H. D. Zhou

The University of Tennessee Medical Center at Knoxville, Knoxville, Tennessee, United States

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Publications (151)500.09 Total impact

  • [Show abstract] [Hide abstract]
    ABSTRACT: We study the low-temperature thermal conductivity (\kappa) of Dy_2Ti_2O_7 and Yb_2Ti_2O_7 single crystals in magnetic fields up to 14 T along the [111], [100] and [110] directions. The main experimental findings for Dy_2Ti_2O_7 are: (i) the low-T \kappa(H) isotherms exhibit not only the step-like decreases at the low-field (< 2 T) magnetic transitions but also obvious field dependencies in high fields (> 7 T); (ii) at T \le 0.5 K, the \kappa(H) curves show anisotropic irreversibility in low fields, that is, the \kappa(H) hysteresis locates at the first-order transition with H \parallel [100] and [110], while it locates between two successive transitions with H \parallel [111]; (iii) the \kappa in the hysteresis loops for H \parallel [100] and [110] show an extremely slow relaxation with the time constant of \sim 1000 min. The main experimental findings for Yb_2Ti_2O_7 are: (i) the zero-field \kappa(T) show a kink-like decrease at the first-order transition (\sim 200 mK) with decreasing temperature; (ii) the low-T \kappa(H) isotherms show a decrease in low field and a large enhancement in high fields; (iii) the low-T \kappa(H) curves show a sharp minimum at 0.5 T for H \parallel [110] and [111]. The roles of monopole excitations, field-induced transitions, spin fluctuations and magnetoelastic coupling are discussed.
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    ABSTRACT: Elastic neutron scattering, ac susceptibility, and specific heat experiments on the pyrochlores Er$_{2}$Ge$_{2}$O$_{7}$ and Yb$_{2}$Ge$_{2}$O$_{7}$ show that both systems are antiferromagnetically ordered in the $\Gamma_5$ manifold. The ground state is a $\psi_{3}$ phase for the Er sample and a $\psi_{2}$ or $\psi_{3}$ phase for the Yb sample, which suggests "Order by Disorder"(ObD) physics. Furthermore, we unify the various magnetic ground states of all known R$_{2}$B$_{2}$O$_{7}$ (R = Er, Yb, B = Sn, Ti, Ge) compounds through the enlarged XY type exchange interaction $J_{\pm}$ under chemical pressure. The mechanism for this evolution is discussed in terms of the phase diagram proposed in the theoretical study [Wong et al., Phys. Rev. B 88, 144402, (2013)].
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    ABSTRACT: We present single-crystal neutron scattering measurements of the spin-1/2 equilateral triangular lattice antiferromagnet Ba$_3$CoSb$_2$O$_9$. Besides confirming that the Co$^{2+}$ magnetic moments lie in the ab plane for zero magnetic field, we determine all the exchange parameters of the minimal quasi-2D spin Hamiltonian, which confirms that Ba$_3$CoSb$_2$O$_9$ is an almost perfect realization of the paradigmatic model of frustrated quantum magnetism. A comparison with linear and nonlinear spin-wave theory reveals that quantum fluctuations induce a strong downward renormalization of the magnon dispersion.
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    ABSTRACT: The ferrimagnetic spinel $CoV_2O_4$ has been a topic of intense recent interest, both as a frustrated insulator with unquenched orbital degeneracy and as a near-itinerant magnet which can be driven metallic with moderate applied pressure. Central outstanding questions include the number and form of magnetic transitions in this material, and the absence of any degeneracy breaking structural phase transition, contrary to all available models. To help address these questions, we have performed a series of neutron diffraction and inelastic scattering measurements on $CoV_2O_4$ powders with minimal cation site disorder. Our data indicate a near ideal cubic spinel structure at all temperatures, and a ferrimagnetic spin state below $T_N = 156 K$, consistent with previous reports. Significantly however, we also provide strong evidence for a weak ($\frac{\Delta a}{a} \sim 10^{-4}$), first order structural phase transition at $T^*$ = 90 K, the same temperature where spin canting is seen in recent single crystal measurements. This transition is characterized by a short-range distortion of oxygen octahedral positions, and a weak $\Delta\sim 1.25 meV$ spin gap is observed in low temperature inelastic data. Together, these findings provide strong support for the local orbital picture and the existence of an orbital glass state below $T^*$. We further rule out in our sample the presence of additional structural or magnetic transitions at lower temperatures, contrary to other studies.
