arXiv subjects

Tyrel M. McQueen

Publications and source records attributed to Tyrel M. McQueen.

70 records · Page 4Linked to original sources

arXiv2018

Charge density wave behavior and order-disorder in the antiferromagnetic metallic series Eu(Ga_1-xAl_x)_4

The solid solution Eu(Ga_1-xAl_x)_4 was grown in single crystal form to reveal a rich variety of crystallographic, magnetic, and electronic properties that differ from the isostructural end compounds EuGa_4 and EuAl_4, despite the similar covalent radii and electronic configurations of Ga and Al. Here we report the onset of magnetic spin reorientation and metamagnetic transitions for x = 0 - 1 evidenced by magnetization and temperature-dependent specific heat measurements. T_N changes non-monotonously with x, and it reaches a maximum around 20 K for x = 0.50, where the a lattice parameter also shows an extreme (minimum) value. Anomalies in the temperature-dependent resistivity consistent with charge density wave behavior exist for x = 0.50 and 1 only. Density functional theory calculations show increased polarization between the Ga-Al covalent bonds in the x = 0.50 structure compared to the end compounds, such that crystallographic order and chemical pressure are proposed as the causes of the charge density wave behavior.

cond-mat.str-el↗

arXiv2017

An effect of Sm vacancies on the hybridization gap in topological Kondo insulator candidate SmB$_6$

A necessary element for the predicted topological state in Kondo insulator SmB$_6$ is the hybridization gap which opens in this compound at low temperatures. In this work, we present a comparative study of the in-gap density of states due to Sm vacancies by Raman scattering spectroscopy and heat capacity for samples where the number of Sm vacancies is equal to or below 1 %. We demonstrate that hybridization gap is very sensitive to the presence of Sm vacancies. At the amount of vacancies above 1 % the gap fills in with impurity states and low temperature heat capacity is enhanced.

cond-mat.str-el↗

arXiv2017

Image registration of low signal-to-noise cryo-STEM data

Combining multiple fast image acquisitions to mitigate scan noise and drift artifacts has proven essential for picometer precision, quantitative analysis of atomic resolution scanning transmission electron microscopy (STEM) data. For very low signal-to-noise ratio (SNR) image stacks - frequently required for undistorted imaging at liquid nitrogen temperatures - image registration is particularly delicate, and standard approaches may either fail, or produce subtly specious reconstructed lattice images. We present an approach which effectively registers and averages image stacks which are challenging due to their low-SNR and propensity for unit cell misalignments. Registering all possible image pairs in a multi-image stack leads to significant information surplus. In combination with a simple physical picture of stage drift, this enables identification of incorrect image registrations, and determination of the optimal image shifts from the complete set of relative shifts. We demonstrate the effectiveness of our approach on experimental, cryogenic STEM datasets, highlighting subtle artifacts endemic to low-SNR lattice images and how they can be avoided. High-SNR average images with information transfer out to 0.72 A are achieved at 300 kV and with the sample cooled to near liquid nitrogen temperature.

eess.SP↗

arXiv2017

Screened moments in a Kondo insulator

Long known to have thermodynamic properties at odds with its insulating electrical transport, SmB6 has been the subject of great debate as it is unclear whether its unusual properties are related to the bulk or novel metallic surface states. We have observed a bulk moment-screening effect in nominally pure and Gd-doped SmB6 via heat capacity, magnetization, and resistivity measurements, and show this new Kondo-impurity like effect provides an unexpected but intuitive explanation for metal-like phenomena stemming from the strongly interacting host system. This affords a coherent understanding for decades of mysteries in strongly-correlated insulators, reveals the expanded utility of techniques previously only utilized for metals, and presents the novel effect of even highly-dilute impurities in strongly correlated insulators.

