Showing 1–50 of 78 results for author: Bozin, E

Fluctuation-enhanced electron-phonon coupling in FeSe

Authors: Jovan Blagojević, Ana Milosavljević, Tea Belojica, Marko Opačić, Andrijana Šolajić, Jelena Pešić, Enrico Di Lucente, Novica Paunović, Milorad V. Milošević, Emil S. Božin, Aifeng Wang, Cedomir Petrović, Zoran V. Popović, Rudi Hackl, Bojana Višić, Nenad Lazarević

Abstract: The interactions among lattice, charge, and spin degrees of freedom fundamentally shape material properties. In FeSe, symmetry-breaking perturbations serve as highly sensitive probes of these couplings. Previous work has shown that defects and isoelectronic substitution can substantially alter these interactions, giving rise to additional phonon modes. In this study, uniaxial strain is employed as… ▽ More The interactions among lattice, charge, and spin degrees of freedom fundamentally shape material properties. In FeSe, symmetry-breaking perturbations serve as highly sensitive probes of these couplings. Previous work has shown that defects and isoelectronic substitution can substantially alter these interactions, giving rise to additional phonon modes. In this study, uniaxial strain is employed as a tunable symmetry-breaking control parameter to probe the intrinsic lattice response in the absence of disorder. The temperature evolution of phonon excitations was examined with fine temperature resolution in the vicinity of the nemato-structural transition temperature $T_s$, under strain applied along the $\langle 110 \rangle$ and $\langle 100 \rangle$ crystallographic directions. A subtle asymmetry of the $A_{1g}^{ph}$ mode appears in the unstrained crystal within a narrow temperature window around $T_s$, originating from the emergence of an additional mode in the fully symmetric channel. With applied strain, this feature becomes more distinctly resolved. The anomaly is attributed to modifications of the coupling between lattice and electronic degrees of freedom driven by the ordering fluctuations right above the nematic transition. These fluctuations enhance susceptibility for phonon-electron-phonon coupling in the vicinity of the X and R points of the Brillouin zone and promote two-phonon scattering close to the $A_{1g}^{ph}$ mode. The presence of this two-phonon scattering depends on both the strength and the direction of the applied strain, indicating a high sensitivity of FeSe to local symmetry breaking. △ Less

Submitted 24 February, 2026; originally announced February 2026.

Phonon frequency comb close to an isolated Einstein mode in InSiTe3

Authors: Tea Belojica, Jovan Blagojević, Sanja Djurdjić Mijin, Andrijana Šolajić, Jelena Pešić, Emil S. Božin, Bojana Višić, Yu Liu, Cedomir Petrovic, Zoran V. Popović, Rudi Hackl, Ana Milosavljević, Nenad Lazarević

Abstract: The emergence of phonon frequency combs exemplifies a rare and intriguing phenomenon in quantum solids. Materials with distinctive phonon band structures are especially promising for hosting such states, as their vibrational dispersion landscape across the Brillouin zone can facilitate the formation of long-lived, collective lattice excitations. In the layered Van der Waals compound InSiTe$_3$, po… ▽ More The emergence of phonon frequency combs exemplifies a rare and intriguing phenomenon in quantum solids. Materials with distinctive phonon band structures are especially promising for hosting such states, as their vibrational dispersion landscape across the Brillouin zone can facilitate the formation of long-lived, collective lattice excitations. In the layered Van der Waals compound InSiTe$_3$, polarization-resolved Raman spectroscopy reveals a pronounced anharmonicity in symmetry-predicted modes and the formation of a self-organized frequency domain structure (coherent-like state), in the range of a localized highenergy A$_{1g}$ phonon mode near 500 cm$^{-1}$. This strong phonon-phonon coupling manifests itself as an anomalous temperature dependence around 200 K, coinciding with the appearance of higher-order excitations within the phonon density of states gap. These findings position InSiTe$_3$ as an unconventional platform where intrinsic highly structured phonon spectral correlations and unusually strong anharmonic effects coexist, opening new avenues for exploring emergent vibrational phenomena in low-dimensional materials. △ Less

Submitted 24 February, 2026; originally announced February 2026.

diffpy.morph: Python tools for model independent comparisons between sets of 1D functions

Authors: Andrew Yang, Christopher L. Farrow, Pavol Juhás, Luis Kitsu Iglesias, Chia-Hao Liu, Samuel D. Marks, Vivian R. K. Wall, Joshua Safin, Sean M. Drewry, Caden Myers, Dillon F. Hanlon, Nicholas Leonard, Cedomir Petrovic, Ahhyun Jeong, Dmitri V. Talapin, Linda F. Nazar, Haidong Zhou, Samuel W. Teitelbaum, Tim B. van Driel, Soham Banerjee, Emil S. Bozin, Michael F. Toney, Katharine Page, Naomi S. Ginsberg, Simon J. L. Billinge

Abstract: diffpy$.$morph addresses a need to gain scientific insights from 1D scientific spectra in model independent ways. A powerful approach for this is to take differences between pairs of spectra and look for meaningful changes that might indicate underlying chemical, structural, or other modifications. The challenge is that the difference curve may contain uninteresting differences such as experimenta… ▽ More diffpy$.$morph addresses a need to gain scientific insights from 1D scientific spectra in model independent ways. A powerful approach for this is to take differences between pairs of spectra and look for meaningful changes that might indicate underlying chemical, structural, or other modifications. The challenge is that the difference curve may contain uninteresting differences such as experimental inconsistencies and benign physical changes such as the effects of thermal expansion. diffpy$.$morph allows researchers to apply simple transformations, or "morphs", to one of the datasets to remove the unwanted differences revealing, when they are present, non-trivial differences. diffpy$.$morph is an open-source Python package available on the Python Package Index and conda-forge. Here, we describe its functionality and apply it to solve a range of experimental challenges on diffraction and PDF data from x-rays and neutrons, though we note that it may be applied to any 1D function in principle. △ Less

Submitted 11 February, 2026; v1 submitted 27 January, 2026; originally announced February 2026.

Comments: 24 pages, 18 figures

Spatio-temporal migration of antiferromagnetic domain walls in Sr2IrO4

Authors: Ian Robinson, David Yang, Ross Harder, Dina Sheyfer, Longlong Wu, Jack Griffiths, Emil Bozin, Mark P. M. Dean, Jialun Liu, Hengdi Zhao, Gang Cao, Angel Rodriguez-Fernandez, Jan-Etienne Pudell, Roman Shayduk, James Wrigley, Alexey Zozulya, Rustam Rysov, Aliaksandr Leonau, Ulrike Boesenberg, Joerg Hallmann, Anders Madsen

Abstract: By laser pump-probe time-resolved coherent magnetic X-ray diffraction imaging, we have measured the migration velocity of antiferromagnetic domain walls in the Mott insulator Sr2IrO4 at 100 K. During the laser-induced demagnetization, we observe domain walls moving at 3x10^6 m/s, significantly faster than acoustic velocities. This is understood to arise from a purely electronic spin contribution t… ▽ More By laser pump-probe time-resolved coherent magnetic X-ray diffraction imaging, we have measured the migration velocity of antiferromagnetic domain walls in the Mott insulator Sr2IrO4 at 100 K. During the laser-induced demagnetization, we observe domain walls moving at 3x10^6 m/s, significantly faster than acoustic velocities. This is understood to arise from a purely electronic spin contribution to the magnetic structure without any role for coupling to the crystal lattice. △ Less

Submitted 7 June, 2026; v1 submitted 10 November, 2025; originally announced November 2025.

Journal ref: Phys. Rev. B 113, L220410 (2026)

High-Quality Ultra-Fast Total Scattering and Pair Distribution Function Data using an X-ray Free Electron Laser

Authors: Adam F. Sapnik, Philip A. Chater, Dean S. Keeble, John S. O. Evans, Federica Bertolotti, Antonietta Guagliardi, Lise J. Støckler, Elodie A. Harbourne, Anders B. Borup, Rebecca S. Silberg, Adrien Descamps, Clemens Prescher, Benjamin D. Klee, Axel Phelipeau, Imran Ullah, Kárel G. Medina, Tobias A. Bird, Viktoria Kaznelson, William Lynn, Andrew L. Goodwin, Bo B. Iversen, Celine Crepisson, Emil S. Bozin, Kirsten M. Ø. Jensen, Emma E. McBride , et al. (26 additional authors not shown)

Abstract: High-quality total scattering data, a key tool for understanding atomic-scale structure in disordered materials, require stable instrumentation and access to high momentum transfers. This is now routine at dedicated synchrotron instrumentation using high-energy X-ray beams, but it is very challenging to measure a total scattering dataset in less than a few microseconds. This limits their effective… ▽ More High-quality total scattering data, a key tool for understanding atomic-scale structure in disordered materials, require stable instrumentation and access to high momentum transfers. This is now routine at dedicated synchrotron instrumentation using high-energy X-ray beams, but it is very challenging to measure a total scattering dataset in less than a few microseconds. This limits their effectiveness for capturing structural changes that occur at the much faster timescales of atomic motion. Current X-ray free-electron lasers (XFELs) provide femtosecond-pulsed X-ray beams with maximum energies of approximately 24 keV, giving the potential to measure total scattering and the attendant pair distribution functions (PDFs) on femtosecond timescales. Here, we show that this potential has been realised using the HED scientific instrument at the European XFEL and present normalised total scattering data for 0.35 Å-1 < Q < 16.6 Å-1 and their PDFs from a broad spectrum of materials, including crystalline, nanocrystalline and amorphous solids, liquids, and clusters in solution. We analyse the data using a variety of methods, including Rietveld refinement, small-box PDF refinement, joint reciprocal-real space refinement, cluster refinement, and Debye scattering analysis. The resolution function of the setup is also characterised. We conclusively show that high-quality data can be obtained from a single approximately 30 fs XFEL pulse. Our efforts not only significantly increase the existing maximum reported Q-range for an S(Q) measured at an XFEL but also mean that XFELs are now a viable X-ray source for the broad community of people using reciprocal space total scattering and PDF methods in their research. △ Less

Submitted 13 June, 2025; v1 submitted 30 April, 2025; originally announced April 2025.