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    ABSTRACT: The frustrated pyrochlore antiferromagnet Gd$_{2}$Ti$_{2}$O$_{7}$ has an unusual partially-ordered magnetic structure at the lowest measurable temperatures. This structure is currently believed to involve four magnetic propagation vectors $\mathbf{k}\in \langle \frac{1}{2} \frac{1}{2} \frac{1}{2} \rangle^*$ in a cubic 4-$\mathbf{k}$ structure, based on analysis of magnetic diffuse-scattering data [J. Phys.: Condens. Matter 16, L321 (2004)]. Here, we present three pieces of evidence against the 4-$\mathbf{k}$ structure. First, we report single-crystal neutron-diffraction measurements as a function of applied magnetic field, which are consistent with the selective field-induced population of non-cubic magnetic domains. Second, we present evidence from high-resolution powder neutron-diffraction measurements that rhombohedral strains exist within magnetic domains, which may be generated by magneto-elastic coupling only for the alternative 1-$\mathbf{k}$ structure. Finally, we show that the argument previously used to rule out the 1-$\mathbf{k}$ structure is flawed, and demonstrate that magnetic diffuse-scattering data can actually be fitted quantitatively by a 1-$\mathbf{k}$ structure in which spin fluctuations on ordered and disordered magnetic sites are strongly coupled. Our results provide an experimental foundation on which to base theoretical descriptions of partially-ordered states.
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    ABSTRACT: The specific heat, the susceptibility under pressure, and the dielectric constant were measured for single crystals ${\mathrm{Y}}_{1$-${}x}{\mathrm{La}}_{x}{\mathrm{TiO}}_{3}$. The observed ${T}^{2}$-dependent specific heat at low temperatures for $0.17$\le${}x$\le${}0.3$ samples shows a spin-orbital liquid state between the ferromagnetic/orbital ordering $(x<0.17)$ and antiferromagnetic/possible orbital liquid phase $(x>0.3)$. The nonmonotonous pressure dependence of ${T}_{\text{C}}$ and the glassy behavior of the dielectric loss for the $x=0.23$ sample suggest that it is approaching a possible quantum critical point. All these properties result from the coupling between the strong spin and orbital fluctuations while approaching the phase boundary.
    Physical Review B 04/2015; 91(16):161106. DOI:10.1103/PhysRevB.91.161106 · 3.74 Impact Factor
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    ABSTRACT: High quality single crystals of BaFe12O19 were grown using the floating zone technique in 100 atm of flowing oxygen. Single crystal neutron diffraction was used to determine the nuclear and magnetic structures of BaFe12O19 at 4 K and 295 K. At both temperatures, there exist local electric dipoles formed by the off-mirror-plane displacements of magnetic Fe3+ ions at the bipyramidal sites. The displacement at 4 K is about half of that at room temperature. The temperature dependence of the specific heat shows no anomaly associated with long range polar ordering in the temperature range from 1.90 to 300 K. The inverse dielectric permittivity, 1/ε, along the c-axis shows a T 2 temperature dependence between 10 K and 20 K, with a significantly reduced temperature dependence displayed below 10 K. Moreover, as the sample is cooled below 1.4 K there is an anomalous sharp upturn in 1/ε. These features resemble those of classic quantum paraelectrics such as SrTiO3. The presence of the upturn in 1/ε indicates that BaFe12O19 is a critical quantum paraelectric system with Fe3+ ions involved in both magnetic and electric dipole formation.
