted to be strongly dependent on the spin Hall effect (SHE) in HM and the magnetic properties of FM. SHE in W is strongly dependent on the crystal phase, which was characterized by X-ray diffraction. We study and characterize SMR in Tungsten(W)/CoFeB(CFB) bilayers with perpendicular magnetic anisotropy at different thermal annealing temperatures using two different experimental technique. Stoner-Wolfharth model was used to explain deviations in our experimental data from the predicted behavior.

Friday, December 8th 2017

12:20 pm:

Unconventional superconductivity and antiferromagnetism often appear close to each other in the phase diagrams of the cuprates and other unconventional superconductors, and it has been proposed that antiferromagnetic (AF) correlations drive the pairing underlying unconventional superconductivity. A quantitative understanding of these correlations is therefore of great importance. We studied AF excitations in the cuprate superconductor HgBa2CuO4+δ (Hg1201) using inelastic neutron scattering. The structural simplicity of Hg1201 makes it an ideal system in which to study the physics of the essential copper-oxygen plane. Our neutron scattering results differ qualitatively from what is seen in other cuprates, suggesting that structural complications in other materials significantly alter the AF excitations.

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