cond-mat.str-el↗

arXiv2017

Seeded Chemical Vapor Transport Growth of Cu$_{2}$OSeO$_{3}$

We present an optimized seeded chemical vapor transport method for the growth of Cu$_{2}$OSeO$_{3}$ that allows for chemical control in a system with many stable phases to selectively produce large phase pure single crystals. This method is shown to consistently produce single crystals in the range of 120 to 180 mg. A Wulff construction model of a representative crystal shows that the minimum energy surface is {1 1 0}, followed by {1 0 0}. Analysis of the lowest index planes revealed that cleavage of Se-O bonds has a large energy cost, leading to an overall high surface energy. The seeded chemical vapor transport demonstrated here shows promise for large single crystal growth of other functional materials such as Weyl semimetals, frustrated magnets, and superconductors.

cond-mat.str-el↗

arXiv2016

Growth and Characterization of Iron Scandium Sulfide (FeSc2S4)

Here we report successful growth of mm scale single crystals of stoichiometric FeSc2S4. Single crystal X-ray diffraction yields a cubic structure, spacegroup Fd-3m, with a=10.5097(2) angstroms at T=110(2) K consistent with previous literature on polycrystallin samples. Models fit to the data reveal no detectable antisite mixing or deviations from the ideal stoichiometry. Heat capacity and dc magnetization measurements on the single crystals match those of high quality powder specimens. The novel traveling solvent growth method presented in this work opens the door to studies requiring sizable single crystals of the candidate spin-orbital liquid FeSc2S4.

cond-mat.mtrl-sci↗

arXiv2016

Structure, Properties, and Disorder in the New Distorted-Hollandite PbIr4Se8

The synthesis and physical properties of the new distorted-Hollandite PbIr4Se8 are reported. Powder X-ray diffraction and transmission electron microscopy show that the structure consists of edge- and corner-sharing IrSe6 octahedra, with one-dimensional channels occupied by Pb. The structure contains Se-Se anion-anion bonding, leading to an electron count of Pb2+(Ir3+)4(Se2)2-(Se2-)6, confirmed by bond-valence sums and diamagnetic behavior. Structural and heat capacity measurements demonstrate disorder on the Pb site, due to the combination of lone-pair effects and the large size of the one-dimensional channels. Comparisons are made to known Hollandite and pseudo-Hollandite structures, which demonstrates that the anion-anion bonding in PbIr4Se8 distorts its structure, to accommodate the Ir3+ state. An electronic structure calculation indicates semiconductor character with a band gap of 0.76(11) eV.

cond-mat.mtrl-sci↗

arXiv2016

Breakdown of the Kondo insulating state in SmB6 by introducing Sm vacancies

SmB6 is a proposed topological Kondo insulator where the presence of topological nontriviality can be tuned by variations in the Sm valence. Experimentally, Sm valence can be changed by tuning stoichiometry of SmB6. We show that Raman scattering can detect vacancies lower than 1% of Sm sites in SmB6 crystal by probing the intensity of defect-induced scattering of the acoustic phonon branch at 10~meV. In the electronic Raman spectra of SmB6 at temperatures below 130~K, we observe features developing in A$_{1g}$ and E$_g$ symmetries at 100 and 41~meV which we assign to excitations between hybridized bands, and depressed spectral weight below 20~meV associated with the hybridization gap. With the increased number of Sm vacancies up to 1% we observe an increase of spectral weight below 20~meV showing that the gap is filling in with electronic states. For the sample with the lowest number of vacancies the in-gap exciton excitations with long lifetimes protected by hybridization gap are observed at 16-18~meV in E$_g$ and T$_{2g}$ symmetries. These excitations broaden as a decrease in the lifetime with increasing number of vacancies and are quenched by the presence of in-gap states at concentration of Sm vacancies of about 1%. Based on this study we suggest that only the most stoichiometric SmB6 samples have a bulk gap necessary for topological Kondo insulators.