On the origin of metal-insulator transitions in the parent compounds of ruthenium-pnictide superconductors

Authors: Niraj Aryal, Emil S. Bozin, Weiguo Yin

Abstract: We study the interplay of the structural phase transition, flat electronic band dispersion, and metal-to-insulator transition (MIT) in the parent compounds of the Ru-pnictide superconductors by using first-principles calculations. Our electron and phonon calculations reveal that Ru(P,As) undergo MIT accompanied by orthorhombic to monoclinic distortion at low temperature, but RuSb stays orthorhombi… ▽ More We study the interplay of the structural phase transition, flat electronic band dispersion, and metal-to-insulator transition (MIT) in the parent compounds of the Ru-pnictide superconductors by using first-principles calculations. Our electron and phonon calculations reveal that Ru(P,As) undergo MIT accompanied by orthorhombic to monoclinic distortion at low temperature, but RuSb stays orthorhombic and metallic in agreement with the experimental findings. We find that although small monoclinic distortion can remove the van Hove singularity at the Fermi level, it does not immediately gap out the Fermi surface and a large value of monoclinic distortion is necessary for a clear MIT suggesting the possibility of an intermediate pseudogapped monoclinic metallic phase. Furthermore, we predict a light-induced two-step insulator-to-metal and structural transitions in the monoclinic phases of RuP and RuAs, which can be tested in future ultrafast pump-probe experiments as an alternative ideal play ground to VO$_2$. △ Less

Submitted 27 January, 2024; originally announced January 2024.

Electronic structure, magnetic and transport properties of antiferromagnetic Weyl semimetal GdAlSi

Authors: Antu Laha, Asish K. Kundu, Niraj Aryal, Emil S. Bozin, Juntao Yao, Sarah Paone, Anil Rajapitamahuni, Elio Vescovo, Tonica Valla, Milinda Abeykoon, Ran Jing, Weiguo Yin, Abhay N. Pasupathy, Mengkun Liu, Qiang Li

Abstract: We report the topological electronic structure, magnetic, and magnetotransport properties of a noncentrosymmetric compound GdAlSi. Magnetic susceptibility shows an antiferromagnetic transition at $T_\mathrm{N}$ = 32 K. In-plane isothermal magnetization exhibits an unusual hysteresis behavior at higher magnetic field, rather than near zero field. Moreover, the hysteresis behavior is asymmetric unde… ▽ More We report the topological electronic structure, magnetic, and magnetotransport properties of a noncentrosymmetric compound GdAlSi. Magnetic susceptibility shows an antiferromagnetic transition at $T_\mathrm{N}$ = 32 K. In-plane isothermal magnetization exhibits an unusual hysteresis behavior at higher magnetic field, rather than near zero field. Moreover, the hysteresis behavior is asymmetric under positive and negative magnetic fields. First-principles calculations were performed on various magnetic configurations, revealing that the antiferromagnetic state is the ground state, and the spiral antiferromagnetic state is a close competing state. The calculations also reveal that GdAlSi hosts multiple Weyl points near the Fermi energy. The band structure measured by angle-resolved photoemission spectroscopy (ARPES) shows relatively good agreement with the theory, with the possibility of Weyl nodes slightly above the Fermi energy. Within the magnetic ordered state, we observe an exceptionally large anomalous Hall conductivity (AHC) of ~ 1310 $Ω^{-1}$cm$^{-1}$ at 2 K. Interestingly, the anomalous Hall effect persists up to room temperature with a significant value of AHC (~ 155 $Ω^{-1}$cm$^{-1}$). Our analysis indicates that the large AHC originates from the Berry curvature associated with the multiple pairs of Weyl points near Fermi energy. △ Less

Submitted 21 December, 2023; originally announced December 2023.

Comments: 10 pages, 6 figures, Accepted for publication in Phys. Rev. B

Resolving Length Scale Dependent Transient Disorder Through an Ultrafast Phase Transition

Authors: Jack Griffiths, Ana Flávia Suzana, Longlong Wu, Samuel D. Marks, Vincent Esposito, Sébastien Boutet, Paul G. Evans, J. F. Mitchell, Mark P. M. Dean, David A. Keen, Ian Robinson, Simon J. L. Billinge, Emil S. Bozin

Abstract: Material functionality can be strongly determined by structure extending only over nanoscale distances. The pair distribution function presents an opportunity to shift structural studies beyond idealized crystal models and investigate structure over varying length scales. Applying this method with ultrafast time resolution has the potential to similarly disrupt the study of structural dynamics and… ▽ More Material functionality can be strongly determined by structure extending only over nanoscale distances. The pair distribution function presents an opportunity to shift structural studies beyond idealized crystal models and investigate structure over varying length scales. Applying this method with ultrafast time resolution has the potential to similarly disrupt the study of structural dynamics and phase transitions. Here, we demonstrate such a measurement of CuIr$_{2}$S$_{4}$ optically pumped from its low temperature Ir-dimerized phase. Dimers are optically suppressed without spatial correlation, generating a structure whose level of disorder depends strongly on length scale. The re-development of structural ordering over tens of picoseconds is directly tracked over both space and time as a transient state is approached. This measurement demonstrates both the crucial role of local structure and disorder in non-equilibrium processes and the feasibility of accessing this information with state-of-the-art XFEL facilities. △ Less

Submitted 12 April, 2024; v1 submitted 5 October, 2023; originally announced October 2023.

Comments: 14 page manuscript with 4 figures. 13 page Supplementary with 17 figures. 27 pages and 21 figures in total

Journal ref: Nature Materials 23, 1041-1047 (2024)

Stacking disorder in $α$-RuCl$_3$ via x-ray three-dimensional difference pair distribution function analysis

Authors: J. Sears, Y. Shen, M. J. Krogstad, H. Miao, Jiaqiang Yan, Subin Kim, W. He, E. S. Bozin, I. K. Robinson, R. Osborn, S. Rosenkranz, Young-June Kim, M. P. M. Dean

Abstract: The van der Waals layered magnet $α$-RuCl$_3$ offers tantalizing prospects for the realization of Majorana quasiparticles. Efforts to understand this are, however, hampered by inconsistent magnetic and thermal transport properties likely coming from the formation of structural disorder during crystal growth, postgrowth processing, or upon cooling through the first order structural transition. Here… ▽ More The van der Waals layered magnet $α$-RuCl$_3$ offers tantalizing prospects for the realization of Majorana quasiparticles. Efforts to understand this are, however, hampered by inconsistent magnetic and thermal transport properties likely coming from the formation of structural disorder during crystal growth, postgrowth processing, or upon cooling through the first order structural transition. Here, we investigate structural disorder in $α$-RuCl$_3$ using x-ray diffuse scattering and three-dimensional difference pair distribution function (3D-$Δ$PDF) analysis. We develop a quantitative model that describes disorder in $α$-RuCl$_3$ in terms of rotational twinning and intermixing of the high and low-temperature structural layer stacking. This disorder may be important to consider when investigating the detailed magnetic and electronic properties of this widely studied material. △ Less

Submitted 25 September, 2023; v1 submitted 30 July, 2023; originally announced July 2023.

Comments: 6 pages; 3 figures; accepted in Physical Review B

Journal ref: Phys. Rev. B 108, 144419 (2023)

Tailoring defects and nanocrystal transformation for optimal heating power in bimagnetic $Co_yFe_{1-y}O@Co_xFe_{3-x}O_4$ particles

Authors: George Antonaropoulos, Marianna Vasilakaki, Kalliopi N. Trohidou, Vincenzo Iannotti, Giovanni Ausanio, Milinda Abeykoon, Emil S. Bozin, Alexandros Lappas

Abstract: The effects of cobalt incorporation in spherical heterostructured iron oxide nanocrystals (NCs) of sub-critical size have been explored by colloidal chemistry methods. Synchrotron X-ray total scattering methods suggest that cobalt (Co) substitution in rock salt iron oxide NCs tends to remedy its vacant iron sites, offering a higher degree of resistance to oxidative conversion. Self-passivation sti… ▽ More The effects of cobalt incorporation in spherical heterostructured iron oxide nanocrystals (NCs) of sub-critical size have been explored by colloidal chemistry methods. Synchrotron X-ray total scattering methods suggest that cobalt (Co) substitution in rock salt iron oxide NCs tends to remedy its vacant iron sites, offering a higher degree of resistance to oxidative conversion. Self-passivation still creates a spinel-like shell, but with higher volume fraction of the rock salt Co-containing phase in the core. The higher divalent metal stoichiometry in the rock salt phase, with increasing Co content, results in a population of unoccupied tetrahedral metal sites in the spinel part, likely through oxidative shell creation, involving an ordered defect-clustering mechanism, directly correlated to the core stabilization. To shed light on the effects of Co-substitution and atomic-scale defects (vacant sites), Monte Carlo simulations suggest that designed NCs, with desirable, enhanced magnetic properties (cf. exchange bias and coercivity), are developed with magnetocrystalline anisotropy raised at relatively low content of Co ions in the lattice. Growth of optimally performing candidates combines also a strongly exchange-coupled system, secured through a high volumetric ratio rock salt phase, interfaced by a not so defective spinel shell. In view of these requirements, Specific Absorption Rate (SAR) calculations demonstrate that the sufficiently protected from oxidation rock salt core and preserved over time heterostructure, play a key role in magnetically-mediated heating efficacies, for potential use of such NCs in magnetic hyperthermia applications. △ Less

Submitted 16 June, 2023; originally announced June 2023.

