The search for delocalization in manganese pnictides

University of West Florida, Department of Physics   Pensacola, FL

2013-03-26

The search for new superconductors: one crystal at a time

Duquesne University, Department of Physics  Pittsburgh, PA

2013-03-14

Search for delocalization in the manganese pnictides

Centro Atomico Bariloche, Instituto Balseiro  S. C. de Bariloche, Argentina

2013-02-23

High-spin/low-spin transition in CaMn2Sb2

2011 Superconductivity Program Review and USAF- China Workshop  Santa Barbara, CA

2011-12-12

Quantum criticality and CaMn2Al10"

Farmingdale State College Physics Colloquium  Farmingdale State College, Farmingdale, NY

2014-11-13

Designing new functional materials

Farmingdale State College Physics Colloquium  Farmingdale State College, Farmingdale, NY

2013-11-12

\Finite, disordered chains of local and correlated moments in transition metal oxides,"

Metropolitan State University  Denver, CO

\Materials design and innovation by solution growth,"

genesis efrc Kickoff Meeting  Stony Brook, NY

\A practical model for developing an applied learning infrastructure,"

4th Annual Applied Learning Conference  Niagara Falls, NY

\Advancing materials discovery,"

Colloquium  Farmingdale State College, NY

"Synthesis and Characterization of Protective Cr-Based Aluminides, Borides, and Carbides"

American Chemical Society Petroleum Research Fund $55,000

2016-06-01

The purpose of our project is to help find new ways to use fossil fuels like coal and natural gas more efficiently, thereby reducing their environmental footprint until carbon-neutral and renewable energy plants can be brought online on a global scale. We plan to take advantage of the fact that the efficiency of a power plant increases with the temperature and pressure of the steam that drives its turbines. Unfortunately, these turbines and other exposed components are fabricated from high strength steels, which are prone to corrosion and pitting as the steam temperature rises. To this end, we propose to develop new materials that can be used to coat these components, protecting them from the hot, dense steam and permitting steam temperature and therefore efficiency to be increased. This project will be carried out predominantly by Farmingdale undergraduate students in our physics research laboratory.

CaMn2Al10: Itinerant Mn magnetism on the verge of ferromagnetic order

2015-07-24

We report the discovery of CaMn2Al10, a metal with strong magnetic anisotropy and moderate electronic correlations. Magnetization measurements find a Curie-Weiss moment of 0.83μB/Mn, significantly reduced from the Hund's rule value, and the magnetic entropy obtained from specific heat measurements is correspondingly small, only ≈9% of Rln2. These results imply that the Mn magnetism is highly itinerant, a conclusion supported by density functional theory calculations that find strong Mn-Al hybridization. Consistent with the layered nature of the crystal structure, the magnetic susceptibility χ is anisotropic below 20 K, with a maximum ratio of χ[010]/χ[001]≈3.5. A strong power-law divergence χ(T)∼T−1.2 below 20 K implies incipient ferromagnetic order with a low Curie temperature TC

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CaMn 2 Sb 2 : Spin waves on a frustrated antiferromagnetic honeycomb lattice

2015-05-22

We present inelastic neutron scattering measurements of the antiferromagnetic insulator CaMn 2 Sb 2 , which consists of corrugated honeycomb layers of Mn. The dispersion of magnetic excitations has been measured along the H and L directions in reciprocal space, with a maximum excitation energy of ≈24 meV. These excitations are well described by spin waves in a Heisenberg model, including first- and second-neighbor exchange interactions J 1 and J 2 in the Mn plane and also an exchange interaction between planes. The determined ratio J 2 /J 1 ≈1/6 suggests that CaMn 2 Sb 2 is an example of a compound that lies very close to the mean field tricritical point, known for the classical Heisenberg model on the honeycomb lattice, where the Néel phase and two different spiral phases coexist. The magnitude of the determined exchange interactions reveals a mean field ordering temperature ≈4 times larger than the reported Néel temperature T N =85 K, suggesting significant frustration arising from proximity to the tricritical point.