cond-mat.str-el↗

arXiv2016

Anion-Anion Bonding and the Topology in Ternary Iridium Seleno-Stannides

The synthesis and physical properties of two new and one known Ir-Sn-Se compound are reported. Their crystal structures are elucidated with transmission electron microscopy and powder X-ray diffraction. IrSn0.45Se1.55 is a pyrite phase which consists of tilted corner-sharing IrX6 octahedra with randomly distributed (Sn-Se)4- and (Se-Se)2- dimers. Ir2Sn3Se3 is a trigonally distorted skutterudite that consists of cooperatively tilted corner-sharing IrSn3Se3 octahedra with ordered (Sn-Se)24- tetramers. Ir2SnSe5 is a layered, distorted \b{eta}-MnO2 (pyrolusite) structure consisting of a double IrSe6 octrahedral row, corner-sharing in the a direction and edge-sharing in the b direction. This distorted pyrolusite contains (Se-Se)2- dimers, Se2- anions, and each double row is "capped" with a (Sn-Se)n polymeric chain. Resistivity, specific heat, and magnetization measurements show that all three have insulating and diamagnetic behavior, indicative of low spin 5d6 Ir3+. Electronic structure calculations on Ir2Sn3Se3 show a single, spherical, non-spin-orbit split valence band, and suggest that Ir2Sn3Se3 is topologically non-trivial under tensile strain, due to inversion of Ir-d and Se-p states.

cond-mat.mtrl-sci↗

arXiv2015

New Honeycomb Iridium (V) Oxides: NaIrO$_3$ and Sr$_3$CaIr$_2$O$_9$

We report the structures and physical properties of two new iridates, NaIrO$_3$ and Sr$_3$CaIr$_2$O$_9$, both of which contain continuous two-dimensional honeycomb connectivity. NaIrO3 is produced by room temperature oxidative deintercalation of sodium from Na$_2$IrO$_3$, and contains edge-sharing IrO6 octahedra that form a planar honeycomb lattice. Sr$_3$CaIr$_2$O$_9$, produced via conventional solid-state synthesis, hosts a buckled honeycomb lattice with novel corner-sharing connectivity between IrO6 octahedra. Both of these new compounds are comprised of Ir$^{5+}$ (5$d^4$) and exhibit negligible magnetic susceptibility. They are thus platforms to investigate the origin of the nonmagnetic behavior exhibited by Ir$^{5+}$ oxides, and provide the first examples of a $J$ = 0 state on a honeycomb lattice.

cond-mat.mtrl-sci↗

arXiv2015

Representational Analysis of Extended Disorder in Atomistic Ensembles Derived from Total Scattering Data

With the increased availability of high intensity time-of-flight neutron and synchrotron X-ray scattering sources that can access wide ranges of momentum transfer, the pair distribution function method has become a standard analysis technique for studying disorder of local coordination spheres and at intermediate atomic separations. In some cases, rational modeling of the total scattering data (Bragg and diffuse) becomes intractable with least-squares approaches and necessitates reverse Monte Carlo (RMC) simulations using large supercells. However, the extraction of meaningful information from the resulting atomistic ensembles is challenging, especially at intermediate length scales. We use representational analysis to describe displacements of atoms in RMC ensembles from an ideal crystallographic structure. Rewriting the displacements in terms of a local basis that is descriptive of the ideal crystallographic symmetry provides a robust approach to characterizing medium-range order (and disorder) and symmetry breaking in complex and disordered crystalline materials. This method enables the extraction of statistically relevant displacement modes (orientation, amplitude, and distribution) of the crystalline disorder and provides directly meaningful information in a symmetry-adapted basis set that is most descriptive of the crystal chemistry and physics.

cond-mat.mtrl-sci↗

arXiv2015

Raman study of magnetic excitations and magneto-elastic coupling in alpha-SrCr2O4

Using Raman spectroscopy, we investigate the lattice phonons, magnetic excitations, and magneto-elastic coupling in the distorted triangular-lattice Heisenberg antiferromagnet alpha-SrCr2O4, which develops helical magnetic order below 43 K. Temperature dependent phonon spectra are compared to predictions from density functional theory calculations which allows us to assign the observed modes and identify weak effects arising from coupled lattice and magnetic degrees of freedom. Raman scattering associated with two-magnon excitations is observed at 20 meV and 40 meV. These energies are in general agreement with our ab-initio calculations of exchange interactions and earlier theoretical predictions of the two-magnon Raman response of triangular-lattice antiferromagnets. The temperature dependence of the two-magnon excitations indicates that spin correlations persist well above the Néel temperature.