Comments: Main text of 23 pages, 12 Figures. Supplementary Information of 17 pages, 9 Figures, 4 Tables

Journal ref: Nanoscale 14 (2022) 382-401

$\rm Li_x(C_5H_5N)_yFe_{2-z}Se_2$: a defect resilient expanded-lattice high-temperature superconductor

Authors: Alexandros Deltsidis, Laura Simonelli, Georgios Vailakis, Izar Capel Berdiell, Georgios Kopidakis, Anna Krzton-Maziopa, Emil S. Bozin, Alexandros Lappas

Abstract: Two-dimensional iron-chalcogenide intercalates display a remarkable correlation of the interlayer spacing with the enhancement of the superconducting critical temperature ($T_c$). In this work, synchrotron x-ray absorption ($XAS$, at Fe and Se K edges) and emission ($XES$) spectroscopies, allow to discuss how the important rise of $T_c$ (44 K) in the molecule intercalated… ▽ More Two-dimensional iron-chalcogenide intercalates display a remarkable correlation of the interlayer spacing with the enhancement of the superconducting critical temperature ($T_c$). In this work, synchrotron x-ray absorption ($XAS$, at Fe and Se K edges) and emission ($XES$) spectroscopies, allow to discuss how the important rise of $T_c$ (44 K) in the molecule intercalated $\rm Li_x(C_5H_5N)_yFe_{2-z}Se_2$ relates to the electronic and local structure changes felt by the inorganic host upon doping ($x$). $XES$ shows that widely-separated layers of edge-sharing $\rm FeSe_4$ tetrahedra, carry low-spin moieties with a local Fe magnetic moment slightly reduced compared to the parent $β$-$\rm Fe_{2-z}Se_2$. Pre-edge $XAS$ advises on the progressively reduced mixing of metal $3d-4p$ states upon lithiation. Doping-mediated local lattice modifications, probed by conventional $T_c$-optimization measures (cf. anion height and $FeSe_4$ tetrahedra regularity), become less relevant when layers are spaced far away. On the basis of extended x-ray absorption fine structure, such distortions are compensated by a softer Fe-network that relates to Fe-site vacancies, alleviating electron-lattice correlations and superconductivity. Density functional theory ($DFT$) guided modification of isolated $\rm Fe_{2-z}Se_2$ ($z$, vacant sites) planes, resembling the host layers, identify that Fe-site deficiency occurs at low energy cost, giving rise to stretched Fe-sheets, in accord with experiments. The robust high-$T_c$ in $\rm Li_x(C_5H_5N)_yFe_{2-z}Se_2$, arises from the interplay of electron donating spacers and the iron-selenide layers tolerance to defect chemistry, a tool to favorably tune its Fermi surface properties. △ Less

Submitted 19 March, 2023; originally announced March 2023.

Comments: Main text of 29 pages, 9 Figures, 1 Table. Supplemental Material of 17 pages, 7 Figures, 1 Table

Journal ref: Inorganic Chemistry 61 (2022) 12797-12808

In-situ visualization of local distortions in the high-$T_c$ molecule-intercalated $Li_x(C_5H_5N)_yFe_{2-z}Se_2$ superconductor

Authors: Izar Capel Berdiell, Edyta Pesko, Elijah Lator, Alexandros Deltsidis, Anna Krzton-Maziopa, A. M. Milinda Abeykoon, Emil S. Bozin, Alexandros Lappas

Abstract: A time-resolved synchrotron X-ray total scattering study sheds light on the evolution of the different structural length scales involved during the intercalation of the layered iron-selenide host by organic molecular donors, aiming at the formation of the expanded lattice $Li_x(C_5H_5N)_yFe_{2-z}Se_2$ hybrid superconductor. The intercalates are found to crystallize in the tetragonal $ThCr_2Si_2$-t… ▽ More A time-resolved synchrotron X-ray total scattering study sheds light on the evolution of the different structural length scales involved during the intercalation of the layered iron-selenide host by organic molecular donors, aiming at the formation of the expanded lattice $Li_x(C_5H_5N)_yFe_{2-z}Se_2$ hybrid superconductor. The intercalates are found to crystallize in the tetragonal $ThCr_2Si_2$-type structure at the average level, however, with an enhanced interlayer iron-selenide spacing (d= 16.2 Å) that accommodates the heterocyclic molecular spacers. Quantitative atomic pair distribution function (PDF) analysis at variable times, suggests distorted $FeSe_4$ tetrahedral local environments that appear swollen with respect to those in the parent $β$-FeSe. Simultaneously acquired, in-situ synchrotron X-ray powder diffraction data disclose that secondary phases ($α$-Fe and $Li_2Se$), grow significantly when higher Li-concentration is used in the solvothermal reaction or when the solution is aged. These observations are in line with the strongly reducing character of the intercalation medium's solvated electrons that mediate the defect chemistry of the expanded lattice superconductor. In the latter, intralayer correlated local distortions indicate electron donating aspects that reflect in somewhat enlarged Fe-Se bonds. They also reveal a degree of relief of chemical pressure associated with a large distance between Fe and Se sheets ('taller' anion height) and a stretched Fe-Fe square planar topology. The elongation of the latter, derived from the in-situ PDF study, speaks for a plausible increase in the Fe-site vacancy concentration. The evolution of the local structural parameters suggests an optimum reaction window where kinetically stabilized phases resemble the distortions of the edge-sharing Fe-Se tetrahedra, required for high-$T_c$ in expanded lattice iron-chalcogenides. △ Less

Submitted 19 March, 2023; originally announced March 2023.

Comments: Main text of 25 pages, 8 Figures. Supplemental Material of 19 pages, 10 Figures, 1 Table

Journal ref: Inorganic Chemistry 61 (2022) 4350-4360

Nanoscale Sn off-centering behind low thermal conductivity in SnSe thermoelectric

Authors: E. S. Bozin, H. Xie, A. M. M. Abeykoon, S. M. Everett, M. G. Tucker, M. G. Kanatzidis, S. J. L. Billinge

Abstract: The local atomic structure of SnSe was characterized across its orthorhmbic-to-orthorhombic structural phase transition using x-ray pair distribution function analysis. Substantial Sn off-centering distortions persist in the high symmetry high temperature phase, with symmetry different from that of ordered distortions below the transition. The analysis implies that the transition is neither order-… ▽ More The local atomic structure of SnSe was characterized across its orthorhmbic-to-orthorhombic structural phase transition using x-ray pair distribution function analysis. Substantial Sn off-centering distortions persist in the high symmetry high temperature phase, with symmetry different from that of ordered distortions below the transition. The analysis implies that the transition is neither order-disorder nor displacive, but rather a complex crossover where the character of coupling changes from 3D-like at low temperature to 2D-like at high temperature. Robust ferro-coupled SnSe intra-layer distortions suggest a ferroelectric-like instability as the driving force. Complex local Sn off-centering is integral to the ultra-low lattice thermal conductivity mechanism in SnSe. △ Less

Submitted 13 January, 2023; originally announced January 2023.

High-temperature polaronic lattice distortions and charge ordering through the charge-density wave and quantum spin liquid phase transitions in 1T-TaS$_{2}$

Authors: E. S. Bozin, M. Abeykoon, S. Conradson, G. Baldinozzi, P. Sutar, D. Mihailovic

Abstract: Interesting emergent behavior in quantum materials arises when the interaction of electrons with the lattice leads to partial localization and ordering of charge at low temperatures. The triangular lattice of some transition metal dichalcogenides additionally presents an interesting case, where spin order is frustrated, leading to an additional complex interplay of interactions involving spin, cha… ▽ More Interesting emergent behavior in quantum materials arises when the interaction of electrons with the lattice leads to partial localization and ordering of charge at low temperatures. The triangular lattice of some transition metal dichalcogenides additionally presents an interesting case, where spin order is frustrated, leading to an additional complex interplay of interactions involving spin, charge and lattice degrees of freedom. Here we present a study of local symmetry breaking of the lattice structure in the layered dichalcogenide material 1T-TaS$_{2}$ using x-ray pair-distribution function measurements. Remarkably, we observe symmetry-breaking polaronic distortions of the lattice structure around individual localized electrons at temperatures well above any of the known long-range ordered phases. These characteristic polaronic signatures remain on cooling through the spin and charge ordered states, eventually revealing a new transition near 50 K to a state displaying partially restored symmetry and significant inter-layer dimerization. The order parameter associated with the observed local atomic displacements and symmetry-allowed polaron spin structure in the ground state suggests that charge ordering is driven by the crystallization of polarons, rather than conventional Fermi surface nesting. Symmetry analysis shows that the distorted structure is consistent with a breakup of the QSL phase at low temperature, concurrent with the disappearance of domains in the charge order. △ Less

Submitted 13 January, 2023; originally announced January 2023.