    APL Materials 03/2015; 3(6). DOI:10.1063/1.4922934 · 2.79 Impact Factor
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    ABSTRACT: We studied the magnetic properties of single-crystal Yb2V2O7 using dc and ac susceptibility measurements, elastic and inelastic neutron-scattering measurements, and linear spin-wave theory. The experimental data show a ferromagnetic ordering of V4+ ions at 70 K, a short-range ordering of Yb3+ ions below 40 K, and finally a long-range noncollinear ordering of Yb3+ ions below 15 K. With external magnetic field oriented along the [111] axis, the Yb sublattice experiences a spin flop transition related to the “three-in one-out” spin structure. By modeling the spin-wave excitations, we extract the Hamiltonian parameters. Our results confirm that although the extra inter-sublattice Yb-V interactions dramatically increase the Yb ordering temperature to 15 K, the intra-sublattice Yb-Yb interactions, based on the pyrochlore lattice, still stabilize the Yb ions' noncollinear spin structure and spin flop transition.
    Physical review. B, Condensed matter 02/2015; 91(6):064425. DOI:10.1103/PhysRevB.91.064425 · 3.66 Impact Factor
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    ABSTRACT: The magnetic phases of the ideal spin-1/2 triangular-lattice antiferromagnet Ba3CoSb2O9 are identified and studied using Ba nuclear magnetic resonance (NMR) spectroscopy in magnetic fields ranging to 30T, oriented parallel and near perpendicular to the crystallographic ab-plane. For both directions, the saturation field is approximately 33T. Notably, the NMR spectra provide microscopic evidence for the stabilization of an up-up-down spin configuration for in-plane fields, giving rise to an one-third magnetization plateau (Msat/3), as well as for a higher field phase transition near to ∼(3/5)Msat for both field orientations. Phase transitions are signaled by the evolution of the NMR spectra, and in some cases through spin-lattice relaxation measurements. The results are compared with expectations obtained from a semi-classical energy density modeling, in which quantum effects are incorporated by effective interactions extracted from the spin-wave analysis of the two-dimensional model. The interlayer coupling also plays a significant role in the outcome. Good agreement between the model and the experimental results is achieved, except for the case of fields approaching the saturation value applied along the c-axis.
    Physical Review B 01/2015; 91(2):024410. DOI:10.1103/PhysRevB.91.024410 · 3.74 Impact Factor
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    ABSTRACT: After nearly 20 years of study, the origin of the spin-liquid state in Tb_{2}Ti_{2}O_{7} remains a challenge for experimentalists and theorists alike. To improve our understanding of the exotic magnetism in Tb_{2}Ti_{2}O_{7}, we synthesize a chemical pressure analog: Tb_{2}Ge_{2}O_{7}. Substitution of titanium by germanium results in a lattice contraction and enhanced exchange interactions. We characterize the magnetic ground state of Tb_{2}Ge_{2}O_{7} with specific heat, ac and dc magnetic susceptibility, and polarized neutron scattering measurements. Akin to Tb_{2}Ti_{2}O_{7}, there is no long-range order in Tb_{2}Ge_{2}O_{7} down to 20 mK. The Weiss temperature of -19.2(1) K, which is more negative than that of Tb_{2}Ti_{2}O_{7}, supports the picture of stronger antiferromagnetic exchange. Polarized neutron scattering of Tb_{2}Ge_{2}O_{7} reveals that liquidlike correlations dominate in this system at 3.5 K. However, below 1 K, the liquidlike correlations give way to intense short-range ferromagnetic correlations with a length scale similar to the Tb-Tb nearest neighbor distance. Despite stronger antiferromagnetic exchange, the ground state of Tb_{2}Ge_{2}O_{7} has ferromagnetic character, in stark contrast to the pressure-induced antiferromagnetic order observed in Tb_{2}Ti_{2}O_{7}.