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Protected Fe valence in quasi-two-dimensional ?-FeSi2

2015-04-02

We report the first comprehensive study of the high temperature form (?-phase) of iron disilicide. Measurements of the magnetic susceptibility, magnetization, heat capacity and resistivity were performed on well characterized single crystals. With a nominal iron d6 configuration and a quasi-two-dimensional crystal structure that strongly resembles that of LiFeAs, ?-FeSi2 is a potential candidate for unconventional superconductivity. Akin to LiFeAs, ?-FeSi2 does not develop any magnetic order and we confirm its metallic state down to the lowest temperatures (T?=?1.8?K). However, our experiments reveal that paramagnetism and electronic correlation effects in ?-FeSi2 are considerably weaker than in the pnictides. Band theory calculations yield small Sommerfeld coefficients of the electronic specific heat ??=?Ce/T that are in excellent agreement with experiment. Additionally, realistic many-body calculations further corroborate that quasi-particle mass enhancements are only modest in ?-FeSi2. Remarkably, we find that the natural tendency to vacancy formation in the iron sublattice has little influence on the iron valence and the density of states at the Fermi level. Moreover, Mn doping does not significantly change the electronic state of the Fe ion. This suggests that the iron valence is protected against hole doping and indeed the substitution of Co for Fe causes a rigid-band like response of the electronic properties. As a key difference from the pnictides, we identify the smaller inter-iron layer spacing, which causes the active orbitals near the Fermi level to be of a different symmetry in ?-FeSi2. This change in orbital character might be responsible for the lack of superconductivity in this system, providing constraints on pairing theories in the iron based pnictides and chalcogenides.

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Origin of the charge gap in LaMnPO

2014-11-08

We present high temperature inelastic neutron scattering and magnetic susceptibility measurements of the antiferromagnetic insulator LaMnPO that are consistent with the presence of two-dimensional magnetic correlations up to a temperature T max ≈700K≫T N =375 K, the Néel temperature. Optical transmission measurements show the T=300 K direct charge gap Δ=1 eV has decreased only marginally by 500 K and suggest it decreases by only 10% at T max . Density functional theory and dynamical mean-field theory calculations reproduce a direct charge gap in paramagnetic LaMnPO only when a strong Hund's coupling J H =0.9 eV is included, as well as on-site Hubbard U=8 eV. Our results show that LaMnPO is a Mott-Hund's insulator, in which the charge gap is rather insensitive to antiferromagnetic exchange coupling

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Intrinsic nanostructure in Zr2?xFe4Si16?y(x?=?0.81, y?=?6.06)

2014-08-28

We present a study of the crystal structure and physical properties of single crystals of a new Fe-based ternary compound, Zr2?xFe4Si16?y(x?=?0.81, y?=?6.06). Zr1.19Fe4Si9.94 is a layered compound, where stoichiometric ?-FeSi2-derived slabs are separated by Zr-Si planes with substantial numbers of vacancies. High resolution transmission electron microscopy (HRTEM) experiments show that these Zr-Si layers consist of 3.5?nm domains where the Zr and Si vacancies are ordered within a supercell sixteen times the volume of the stoichiometric cell. Within these domains, the occupancies of the Zr and Si sites obey symmetry rules that permit only certain compositions, none of which by themselves reproduce the average composition found in x-ray diffraction experiments. Magnetic susceptibility and magnetization measurements reveal a small but appreciable number of magnetic moments that remain freely fluctuating to 1.8?K, while neutron diffraction confirms the absence of bulk magnetic order with a moment of 0.2?B or larger down to 1.5?K. Electrical resistivity measurements find that Zr1.19Fe4Si9.94 is metallic, and the modest value of the Sommerfeld coefficient of the specific heat ??=?C/T suggests that quasi-particle masses are not particularly strongly enhanced. The onset of superconductivity at Tc??6?K results in a partial resistive transition and a small Meissner signal, although a bulk-like transition is found in the specific heat. Sharp peaks in the ac susceptibility signal the interplay of the normal skin depth and the London penetration depth, typical of a system in which nano-sized superconducting grains are separated by a non-superconducting host. Ultra low field differential magnetic susceptibility measurements reveal the presence of a surprisingly large number of trace magnetic and superconducting phases, suggesting that the Zr-Fe-Si ternary system could be a potentially rich source of new bulk superconductors.