Comments: 16 pages, 4 figures

Structure of Charge Density Waves in La$_{1.875}$Ba$_{0.125}$CuO$_4$

Authors: J. Sears, Y. Shen, M. J. Krogstad, H. Miao, E. S. Bozin, I. K. Robinson, G. D. Gu, R. Osborn, S. Rosenkranz, J. M. Tranquada, M. P. M. Dean

Abstract: Although charge-density wave (CDW) correlations exist in several families of cuprate supercon-ductors, they exhibit substantial variation in CDW wavevector and correlation length, indicating a key role for CDW-lattice interactions. We investigated this interaction in La$_{1.875}$Ba$_{0.125}$CuO$_4$ using single crystal x-ray diffraction to collect a large number of CDW peak intensities, and determ… ▽ More Although charge-density wave (CDW) correlations exist in several families of cuprate supercon-ductors, they exhibit substantial variation in CDW wavevector and correlation length, indicating a key role for CDW-lattice interactions. We investigated this interaction in La$_{1.875}$Ba$_{0.125}$CuO$_4$ using single crystal x-ray diffraction to collect a large number of CDW peak intensities, and determined the Cu and La/Ba atomic distortions induced by the formation of CDW order. Within the CuO$_2$ planes, the distortions involve a periodic modulation of the Cu-Cu spacing along the direction of the ordering wave vector. The charge ordering within the copper-oxygen layer induces an out-of-plane breathing modulation of the surrounding lanthanum layers, which leads to a related distortion on the adjacent copper-oxygen layer. Our result implies that the CDW-related structural distortions do not remain confined to a single layer but rather propagate an appreciable distance through the crystal. This leads to overlapping structural modulations, in which CuO$_2$ planes exhibit distortions arising from the orthogonal CDWs in adjacent layers as well as distortions from the CDW within the layer itself. We attribute this striking effect to the weak c-axis charge screening in cuprates and suggest this effect could help couple the CDW between adjacent planes in the crystal. △ Less

Submitted 20 February, 2023; v1 submitted 22 December, 2022; originally announced December 2022.

Comments: 9 pages; Accepted in Phys. Rev. B

Journal ref: Phys. Rev. B 107, 115125 (2023)

Role of Local Ru Hexamers in Superconductivity of Ruthenium Phosphide

Authors: Robert J. Koch, Niraj Aryal, Oleh Ivashko, Yu Liu, Milinda Abeykoon, Eric D. Bauer, Martin v. Zimmermann, Weiguo Yin, Cedomir Petrovic, Emil S. Bozin

Abstract: Superconductivity in binary ruthenium pnictides occurs proximal to and upon suppression of a mysterious non-magnetic ground state, preceded by a pseudogap phase associated with Fermi surface instability, and its critical temperature, T$_{c}$, is maximized around the pseudogap quantum critical point. By analogy with isoelectronic iron based counterparts, antiferromagnetic fluctuations became "usual… ▽ More Superconductivity in binary ruthenium pnictides occurs proximal to and upon suppression of a mysterious non-magnetic ground state, preceded by a pseudogap phase associated with Fermi surface instability, and its critical temperature, T$_{c}$, is maximized around the pseudogap quantum critical point. By analogy with isoelectronic iron based counterparts, antiferromagnetic fluctuations became "usual suspects" as putative mediators of superconducting pairing. Here we report on a high temperature local symmetry breaking in RuP, the parent of the maximum-Tc branch of these novel superconductors, revealed by combined nanostructure-sensitive powder and single crystal X-ray total scattering experiments. Large local Ru$_{6}$ hexamer distortions associated with orbital-charge trimerization form above the two-stage electronic transition in RuP. While hexamer ordering enables the nonmagnetic ground state and presumed complex oligomerization, the relevance of pseudogap fluctuations for superconductivity emerges as a distinct prospect. As a transition metal system in which partial d-manifold filling combined with high crystal symmetry promotes electronic instabilities, this represents a further example of local electronic precursors underpinning the macroscopic collective behavior of quantum materials. △ Less

Submitted 19 August, 2022; originally announced August 2022.

Comments: 10 pages 6 figures

Journal ref: Phys. Rev. B 106, 214516 (2022)

Strongly-overdoped La$_{2-x}$Sr$_x$CuO$_4$: Evidence for Josephson-coupled grains of strongly-correlated superconductor

Authors: Yangmu Li, A. Sapkota, P. M. Lozano, Zengyi Du, Hui Li, Zebin Wu, Asish K. Kundu, R. J. Koch, Lijun Wu, B. L. Winn, Songxue Chi, M. Matsuda, M. Frontzek, E. S. Bozin, Yimei Zhu, I. Bozovic, Abhay N. Pasupathy, Ilya K. Drozdov, Kazuhiro Fujita, G. D. Gu, Igor Zaliznyak, Qiang Li, J. M. Tranquada

Abstract: The interpretation of how superconductivity disappears in cuprates at large hole doping has been controversial. To address this issue, we present an experimental study of single-crystal and thin film samples of La$_{2-x}$Sr$_x$CuO$_4$ (LSCO) with $x\ge0.25$. In particular, measurements of bulk susceptibility on LSCO crystals with $x=0.25$ indicate an onset of diamagnetism at $T_{c1}=38.5$ K, with… ▽ More The interpretation of how superconductivity disappears in cuprates at large hole doping has been controversial. To address this issue, we present an experimental study of single-crystal and thin film samples of La$_{2-x}$Sr$_x$CuO$_4$ (LSCO) with $x\ge0.25$. In particular, measurements of bulk susceptibility on LSCO crystals with $x=0.25$ indicate an onset of diamagnetism at $T_{c1}=38.5$ K, with a sharp transition to a phase with full bulk shielding at $T_{c2}=18$ K, independent of field direction. Strikingly, the in-plane resistivity only goes to zero at $T_{c2}$. Inelastic neutron scattering on $x=0.25$ crystals confirms the presence of low-energy incommensurate magnetic excitations with reduced strength compared to lower doping levels. The ratio of the spin gap to $T_{c2}$ is anomalously large. Our results are consistent with a theoretical prediction for strongly overdoped cuprates by Spivak, Oreto, and Kivelson, in which superconductivity initially develops within disconnected self-organized grains characterized by a reduced hole concentration, with bulk superconductivity occurring only after superconductivity is induced by proximity effect in the surrounding medium of higher hole concentration. Beyond the superconducting-to-metal transition, local differential conductance measurements on an LSCO thin film suggest that regions with pairing correlations survive, but are too dilute to support superconducting order. Future experiments will be needed to test the degree to which these results apply to overdoped cuprates in general. △ Less

Submitted 21 December, 2022; v1 submitted 3 May, 2022; originally announced May 2022.

Comments: 21 pages, 15 figures; added more sample characterizations, results for another sample, more discussion; accepted version

Journal ref: Phys. Rev. B 106, 224515 (2022)

On single crystal total scattering data reduction and correction protocols for analysis in direct space

Authors: Robert J. Koch, Nikolaj Roth, Yiu Liu, Oleh Ivashko, Ann-Christin Dippel, Cedomir Petrovic, Bo B. Iversen, Martin v. Zimmermann, Emil S. Bozin

Abstract: We explore data reduction and correction steps and processed data reproducibility in the emerging single crystal total scattering based technique of three-dimensional differential atomic pair distribution function (3D-$Δ$PDF) analysis. All steps from sample measurement to data-processing are outlined in detail using a CuIr$_2$S$_4$ example crystal studied in a setup equipped with a high-energy x-r… ▽ More We explore data reduction and correction steps and processed data reproducibility in the emerging single crystal total scattering based technique of three-dimensional differential atomic pair distribution function (3D-$Δ$PDF) analysis. All steps from sample measurement to data-processing are outlined in detail using a CuIr$_2$S$_4$ example crystal studied in a setup equipped with a high-energy x-ray beam and a flat panel area detector. Computational overhead as it pertains to data-sampling and the associated data processing steps is also discussed. Various aspects of the final 3D-$Δ$PDF reproducibility are explicitly tested by varying data-processing order and included steps, and by carrying out a crystal-to-crystal data comparison. We identify situations in which the 3D-$Δ$PDF is robust, and caution against a few particular cases which can lead to inconsistent 3D-$Δ$PDFs. Although not all the approaches applied here-in will be valid across all systems, and a more in-depth analysis of some of the effects of the data processing steps may still needed, the methods collected here-in represent the start of a more systematic discussion about data processing and corrections in this field. △ Less

Submitted 20 July, 2021; v1 submitted 14 May, 2021; originally announced May 2021.