    Physical Review Letters 12/2014; 113(26):267205. · 7.51 Impact Factor
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    ABSTRACT: The origin of the spin liquid state in Tb$_2$Ti$_2$O$_7$ has challenged experimentalists and theorists alike for nearly 20 years. To improve our understanding of the exotic magnetism in Tb$_2$Ti$_2$O$_7$, we have synthesized a chemical pressure analog, Tb$_2$Ge$_2$O$_7$. Germanium substitution results in a lattice contraction and enhanced exchange interactions. We have characterized the magnetic ground state of Tb$_2$Ge$_2$O$_7$ with specific heat, ac and dc magnetic susceptibility, and polarized neutron scattering measurements. Akin to Tb$_2$Ti$_2$O$_7$, there is no long-range order in Tb$_2$Ge$_2$O$_7$ down to 20 mK. The Curie-Weiss temperature of $-19.2(1)$ K, which is more negative than that of Tb$_2$Ti$_2$O$_7$, supports the picture of stronger antiferromagnetic exchange. Polarized neutron scattering of Tb$_2$Ge$_2$O$_7$ reveals that at 3.5 K liquid-like correlations dominate in this system. However, below 1 K, the liquid-like correlations give way to intense short-range ferromagnetic correlations with a length scale related to the Tb-Tb distance. Despite stronger antiferromagnetic exchange, the ground state of Tb$_2$Ge$_2$O$_7$ has ferromagnetic character, in stark contrast to the pressure-induced antiferromagnetic order observed in Tb$_2$Ti$_2$O$_7$.
    Physical Review Letters 12/2014; 113:267205. DOI:10.1103/PhysRevLett.113.267205 · 7.51 Impact Factor
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    ABSTRACT: We have performed magnetic, electric, thermal, and neutron powder diffraction (NPD) experiments as well as density functional theory (DFT) calculations on Ba3MnNb2O9. All results suggest that Ba3MnNb2O9 is a spin-5/2 triangular lattice antiferromagnet (TLAF) with weak easy-axis anisotropy. At zero field, we observed a narrow two-step transition at TN1 = 3.4 K and TN2 = 3.0 K. The neutron diffraction measurement and the DFT calculation indicate a 120◦ spin structure in the ab plane with out-of-plane canting at low temperatures. With increasing magnetic field, the 120◦ spin structure evolves into up-up-down (uud) and oblique phases showing successive magnetic phase transitions, which fits well to the theoretical prediction for the 2D Heisenberg TLAF with classical spins. Multiferroicity is observed when the spins are not collinear but suppressed in the uud and oblique phases.
    Physical Review B 12/2014; 90(22):224402. DOI:10.1103/PhysRevB.90.224402 · 3.74 Impact Factor
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    ABSTRACT: We present a detailed local probe study of the magnetic order in the oxychalcogenide La2O2Fe2OSe2 utilizing Fe57 Mössbauer, La139 NMR, and muon-spin relaxation spectroscopy. This system can be regarded as an insulating reference system of the Fe arsenide and chalcogenide superconductors. From the combination of the local probe techniques we identify a noncollinear magnetic structure similar to Sr2F2Fe2OS2. The analysis of the magnetic order parameter yields an ordering temperature TN=90.1K and a critical exponent of β=0.133, which is close to the two-dimensional Ising universality class as reported in the related oxychalcogenide family.
    Physical Review B 11/2014; 90(18):184408. DOI:10.1103/PhysRevB.90.184408 · 3.74 Impact Factor
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    ABSTRACT: We have carried out 63,65Cu NMR spectra measurements in a magnetic field up to about 15.5 T on a single crystal of the multiferroic triangular-lattice antiferromagnet CuCrO2. The measurements were performed for perpendicular and parallel orientations of the magnetic field with respect to the c axis of the crystal, and the detailed angle dependence of the spectra on the magnetic field direction in the ab plane was studied. The shape of the spectra can be well described in the model of spiral spin structure proposed by recent neutron diffraction experiments. When the field is rotated perpendicular to the crystal c axis, we observed, directly for the first time, a remarkable reorientation of the spin plane simultaneous with rotation of the incommensurate wavevector, by quantitatively deducing the conversion of the energetically less favorable domain to a more favorable one. At high enough fields parallel to the c axis, the data are consistent with either a field-induced commensurate spiral magnetic structure or an incommensurate spiral magnetic structure with a disorder in the c direction, suggesting that high fields may have influence on interplanar ordering.