cond-mat.str-el↗

arXiv2013

Local increase of symmetry on cooling in KNi$_2$Se$_2

KNi$_2$Se$_2$ exhibits an increase of symmetry on cooling below $T\le50$ K, as observed by Raman spectroscopy and synchrotron x-ray diffraction. X-ray absorption spectroscopy confirms that the symmetry increase is due to changes in nickel-nickel interactions and suppression of charge density wave fluctuations. Density functional theory calculations reveal a zone- boundary lattice instability that provides a model of the room-temperature x-ray pair distribution function data, but fails to describe the higher local symmetry observed for $T\le50$K. Together, these results support many- body correlation effects as drivers for the unusual heavy fermion electronic ground state in KNi$_2$Se$_2$.

cond-mat.str-el↗

arXiv2013

Possible valence-bond condensation in the frustrated cluster magnet LiZn2Mo3O8

The emergence of complex electronic behaviour from simple ingredients has resulted in the discovery of numerous states of matter. Many examples are found in systems exhibiting geometric magnetic frustration, which prevents simultaneous satisfaction of all magnetic interactions. This frustration gives rise to complex magnetic properties such as chiral spin structures orbitally-driven magnetism, spin-ice behavior exhibiting Dirac strings with magnetic monopoles, valence bond solids, and spin liquids. Here we report the synthesis and characterization of LiZn2Mo3O8, a geometrically frustrated antiferromagnet in which the magnetic moments are localized on small transition metal clusters rather than individual ions. By doing so, first order Jahn-Teller instabilities and orbital ordering are prevented, allowing the strongly interacting magnetic clusters in LiZn2Mo3O8 to probably give rise to an exotic condensed valence-bond ground state reminiscent of the proposed resonating valence bond state. Our results also link magnetism on clusters to geometric magnetic frustration in extended solids, demonstrating a new approach for unparalleled chemical control and tunability in the search for collective, emergent electronic states of matter.

cond-mat.mtrl-sci↗

arXiv2012

Charge density wave fluctuations, heavy electrons, and superconductivity in KNi$_2$S$_2$

Understanding the complexities of electronic and magnetic ground states in solids is one of the main goals of solid-state physics. Materials with the canonical ThCr$_2$Si$_2$-type structure have proved particularly fruitful in this regards, as they exhibit a wide range of technologically advantageous physical properties described by "many-body physics," including high-temperature superconductivity and heavy fermion behavior. Here, using high-resolution synchrotron X-ray diffraction and time-of-flight neutron scattering, we show that the isostructural mixed valence compound, KNi$_2$S$_2$, displays a number of highly unusual structural transitions, most notably the presence of charge density wave fluctuations that disappear on cooling. This behavior occurs without magnetic or charge order, in contrast to expectations based on all other known materials. Furthermore, the low-temperature electronic state of KNi$_2$S$_2$ is found to exhibit many characteristics of heavy-fermion behavior, including a heavy electron state ($m^*/m_e \sim$ 24), with a negative coefficient of thermal expansion, and superconductivity below $T_c$ = 0.46(2) K. In the potassium nickel sulfide, these behaviors arise in the absence of localized magnetism, and instead appear to originate in proximity to charge order.

cond-mat.str-el↗

arXiv2012

Mixed-Valence-Driven Heavy-Fermion Behavior and Superconductivity in KNi$_2$Se$_2$

Based on specific heat and magnetoresistance measurements, we report that a "heavy" electronic state exists below $T \approx$ 20 K in KNi$_2$Se$_2$, with an increased carrier mobility and enhanced effective electronic band mass, $m$* = 6$m_b$ to 18$m_b$. This "heavy" state evolves into superconductivity at $T_c$ = 0.80(1) K. These properties resemble that of a many-body heavy-fermion state, which derives from the hybridization between localized magnetic states and conduction electrons. Yet, no evidence for localized magnetism or magnetic order is found in KNi$_2$Se$_2$ from magnetization measurements or neutron diffraction. Instead, neutron pair-distribution-function analysis reveals the presence of local charge-density-wave distortions that disappear on cooling, an effect opposite to what is typically observed, suggesting that the low-temperature electronic state of KNi$_2$Se$_2$ arises from cooperative Coulomb interactions and proximity to, but avoidance of, charge order.

cond-mat.supr-con↗