Journal ref: Acta Cryst. A77, 611 (2021)

Dual Orbital Degeneracy Lifting in a Strongly Correlated Electron System

Authors: R. J. Koch, R. Sinclair, M. T. McDonnell, R. Yu, M. Abeykoon, M. G. Tucker, A. M. Tsvelik, S. J. L. Billinge, H. D. Zhou, W. -G. Yin, E. S. Bozin

Abstract: The local structure of NaTiSi$_{2}$O$_{6}$ is examined across its Ti-dimerization orbital-assisted Peierls transition at 210 K. An atomic pair distribution function approach evidences local symmetry breaking preexisting far above the transition. The analysis unravels that on warming the dimers evolve into a short range orbital degeneracy lifted (ODL) state of dual orbital character, persisting up… ▽ More The local structure of NaTiSi$_{2}$O$_{6}$ is examined across its Ti-dimerization orbital-assisted Peierls transition at 210 K. An atomic pair distribution function approach evidences local symmetry breaking preexisting far above the transition. The analysis unravels that on warming the dimers evolve into a short range orbital degeneracy lifted (ODL) state of dual orbital character, persisting up to at least 490 K. The ODL state is correlated over the length scale spanning $\sim$6 sites of the Ti zigzag chains. Results imply that the ODL phenomenology extends to strongly correlated electron systems. △ Less

Submitted 29 September, 2020; originally announced September 2020.

Comments: 7 pages, 5 figures

Journal ref: Phys. Rev. Lett. 126, 186402 (2021)

Comment on "Colossal Pressure-Induced Softening in Scandium Fluoride"

Authors: I. A. Zaliznyak, E. Bozin, A. V. Tkachenko

Abstract: The results reported by Wei et al. [Phys. Rev. Lett. 124, 255502 (2020)] can be confronted with predictive, quantitative theories of negative thermal expansion (NTE) and pressure-induced softening, allowing to corroborate, or invalidate certain approaches. Motivated to corroborate the quantitative predictions of the recent Coulomb Floppy Network (CFN) microscopic theory of vibrational and thermome… ▽ More The results reported by Wei et al. [Phys. Rev. Lett. 124, 255502 (2020)] can be confronted with predictive, quantitative theories of negative thermal expansion (NTE) and pressure-induced softening, allowing to corroborate, or invalidate certain approaches. Motivated to corroborate the quantitative predictions of the recent Coulomb Floppy Network (CFN) microscopic theory of vibrational and thermomechanical properties of empty perovskite crystals [Tkachenko and Zaliznyak, arXiv:1908.11643 (2019)], we compared theory prediction for the mean-squared transverse displacement of the F atoms, U$_{perp}$, with that reported in Fig. 5 of Wei et al. and observed a marked discrepancy (an order-of-magnitude larger than the error bar). We then compared these results with the previously published Xray diffraction data of Greve, et al. [JACS 132, 15496 (2010)] and the neutron diffraction data of Wendt, et al. [Science Advances 5 (2019), 10.1126/sciadv.aay2748]. We found the latter two data sets to be in a good agreement with each other, as well as with the prediction of CFN theory. We thus conclude that U$_{perp}$ values reported in Fig. 5 of Wei et al. are substantially incorrect. The purpose of this Comment is twofold: (i) to caution the researchers against using the U$_{perp}$ data of Wei et al. for quantitative comparisons with theory, and (ii) to encourage Wei et al. to reconsider their analysis and obtain a reliable U$_{perp}$ data by better accounting for the beam transmission and attenuation effects. △ Less

Submitted 24 September, 2020; originally announced September 2020.

Comments: 1 page, 1 figure

Journal ref: Phys. Rev. Lett. 126, 179601 (2021)

Two-orbital degeneracy lifted state as a local precursor to a metal-insulator transition

Authors: Long Yang, Robert J. Koch, Hong Zheng, J. F. Mitchell, Weiguo Yin, Matthew G. Tucker, Simon J. L. Billinge, Emil S. Bozin

Abstract: The recent discovery of a local fluctuating t2g orbital-degeneracy-lifted (ODL) state in CuIr2S4 as a high temperature precursor to the metal-insulator transition (MIT) opens the door to a possible widespread presence of precursor states in scarcely studied high-temperature regimes of transition metal based quantum materials. Although in CuIr2S4 the ODL state comprises one orbital per Ir, there is… ▽ More The recent discovery of a local fluctuating t2g orbital-degeneracy-lifted (ODL) state in CuIr2S4 as a high temperature precursor to the metal-insulator transition (MIT) opens the door to a possible widespread presence of precursor states in scarcely studied high-temperature regimes of transition metal based quantum materials. Although in CuIr2S4 the ODL state comprises one orbital per Ir, there is no fundamental reason to exclude multi-orbital ODL states in general. The MgTi2O4 spinel exhibits a MIT on cooling at Ts ~250 K, accompanied by Ti t2g orbital ordering (OO) and spin dimerization with the average symmetry reducing to tetragonal. It shares with CuIr2S4 the pyrochlore transition metal sublattice with active t2g orbitals. This, together with its different orbital filling (t2g1 vs t2g5.5) make it a candidate for hosting a multi-orbital ODL precursor state. By combining x-ray and neutron pair distribution function analyses to track the evolution of the local atomic structure across the MIT we find that local tetragonality already exists in the metallic globally cubic phase at high temperature. Local distortions exist up to at least 500 K. Significantly, the high temperature local state is not continuously connected to the OO band insulator ground state, and so the transition cannot be characterized as a trivial order-disorder type. The shortest Ti-Ti spin singlet dimer bonds expand abruptly on warming across the transition but remain shorter than those seen in the cubic structure. These seemingly contradictory observations can be understood within the model of a local fluctuating two-orbital t2g ODL precursor state. The ODL state in MgTi2O4 has a correlation length of about 1 nm at high temperature. We discuss that this extended character of the local distortions is consistent with the two-orbital nature of the ODL state imposed by the charge filling and the bond charge repulsion. △ Less

Submitted 18 September, 2020; originally announced September 2020.

Comments: 13 pages, 8 figures

Journal ref: Phys. Rev. B 102, 235128 (2020)

Three-dimensional Ising Ferrimagnetism of Cr-Fe-Cr trimers in FeCr2Te4

Authors: Yu Liu, R. J. Koch, Zhixiang Hu, Niraj Aryal, Eli Stavitski, Xiao Tong, Klaus Attenkofer, E. S. Bozin, Weiguo Yin, C. Petrovic

Abstract: We carried out a comprehensive study of magnetic critical behavior in single crystals of ternary chalcogenide FeCr$_2$Te$_4$ that undergoes a ferrimagnetic transition below $T_c$ $\sim$ 123 K. Detailed critical behavior analysis and scaled magnetic entropy change indicate a second-order ferrimagentic transition. Critical exponents $β= 0.30(1)$ with $T_c = 122.4(5)$ K, $γ= 1.22(1)$ with… ▽ More We carried out a comprehensive study of magnetic critical behavior in single crystals of ternary chalcogenide FeCr$_2$Te$_4$ that undergoes a ferrimagnetic transition below $T_c$ $\sim$ 123 K. Detailed critical behavior analysis and scaled magnetic entropy change indicate a second-order ferrimagentic transition. Critical exponents $β= 0.30(1)$ with $T_c = 122.4(5)$ K, $γ= 1.22(1)$ with $T_c = 122.8(1)$ K, and $δ= 4.24(2)$ at $T_c$ $\sim$ 123 K suggest that the spins approach three-dimensional Ising ($β$ = 0.325, $γ$ = 1.24, and $δ$ = 4.82) model coupled with the attractive long-range interactions between spins that decay as $J(r)\approx r^{-4.88}$. Our results suggest that the ferrimagnetism in FeCr$_2$Te$_4$ is due to itinerant ferromagnetism among the antiferromagnetically coupled Cr-Fe-Cr trimers. △ Less

Submitted 18 August, 2020; v1 submitted 14 July, 2020; originally announced July 2020.

Comments: Accepted in PRB

Journal ref: Phys. Rev. B 102, 085158 (2020)

Vacancy-driven non-cubic local structure and magnetic anisotropy tailoring in Fe$_x$O-Fe$_{3-δ}$O$_4$ nanocrystals

Authors: Alexandros Lappas, George Antonaropoulos, Konstantinos Brintakis, Marianna Vasilakaki, Kalliopi N. Trohidou, Vincenzo Iannotti, Giovanni Ausanio, Athanasia Kostopoulou, Milinda Abeykoon, Ian K. Robinson, Emil S. Bozin

Abstract: In contrast to bulk materials, nanoscale crystal growth is critically influenced by size- and shape-dependent properties. However, it is challenging to decipher how stoichiometry, in the realm of mixed-valence elements, can act to control physical properties, especially when complex bonding is implicated by short and long-range ordering of structural defects. Here, solution-grown iron-oxide nanocr… ▽ More In contrast to bulk materials, nanoscale crystal growth is critically influenced by size- and shape-dependent properties. However, it is challenging to decipher how stoichiometry, in the realm of mixed-valence elements, can act to control physical properties, especially when complex bonding is implicated by short and long-range ordering of structural defects. Here, solution-grown iron-oxide nanocrystals (NCs) of the pilot wustite system are found to convert into iron-deficient rock-salt and ferro-spinel sub-domains, but attain a surprising tetragonally distorted local structure. Cationic vacancies within chemically uniform NCs are portrayed as the parameter to tweak the underlying properties. These lattice imperfections are shown to produce local exchange-anisotropy fields that reinforce the nanoparticles magnetization and overcome the influence of finite-size effects. The concept of atomic-scale defect control in subcritical size NCs, aspires to become a pathway to tailor-made properties with improved performance for hyperthermia heating over defect-free NCs. △ Less

Submitted 16 May, 2020; originally announced May 2020.