    Journal of Experimental and Theoretical Physics 11/2014; 119(5):880-890. DOI:10.1134/S1063776114110181 · 0.93 Impact Factor
  • A. Kiswandhi · J. Ma · J. S. Brooks · H. D. Zhou
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    ABSTRACT: We present temperature dependent powder x-ray diffraction measurements of the spinel $A{\mathrm{V}}_{2}{\mathrm{O}}_{4}$ ($A=\mathrm{Cd}, \mathrm{Mg}, \mathrm{Zn}$, and ${\mathrm{Mn}}_{1$-${}x}{\mathrm{Co}}_{x}{\mathrm{V}}_{2}{\mathrm{O}}_{4}$). The result shows that even though the V-V distance is an important governing parameter, the $A$-site magnetism also has a significant effect on the physical properties of $A{\mathrm{V}}_{2}{\mathrm{O}}_{4}$. This is demonstrated by comparing the structural and transport properties of nonmagnetic $A$-site ions $A{\mathrm{V}}_{2}{\mathrm{O}}_{4}$ ($A$ $=\mathrm{Cd}, \mathrm{Mg}, \mathrm{Zn}$) with the magnetic $A$-site ion substitutional sequence ${\mathrm{Mn}}_{1$-${}x}{\mathrm{Co}}_{x}{\mathrm{V}}_{2}{\mathrm{O}}_{4}$.
    Physical Review B 10/2014; 90(15). DOI:10.1103/PhysRevB.90.155132 · 3.74 Impact Factor
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    ABSTRACT: A flop of electric polarization from $P$$\parallel$$c$ ($P_c$) to $P$$\parallel$$a$ ($P_a$) is observed in MnTiO$_3$ as a spin flop transition is triggered by a $c$-axis magnetic field, $H_{\|c}$=7 T. The critical magnetic field $H_{\|c}$ for $P_a$ is significantly reduced in Mn$_{1-x}$Ni$_x$TiO$_3$ (x=0.33). $P_a$ and $P_c$ have been observed with both $H_{\|c}$ and $H_{\|a}$. Neutron diffraction measurements revealed similar magnetic arrangements for the two compositions where the ordered spins couple antiferromagnetically with their nearest intra- and inter-planar neighbors. In the x=0.33 system, the uniaxial and planar anisotropies of Mn$^{2+}$ and Ni$^{2+}$ compete and give rise to a spin reorientation transition at $T_R$. A magnetic field, $H_{\|c}$, aligns the spins along $c$ for $T_R$$<$$T$$<$$T_N$. The rotation of the collinear spins away from the $c$-axis for $T$$<$$T_R$ alters the magnetic point symmetry and gives rise to a new ME susceptibility tensor form. Such linear ME response provides satisfactory explanation for the behavior of the field-induced electric polarization in both compositions. As the Ni content increases to x=0.5 and 0.68, the ME effect disappears as a new magnetic phase emerges.
    Physical Review B 09/2014; 90:144429. DOI:10.1103/PhysRevB.90.144429 · 3.74 Impact Factor
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    M. Zhu · D. Do · C. R. Dela Cruz · Z. Dun · H. D. Zhou · S. D. Mahanti · X. Ke
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    ABSTRACT: We report the complex magnetic phase diagram and electronic structure of Cr2(Te1-xWx)O6 systems. While compounds with different x values possess the same crystal structure, they display different magnetic structures below and above xc = 0.7, where both the transition temperature TN and sublattice magnetization (Ms) reach a minimum. Unlike many known cases where magnetic interactions are controlled either by injection of charge carriers or by structural distortion induced via chemical doping, in the present case it is achieved by tuning the orbital hybridization between Cr 3d and O 2p orbitals through W 5d states. The result is supported by ab-initio electronic structure calculations. Through this concept, we introduce a new approach to tune magnetic and electronic properties via chemical doping.