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Observation of Ferroelectricity and Structure-Dependent Magnetic Behavior in Novel One-Dimensional Motifs of Pure, Crystalline Yttrium Manganese Oxides

2014-09-18

Multiferroic materials, such as nanostructured h-YMnO3, are expected to fulfill a crucial role as active components of technological devices, particularly for information storage. Herein, we report on the template mediated sol–gel synthesis of unique one-dimensional nanostructured motifs of hexagonal phase YMnO3, possessing a space group of P63cm. We found that the inherent morphology of the as-obtained h-YMnO3 nanostructures was directly impacted by the chemical composition of the employed membrane. Specifically, the use of anodic alumina and polycarbonate templates promoted nanotube and nanowire formation, respectively. Isolated polycrystalline nanotubes and single crystalline nanowires possessed diameters of 276 ± 52 nm, composed of 17 nm particulate constituent grains, and 125 ± 21 nm, respectively, with lengths of up to several microns. The structures and compositions of all our as-prepared products were probed by XRD, SEM, HRTEM, EXAFS, XANES, SAED, and far-IR spectroscopy. In the specific case of nanowires, we determined that the growth direction was mainly along the c-axis and that discrete, individual structures gave rise to expected ferroelectric behavior. Overall, our YMnO3 samples evinced the onset of a spin-glass transition at 41 ± 1 K for both templateless bulk control and nanowire samples but at 26 ± 3 K for nanotubes. Interestingly, only the as-synthesized crystalline nanotubular mesh gave rise to noticeably enhanced magnetic properties (i.e., a higher magnetic moment of 3.0 μB/Mn) as well as a lower spin-glass transition temperature, attributable to a smaller constituent crystallite size. Therefore, this work not only demonstrates our ability to generate viable one-dimensional nanostructures of a significant and commercially relevant metal oxide but also contributes to an understanding of structure–property correlations in these systems.

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Synthesis and Properties of Charge-Ordered Thallium Halide Perovskites, CsTl+0.5Tl3+0.5X3 (X = F or Cl): Theoretical Precursors for Superconductivity?

2013-10-22

Recently, CsTlCl3 and CsTlF3 perovskites were theoretically predicted to be potential superconductors if they were optimally doped. The syntheses of these two compounds together with a complete characterization of the samples are reported. CsTlCl3 was obtained as orange crystals in two different polymorphs: a tetragonal phase (I4/m) and a cubic phase (Fm3̅m). CsTlF3 was formed as a light brown powder, and also as a double cubic perovskite (Fm3̅m). In all three CsTlX3 phases, Tl+ and Tl3+ were located in two different crystallographic positions that accommodate their different bond lengths. In CsTlCl3, some Tl vacancies were found in the Tl+ position. The charge ordering between Tl+ and Tl3+ was confirmed by X-ray absorption and Raman spectroscopy. The Raman spectroscopy of CsTlCl3 at high pressure (58 GPa) did not indicate any phase transition to a possible single Tl2+ state. However, the highly insulating material became less resistive with an increasing high pressure, while it underwent a change in its optical properties, from transparent to deeply opaque red, indicative of a decrease in the magnitude of the band gap. The theoretical design and experimental validation of the existence of CsTlF3 and CsTlCl3 cubic perovskites are the necessary first steps in confirming the theoretical prediction of superconductivity in these materials.

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Observation of antiferromagnetic order collapse in the pressurized insulator LaMnPO

2013-08-30

The emergence of superconductivity in the iron pnictide or cuprate high temperature superconductors usually accompanies the suppression of a long-ranged antiferromagnetic (AFM) order state in a corresponding parent compound by doping or pressurizing. A great deal of effort by doping has been made to find superconductivity in Mn-based compounds, which are thought to bridge the gap between the two families of high temperature superconductors, but the AFM order was not successfully suppressed. Here we report the first observations of the pressure-induced elimination of long-ranged AFM order at ~ 34 GPa and a crossover from an AFM insulating to an AFM metallic state at ~ 20 GPa in LaMnPO single crystals that are iso-structural to the LaFeAsO superconductor by in-situ high pressure resistance and ac susceptibility measurements. These findings are of importance to explore potential superconductivity in Mn-based compounds and to shed new light on the underlying mechanism of high temperature superconductivity.