Comments: 20 pages, 9 figures, Supplemental Material

Journal ref: Phys. Rev. X 9, 041044 (2019)

Pressure induced topological quantum phase transition in Weyl semimetal T_d-MoTe_2

Authors: Z. Guguchia, A. M. dos Santos, F. O. von Rohr, J. J. Molaison, S. Banerjee, D. Rhodes, J. -X. Yin, R. Khasanov, J. Hone, Y. J. Uemura, M. -Z. Hasan, H. Luetkens, E. S. Bozin, A. Amato

Abstract: We report the pressure (p_max = 1.5 GPa) evolution of the crystal structure of the Weyl semimetal T_d-MoTe_2 by means of neutron diffraction experiments. We find that the fundamental non-centrosymmetric structure T_d is fully suppressed and transforms into a centrosymmertic 1T' structure at a critical pressure of p_cr = 1.2 GPa. This is strong evidence for a pressure induced quantum phase transiti… ▽ More We report the pressure (p_max = 1.5 GPa) evolution of the crystal structure of the Weyl semimetal T_d-MoTe_2 by means of neutron diffraction experiments. We find that the fundamental non-centrosymmetric structure T_d is fully suppressed and transforms into a centrosymmertic 1T' structure at a critical pressure of p_cr = 1.2 GPa. This is strong evidence for a pressure induced quantum phase transition (QPT) between topological to a trivial electronic state. Although the topological QPT has strong effect on magnetoresistance, it is interesting that the superconducting critical temperature T_c, the superfluid density, and the SC gap all change smoothly and continuously across p_cr and no sudden effects are seen concomitantly with the suppression of the T_d structure. This implies that the T_c, and thus the SC pairing strength, is unaffected by the topological QPT. However, the QPT requires the change in the SC gap symmetry from non-trivial s+- to a trivial s++ state, which we discuss in this work. Our systematic characterizations of the structure and superconducting properties associated with the topological QPT provide deep insight into the pressure induced phase diagram in this topological quantum material. △ Less

Submitted 28 March, 2020; originally announced March 2020.

Comments: 9 pages, 4 figures

Journal ref: J. Phys. Soc. Jpn. 89, 094707 (2020)

Charge density waves in cuprate superconductors beyond the critical doping

Authors: H. Miao, G. Fabbris, R. J. Koch, D. G. Mazzone, C. S. Nelson, R. Acevedo-Esteves, Y. Li, G. D. Gu, T. Yilmaz, K. Kaznatcheev, E. Vescovo, M. Oda, K. Kurosawa, N. Momono, T. A. Assefa, I. K. Robinson, E. Bozin, J. M. Tranquada, P. D. Johnson, M. P. M. Dean

Abstract: The unconventional normal-state properties of the cuprates are often discussed in terms of emergent electronic order that onsets below a putative critical doping of xc = 0.19. Charge-density wave (CDW) correlations represent one such order; however, experimental evidence for such order generally spans a limited range of doping that falls short of the critical value xc, leading to questions regardi… ▽ More The unconventional normal-state properties of the cuprates are often discussed in terms of emergent electronic order that onsets below a putative critical doping of xc = 0.19. Charge-density wave (CDW) correlations represent one such order; however, experimental evidence for such order generally spans a limited range of doping that falls short of the critical value xc, leading to questions regarding its essential relevance. Here, we use x-ray diffraction to demonstrate that CDW correlations in La2-xSrxCuO4 persist up to a doping of at least x = 0.21. The correlations show strong changes through the superconducting transition, but no obvious discontinuity through xc = 0.19, despite changes in Fermi surface topology and electronic transport at this doping. These results demonstrate the interaction between CDWs and superconductivity even in overdoped cuprates and prompt a reconsideration of the role of CDW correlations in the high-temperature cuprate phase diagram. △ Less

Submitted 20 February, 2021; v1 submitted 28 January, 2020; originally announced January 2020.

Comments: 8 pages + 5 pages of supplemental material; accepted in npj Quantum Materials

Journal ref: npj Quantum Materials 6, 31 (2021)

Nanoscale degeneracy lifting in a geometrically frustrated antiferromagnet

Authors: Benjamin A. Frandsen, Emil S. Bozin, Eleni Aza, Antonio Fernandez Martinez, Mikhail Feygenson, Katharine Page, Alexandros Lappas

Abstract: The local atomic and magnetic structures of the compounds $A$MnO$_2$ ($A$ = Na, Cu), which realize a geometrically frustrated, spatially anisotropic triangular lattice of Mn spins, have been investigated by atomic and magnetic pair distribution function analysis of neutron total scattering data. Relief of frustration in CuMnO$_2$ is accompanied by a conventional cooperative symmetry-lowering latti… ▽ More The local atomic and magnetic structures of the compounds $A$MnO$_2$ ($A$ = Na, Cu), which realize a geometrically frustrated, spatially anisotropic triangular lattice of Mn spins, have been investigated by atomic and magnetic pair distribution function analysis of neutron total scattering data. Relief of frustration in CuMnO$_2$ is accompanied by a conventional cooperative symmetry-lowering lattice distortion driven by Néel order. In NaMnO$_2$, however, the distortion has a short-range nature. A cooperative interaction between the locally broken symmetry and short-range magnetic correlations lifts the magnetic degeneracy on a nanometer length scale, enabling long-range magnetic order in the Na-derivative. The degree of frustration, mediated by residual disorder, contributes to the rather differing pathways to a single, stable magnetic ground state in these two related compounds. This study demonstrates how nanoscale structural distortions that cause local-scale perturbations can lift the ground state degeneracy and trigger macroscopic magnetic order. △ Less

Submitted 27 January, 2020; originally announced January 2020.

Journal ref: Phys. Rev. B 101, 024423 (2020)

Quantum liquid with strong orbital fluctuations: the case of a pyroxene family

Authors: A. Feiguin, A. M. Tsvelik, Weiguo Yin, E. S. Bozin

Abstract: We discuss quasi one-dimensional magnetic Mott insulators of the pyroxene family where spin and orbital degrees of freedom remain tightly bound. We analyze their excitation spectrum and outline the conditions under which the orbital degrees of freedom become liberated so that the excitations become dispersive and the spectral weight shifts to energies much smaller than the exchange integral. We discuss quasi one-dimensional magnetic Mott insulators of the pyroxene family where spin and orbital degrees of freedom remain tightly bound. We analyze their excitation spectrum and outline the conditions under which the orbital degrees of freedom become liberated so that the excitations become dispersive and the spectral weight shifts to energies much smaller than the exchange integral. △ Less

Submitted 18 October, 2019; v1 submitted 22 August, 2019; originally announced August 2019.

Comments: 5 pages, 5 figures, Supplemental Material, revised version

Entropic elasticity and negative thermal expansion in a simple cubic crystal

Authors: David Wendt, Emil Bozin, Joerg Neuefeind, Katharine Page, Wei Ku, Limin Wang, Brent Fultz, Alexei Tkachenko, Igor Zaliznyak

Abstract: While most solids expand when heated, some materials show the opposite behavior: negative thermal expansion (NTE). In polymers and biomolecules, NTE originates from the entropic elasticity of an ideal, freely-jointed chain. The origin of NTE in solids has been widely believed to be different. Our neutron scattering study of a simple cubic NTE material, ScF3, overturns this consensus. We observe th… ▽ More While most solids expand when heated, some materials show the opposite behavior: negative thermal expansion (NTE). In polymers and biomolecules, NTE originates from the entropic elasticity of an ideal, freely-jointed chain. The origin of NTE in solids has been widely believed to be different. Our neutron scattering study of a simple cubic NTE material, ScF3, overturns this consensus. We observe that the correlation in the positions of the neighboring fluorine atoms rapidly fades on warming, indicating an uncorrelated thermal motion constrained by the rigid Sc-F bonds. This leads us to a quantitative theory of NTE in terms of entropic elasticity of a floppy network crystal, which is in remarkable agreement with experimental results. We thus reveal the formidable universality of the NTE phenomenon in soft and hard matter. △ Less

Submitted 12 June, 2019; originally announced June 2019.