    Physical Review Letters 09/2014; 113(7). DOI:10.1103/PhysRevLett.113.076406 · 7.51 Impact Factor
  • S.-H. Do · J. van Tol · H. D. Zhou · K.-Y. Choi
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    ABSTRACT: High-field magnetization and high-frequency electron spin resonance (ESR) are employed to differentiate magnetism between an orthorhombic and a hexagonal majority phase of Ba3CuSb2O9. For the orthorhombic sample, an ESR signal changes its temperature dependence at T-S similar to 200 K, suggesting a static Jahn-Teller (JT) ordering. A magnetization curve follows a power-law behavior M similar to H-alpha m with the exponent alpha(m) = 0.72 +/- 0.06 for 8 < H < 26 T and alpha(m) = 1.06 +/- 0.04 for H > 26 T. The ESR linewidth exhibits a critical-like divergence, Delta H-pp(T) proportional to T-alpha with the exponents of alpha = 0.22 +/- 0.07 and 0.32 +/- 0.04. The sublinear magnetization and the critical ESR line broadening are taken as evidence of a random singlet state. For the hexagonal sample, both Delta H-pp(T) and g factor are described by the same thermally activated process with the energy barrier of 300 K. This evidences intrinsic coupling of spins to orbital degrees of freedom and thereby gives support for a dynamic spin-orbital entangled state. Our results demonstrate that magnetism in the spin-orbital coupled compound Ba3CuSb2O9 is dictated by a spatiotemporal structure of the JT distortions.
    Physical Review B 09/2014; 90(10). DOI:10.1103/PhysRevB.90.104426 · 3.74 Impact Factor
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    ABSTRACT: BiCu2PO6, a spin-gapped material in a frustrated two-leg ladder lattice, exhibits a complex (H, T) phase diagram in external magnetic fields. We have characterized the field-induced phases through specific heat, magnetocaloric effect, Faraday rotation, and magnetization measurements in fields up to 120 T. In addition to observing a magnetic phase above 60 T which we attribute to the crystallization of triplet excitations, our measurements reveal the emergence and the subsequent disappearance of a high-entropy (short-range-ordered) phase at first and second critical magnetic fields, respectively. This is in good agreement with the theoretical prediction of a quantum soliton lattice at intermediate fields.
    Physical Review B 08/2014; 90(6-6). DOI:10.1103/PhysRevB.90.060408 · 3.74 Impact Factor
  • M Zhu · D Do · C R Dela Cruz · Z Dun · H D Zhou · S D Mahanti · X Ke
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    ABSTRACT: We report the complex magnetic phase diagram and electronic structure of Cr_{2}(Te_{1-x}W_{x})O_{6} systems. While compounds with different x values possess the same crystal structure, they display different magnetic structures below and above x_{c}=0.7, where both the transition temperature T_{N} and sublattice magnetization (M_{s}) reach a minimum. Unlike many known cases where magnetic interactions are controlled either by injection of charge carriers or by structural distortion induced via chemical doping, in the present case it is achieved by tuning the orbital hybridization between Cr 3d and O 2p orbitals through W 5d states. The result is supported by ab initio electronic structure calculations. Through this concept, we introduce a new approach to tune magnetic and electronic properties via chemical doping.
    Physical Review Letters 08/2014; 113(7):076406. · 7.51 Impact Factor

Publication Stats

891 Citations
500.09 Total Impact Points

Institutions

  • 2012–2015
    • The University of Tennessee Medical Center at Knoxville
      Knoxville, Tennessee, United States
  • 2006–2014
    • Florida State University
      • • Department of Physics
      • • Department of Chemistry and Biochemistry
      Tallahassee, Florida, United States
  • 2013
    • University of Tennessee
      • Department of Physics & Astronomy
      Knoxville, Tennessee, United States
  • 2007–2012
    • National High Magnetic Field Laboratory
      Tallahassee, Florida, United States
  • 2004–2007
    • University of Texas at Austin
      • Department of Materials Science and Engineering
      Austin, Texas, United States