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Local moments and suppression of antiferromagnetism in correlated Zr 4 Fe 4 Si 7

2013-08-29

We report magnetic, transport, and neutron diffraction measurements as well as a doping study of the V-phase compound Zr 4 Fe 4 Si 7 . This compound exhibits collinear antiferromagnetic order below T N =98±1 K with a staggered moment of 0.57(3)μ B / Fe as T→ 0. The magnetic order can be quenched with Co substitution to the Fe site, but even then a 1.5μ B / Fe paramagnetic moment remains. The resistivity and heat capacity of Zr 4 Fe 4 Si 7 are Fermi-liquid-like below 16 and 7 K, respectively, and reveal correlations on the scale of those observed in superconducting Fe pnictides and chalcogenides. Electronic structure calculations overestimate the ordered moment, suggesting the importance of dynamical effects. The existence of magnetic order, electronic correlations, and spin fluctuations make Zr 4 Fe 4 Si 7 distinct from the majority of Fe-Si compounds, fostering comparison instead with the parent compounds of Fe-based superconductors.

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Magnetic structure of Yb2Pt2Pb: Ising moments on the Shastry-Sutherland lattice

2016-01-01

Magnetic structure of Yb2Pt2Pb: Ising moments on the Shastry-Sutherland lattice

Optimized in situ crystal growth and disordered quasi-one dimensional magnetism in Li2Mn2(MoO4)3,

Optimized in situ crystal growth and disordered quasi-one dimensional magnetism in Li2Mn2(MoO4)3,

\The disordered and correlated insulator Bi2CrAl3O9,"

\The disordered and correlated insulator Bi2CrAl3O9,"

\Structural chemistry of akdalaite, Al10O14(OH)2, the isostructural aluminum analogue of ferrihydrite,"

\Structural chemistry of akdalaite, Al10O14(OH)2, the isostructural aluminum analogue of ferrihydrite,"

\Combined computational and experimental investigation of the La2CuO4

\Combined computational and experimental investigation of the La2CuO4

\Pseudo-icosahedral Cr55Al232

\Pseudo-icosahedral Cr55Al232

PHY 125L - Physical Science Laboratory I

A Physical Science Laboratory to accompany any of the Physical Science Theory courses listed above. Experiments will be in the area of Physics, Astronomy, Meteorology, the Environment and Technology.

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PHY 135 - College Physics I

An integrated theory/laboratory general college physics course without calculus. Topics will include fundamental concepts of units, vectors, equilibrium, velocity and acceleration in linear and rotational motion, force, energy, momentum, fluids at rest and in motion, and oscillatory motion. Laboratory problems, experiments and report writing associated with the topics studied in the theory are performed.

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PHY 136 - College Physics II

A continuation of PHY 135. Topics will include heat, electricity, magnetism, light and optics.

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PHY 143 - General Physics I (Calculus)

A fundamental, calculus based, physics course with laboratory offered primarily for students in Science curricula. Topics discussed include Mechanics, Wave Motion, Kinetic Theory, and Thermodynamics.

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PHY 323 - Electromagnetic Theory

This course is an introduction to electromagnetic theory. Topics covered are Vector Analysis; Coulomb's Law; Gauss's Law; the Del Operator; the Divergence and Gradient; the Potential; Potential Gradient; Conductors, Dielectrics and Capacitors; the Magnetic Field; the Biot-Savart Law; Ampere's Law; the Curl of E and H; Faraday's Law; Maxwell's Equations.

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PHY 480 - Physics Research I

Physics Research I represents substantial projects or work experiences for 135 hours earning 3 credits. Students will work alongside physics faculty in their professional research. Registration requires submission of resume three months in advance, physics faculty invitation or recommendation, and department Chair approval.

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