Comments: 6 pages, 4 figures. Work reported at 2019 APS March meeting (http://meetings.aps.org/Meeting/MAR19/Session/B33.8)

Journal ref: Science Advances 5 (11), eaay2748 (2019)

Photoinduced ultrafast dynamics of local nematicity and lattice distortions in FeSe crystals

Authors: T. Konstantinova, L. Wu, M. Abeykoon, R. J. Koch, A. F. Wang, R. K. Li, X. Shen, J. Li, J. Tao, I. A. Zaliznyak, C. Petrovic, S. J. L. Billinge, X. J. Wang, E. S. Bozin, Y. Zhu

Abstract: Formation of electronic nematicity is a common thread of unconventional superconductors. In iron-based materials, the long-range nematic order is revealed by the orthorhombic lattice distortion, which importance is a highly controversial topic due the small magnitude of the distortion. Here, we study the local crystal structure of FeSe and its interaction with electronic degrees of freedom using u… ▽ More Formation of electronic nematicity is a common thread of unconventional superconductors. In iron-based materials, the long-range nematic order is revealed by the orthorhombic lattice distortion, which importance is a highly controversial topic due the small magnitude of the distortion. Here, we study the local crystal structure of FeSe and its interaction with electronic degrees of freedom using ultrafast electron diffraction, x-ray pair distribution function analysis, and transmission electron microscopy and find a significant lattice response to local nematicity. The study demonstrates how local lattice distortions, which exist even at temperatures above the nematic phase transition, can be released by photoexcitation, leading to an enhancement of the crystalline order. The observed local atomic structures and their out-of-equilibrium behavior unravel a sophisticated coupling between the lattice and nematic order parameter in FeSe. △ Less

Submitted 2 March, 2019; originally announced March 2019.

Journal ref: Phys. Rev. B 99, 180102 (2019)

Correlated disorder to order crossover in the local structure of K$_x$Fe$_{2-y}$Se$_{2-z}$S$_z$

Authors: P. Mangelis, R. J. Koch, H. Lei, R. B. Neder, M. T. McDonnell, M. Feygenson, C. Petrovic, A. Lappas, E. S. Bozin

Abstract: A detailed account of the local atomic structure and disorder at 5~K across the phase diagram of the high temperature superconductor K$_x$Fe$_{2-y}$Se$_{2-z}$S$_z$ $(0 \leq z \leq 2)$ is obtained from neutron total scattering and associated atomic pair distribution function (PDF) approaches. Various model independent and model dependent aspects of the analysis reveal a high level of structural com… ▽ More A detailed account of the local atomic structure and disorder at 5~K across the phase diagram of the high temperature superconductor K$_x$Fe$_{2-y}$Se$_{2-z}$S$_z$ $(0 \leq z \leq 2)$ is obtained from neutron total scattering and associated atomic pair distribution function (PDF) approaches. Various model independent and model dependent aspects of the analysis reveal a high level of structural complexity on the nanometer length-scale. Evidence is found for considerable disorder in the $c$-axis stacking of the FeSe$_{1-x}$S$_{x}$ slabs without observable signs of turbostratic character of the disorder. In contrast to the related FeCh (Ch = S, Se) type superconductors, substantial Fe-vacancies are present in K$_x$Fe$_{2-y}$Se$_{2-z}$S$_z$, deemed detrimental for superconductivity when ordered. Our study suggests that the distribution of vacancies significantly modifies the iron-chalcogen bond-length distribution, in agreement with observed evolution of the PDF signal. A crossover like transition is observed at a composition of $z\approx1$, from a predominantly vacancy-disordered state at the selenium end to a more vacancy-ordered (VO) phase closer to the sulfur end of the phase diagram. The S-content dependent measures of the local structure are found to exhibit distinct behavior on either side of this crossover, correlating well with the evolution of the superconducting state to that of a magnetic semiconductor towards the $z\approx2$ end. The behavior reinforces the idea of the intimate relationship of correlated Fe-vacancies order in the local structure and the emergent electronic properties. △ Less

Submitted 19 September, 2019; v1 submitted 28 February, 2019; originally announced March 2019.

Comments: 15 pages, 11 figures

Journal ref: Phys. Rev. B 100, 094108 (2019)

Room temperature local nematicity in FeSe superconductor

Authors: R. J. Koch, T. Konstantinova, M. Abeykoon, A. Wang, C. Petrovic, Y. Zhu, E. S. Bozin, S. J. L. Billinge

Abstract: We report pair distribution function measurements of the iron-based superconductor FeSe above and below the structural transition temperature. Structural analysis reveals a local orthorhombic distortion with a correlation length of about 4 nm at temperatures where an average tetragonal symmetry is observed. The analysis further demonstrates that the local distortion is larger than the distortion a… ▽ More We report pair distribution function measurements of the iron-based superconductor FeSe above and below the structural transition temperature. Structural analysis reveals a local orthorhombic distortion with a correlation length of about 4 nm at temperatures where an average tetragonal symmetry is observed. The analysis further demonstrates that the local distortion is larger than the distortion at temperatures where the average observed symmetry is orthorhombic. Our results suggest that the low-temperature macroscopic nematic state in FeSe forms from an imperfect ordering of orbital-degeneracy-lifted nematic fluctuations which persist up to at least 300 K. △ Less

Submitted 18 June, 2019; v1 submitted 22 February, 2019; originally announced February 2019.

Comments: 6 pages, 4 figures

Journal ref: Phys. Rev. B 100, 020501 (2019)

Local orbital degeneracy lifting as a precursor to an orbital-selective Peierls transition

Authors: E. S. Bozin, W. G. Yin, R. J. Koch, M. Abeykoon, Y. S. Hor, H. Zheng, H. C. Lei, C. Petrovic, J. F. Mitchell, S. J. L. Billinge

Abstract: Fundamental electronic principles underlying all transition metal compounds are the symmetry and filling of the $d$-electron orbitals and the influence of this filling on structural configurations and responses. Here we use a sensitive local structural technique, x-ray atomic pair distribution function analysis, to reveal the presence of fluctuating local-structural distortions at high temperature… ▽ More Fundamental electronic principles underlying all transition metal compounds are the symmetry and filling of the $d$-electron orbitals and the influence of this filling on structural configurations and responses. Here we use a sensitive local structural technique, x-ray atomic pair distribution function analysis, to reveal the presence of fluctuating local-structural distortions at high temperature of one such compound, \cis . We show that this hitherto overlooked fluctuating symmetry lowering is electronic in origin and will significantly modify the energy-level spectrum and electronic and magnetic properties. The explanation is a local, fluctuating, orbital-degeneracy-lifted state. The natural extension of our result would be that this phenomenon is likely to be widespread amongst diverse classes of partially filled nominally degenerate d-electron systems, with potentially broad implications for our understanding of their properties. △ Less

Submitted 9 March, 2019; v1 submitted 29 January, 2019; originally announced January 2019.

Comments: 10 pages 3 figures

Thermoelectric studies of Ir$_{1-x}$Rh$_{x}$Te$_{2}$ (0 $\leqslant x \leqslant $ 0.3)

Authors: Yu Liu, $^{1}$ Hechang Lei, $^{1}$ Kefeng Wang, $^{1}$ Milinda Abeykoon, $^{2}$ J. B. Warren, $^{3}$ Emil Bozin, $^{1}$, C. Petrovic$^{1}$

Abstract: We report thermoelectric properties of Ir$_{1-x}$Rh$_x$Te$_2$ ($0 \leqslant x \leqslant 0.3$) alloy series where superconductivity at low temperatures emerges as the high-temperature structural transition ($T_s$) is suppressed. The isovalent ionic substitution of Rh into Ir has different effects on physical properties when compared to the anionic substitution of Se into Te, in which the structural… ▽ More We report thermoelectric properties of Ir$_{1-x}$Rh$_x$Te$_2$ ($0 \leqslant x \leqslant 0.3$) alloy series where superconductivity at low temperatures emerges as the high-temperature structural transition ($T_s$) is suppressed. The isovalent ionic substitution of Rh into Ir has different effects on physical properties when compared to the anionic substitution of Se into Te, in which the structural transition is more stable with Se substitution. Rh substitution results in a slight reduction of lattice parameters and in an increase of number of carriers per unit cell. Weak-coupled BCS superconductivity in Ir$_{0.8}$Rh$_{0.2}$Te$_2$ that emerges at low temperature ($T_c^{zero}$ = 2.45 K) is most likely driven by electron-phonon coupling rather than dimer fluctuations mediated pairing. △ Less

Submitted 26 September, 2018; originally announced September 2018.

Journal ref: Physical Review B 98, 094519 (2018)

Emphanitic anharmonicity in PbSe at high temperature and the anomalous electronic properties in the PbQ (Q=S, Se, Te) system

Authors: Runze Yu, Emil S. Bozin, Milinda Abeykoon, Boris Sangiorgio, Nicola A. Spaldin, Christos D. Malliakas, Mercouri G. Kanatzidis, Simon J. L. Billinge

Abstract: The temperature dependence of the local structure of PbSe has been investigated using pair distribution function (PDF) analysis of x-ray and neutron powder diffraction data and density functional theory (DFT) calculations. Observation of non-Gaussian PDF peaks at high temperature indicates the presence of significant anharmonicity, which can be modeled as Pb off-centering along [100] directions th… ▽ More The temperature dependence of the local structure of PbSe has been investigated using pair distribution function (PDF) analysis of x-ray and neutron powder diffraction data and density functional theory (DFT) calculations. Observation of non-Gaussian PDF peaks at high temperature indicates the presence of significant anharmonicity, which can be modeled as Pb off-centering along [100] directions that grows on warming similar to the behavior seen in PbTe and PbS and sometimes called emphanisis. Interestingly, the emphanitic response is smaller in PbSe than in both PbS and PbTe indicating a non-monotonic response with chalcogen atomic number in the PbQ (Q=S, Se, Te) series. The DFT calculations indicate a correlation between band gap and the amplitude of [100] dipolar distortion, suggesting that emphanisis may be behind the anomalous composition and temperature dependencies of the band gaps in this series. △ Less

Submitted 2 May, 2018; originally announced May 2018.

Comments: 4 pages, 4 figures, includes supplemental information

Journal ref: Phys. Rev. B 98, 144108 (2018)

Absence of Local Fluctuating Dimers in Superconducting Ir$_{1-x}$(Pt,Rh)$_x$Te$_2$

Authors: R. Yu, S. Banerjee, H. Lei, R. Sinclair, M. Abeykoon, H. D. Zhou, C. Petrovic, Z. Guguchia, E. S. Bozin

Abstract: The compound IrTe2 is known to exhibit a transition to a modulated state featuring Ir-Ir dimers, with large associated atomic displacements. Partial substitution of Pt or Rh for Ir destabilizes the modulated structure and induces superconductivity. It has been proposed that quantum critical dimer fluctuations might be associated with the superconductivity. Here we test for such local dimer correla… ▽ More The compound IrTe2 is known to exhibit a transition to a modulated state featuring Ir-Ir dimers, with large associated atomic displacements. Partial substitution of Pt or Rh for Ir destabilizes the modulated structure and induces superconductivity. It has been proposed that quantum critical dimer fluctuations might be associated with the superconductivity. Here we test for such local dimer correlations and demonstrate their absence. X-ray pair distribution function approach reveals that the local structure of Ir0.95Pt0.05Te2 and Ir0.8Rh0.2Te2 dichalcogenide superconductors with compositions just past the dimer/superconductor boundary is explained well by a dimer-free model down to 10 K, ruling out the possibility of there being nanoscale dimer fluctuations in this regime. This is inconsistent with the proposed quantum-critical-point-like interplay of the dimer state and △ Less

Submitted 29 December, 2017; originally announced January 2018.

Comments: 4 pages, 4 figures

Journal ref: Phys. Rev. B 97, 174515 (2018)

Correlated local dipoles in PbTe

Authors: Boris Sangiorgio, Emil S. Bozin, Christos D. Malliakas, Michael Fechner, Arkadiy Simonov, Mercouri G. Kanatzidis, Simon J. L. Billinge, Nicola A. Spaldin, Thomas Weber

Abstract: We present a combined single-crystal x-ray diffuse scattering and ab-initio molecular dynamics study of lead telluride, PbTe. Well-known for its thermoelectric and narrow-gap semiconducting properties, PbTe recently achieved further notoriety following the report of an unusual off-centering of the lead atoms, accompanied by a local symmetry breaking, on heating. This observation, which was named e… ▽ More We present a combined single-crystal x-ray diffuse scattering and ab-initio molecular dynamics study of lead telluride, PbTe. Well-known for its thermoelectric and narrow-gap semiconducting properties, PbTe recently achieved further notoriety following the report of an unusual off-centering of the lead atoms, accompanied by a local symmetry breaking, on heating. This observation, which was named emphanisis, ignited considerable controversy regarding the details of the underlying local structure and the appropriate interpretation of the total scattering experiments. In this study, we demonstrate close agreement between our diffuse scattering measurements and our calculations, which allows us to analyze features such as higher-order correlations that are accessible in the simulations but not experimentally. This allowed us to discover an unusual correlated local dipole formation extending over several unit cells with an associated local reduction of the cubic symmetry in both our x-ray diffuse scattering measurements and our molecular dynamics simulations. Importantly, when averaged spatially or temporally, the most probable positions for the ions are at the centers of their coordination polyhedra. Our results therefore clarify the nature of the local symmetry breaking, and reveal the source of the earlier controversy regarding the existence or absence of off-centering. Finally, we provide an interpretation of the behavior in terms of coupled soft optical and acoustic modes, which is linked also to the high thermoelectric performance of PbTe. △ Less

Submitted 25 November, 2017; v1 submitted 8 September, 2017; originally announced September 2017.

Comments: Revision of the paper, results unchanged, improved clarity of the arguments and results

Journal ref: Phys. Rev. Materials 2, 085402 (2018)

Unconventional order-disorder phase transition in improper ferroelectric hexagonal manganites

Authors: Sandra H. Skjærvø, Quintin Meier, Mikhail Feygenson, Nicola A. Spaldin, Simon J. L. Billinge, Emil S. Bozin, Sverre M. Selbach

Abstract: The improper ferroelectricity in YMnO$_3$ and other related multiferroic hexagonal manganites are known to cause topologically protected ferroelectric domains that give rise to rich and diverse physical phenomena. The local structure and structural coherence across the ferroelectric transition, however, were previously not well understood. Here we reveal the evolution of the local structure with t… ▽ More The improper ferroelectricity in YMnO$_3$ and other related multiferroic hexagonal manganites are known to cause topologically protected ferroelectric domains that give rise to rich and diverse physical phenomena. The local structure and structural coherence across the ferroelectric transition, however, were previously not well understood. Here we reveal the evolution of the local structure with temperature in YMnO$_3$ using neutron total scattering techniques, and interpret them with the help of first-principles calculations. The results show that, at room temperature, the local and average structures are consistent with the established ferroelectric $P6_3cm$ symmetry. On heating, both local and average structural analyses show striking anomalies from $\sim 800$ K up to the Curie temperature consistent with increasing fluctuations of the order parameter angle. These fluctuations result in an unusual local symmetry lowering into a \textit{continuum of structures} on heating. This local symmetry breaking persists into the high-symmetry non-polar phase, constituting an unconventional type of order-disorder transition. △ Less

Submitted 11 April, 2018; v1 submitted 30 July, 2017; originally announced July 2017.

Comments: 10 pages, 5 figures

Journal ref: Phys. Rev. X 9, 031001 (2019)

Pressure tuning of structure, superconductivity and novel magnetic order in the Ce-underdoped electron-doped cuprate T'-Pr_1.3-xLa_0.7Ce_xCuO_4 (x = 0.1)

Authors: Z. Guguchia, T. Adachi, Z. Shermadini, T. Ohgi, J. Chang, E. Bozin, F. von Rohr, A. M. dos Santos, J. J. Molaison, R. Boehler, Y. Koike, A. R. Wieteska, B. A. Frandsen, E. Morenzoni, A. Amato, S. J. L. Billinge, Y. J. Uemura, R. Khasanov

Abstract: High-pressure neutron powder diffraction, muon-spin rotation and magnetization studies of the structural, magnetic and the superconducting properties of the Ce-underdoped superconducting (SC) electron-doped cuprate system T'-Pr_1.3-xLa_0.7Ce_xCuO_4 with x = 0.1 are reported. A strong reduction of the lattice constants a and c is observed under pressure. However, no indication of any pressure induc… ▽ More High-pressure neutron powder diffraction, muon-spin rotation and magnetization studies of the structural, magnetic and the superconducting properties of the Ce-underdoped superconducting (SC) electron-doped cuprate system T'-Pr_1.3-xLa_0.7Ce_xCuO_4 with x = 0.1 are reported. A strong reduction of the lattice constants a and c is observed under pressure. However, no indication of any pressure induced phase transition from T' to T structure is observed up to the maximum applied pressure of p = 11 GPa. Large and non-linear increase of the short-range magnetic order temperature T_so in T'-Pr_1.3-xLa_0.7Ce_xCuO_4 (x = 0.1) was observed under pressure. Simultaneously pressure causes a non-linear decrease of the SC transition temperature T_c. All these experiments establish the short-range magnetic order as an intrinsic and a new competing phase in SC T'-Pr_1.2La_0.7Ce_0.1CuO_4. The observed pressure effects may be interpreted in terms of the improved nesting conditions through the reduction of the in-plane and out-of-plane lattice constants upon hydrostatic pressure. △ Less

Submitted 23 April, 2017; originally announced April 2017.

Comments: 11 pages, 10 figures

Journal ref: Phys. Rev. B 96, 094515 (2017)

Probing IrTe2 crystal symmetry by polarized Raman scattering

Authors: N. Lazarević, E. S. Bozin, M. Šćepanović, M. Opačić, Hechang Lei, C. Petrovic, Z. V. Popović

Abstract: Polarized Raman scattering measurements on IrTe$_2$ single crystals carried out over 15~K - 640~K temperature range, and across the structural phase transition, reveal new insights regarding the crystal symmetry.\ In the high temperature regime three Raman active modes are observed at all studied temperatures above the structural phase transition, rather than two as predicted by the factor group a… ▽ More Polarized Raman scattering measurements on IrTe$_2$ single crystals carried out over 15~K - 640~K temperature range, and across the structural phase transition, reveal new insights regarding the crystal symmetry.\ In the high temperature regime three Raman active modes are observed at all studied temperatures above the structural phase transition, rather than two as predicted by the factor group analysis for the assumed $P\bar{3}m1$ symmetry. This indicates that the actual symmetry of the high temperature phase is lower than previously thought.\ Observation of an additional E$_g$ mode at high temperature can be explained by doubling of the original trigonal unit cell along the $c$-axis and within the $P\bar{3}c1$ symmetry.\ In the low temperature regime (below 245 K) the new Raman modes appear as a consequence of the symmetry lowering phase transition and corresponding increase of the primitive cell.\ All the modes observed below the phase transition temperature can be assigned within the monoclinic crystal symmetry.\ Temperature dependence of the Raman active phonons in both phases are mainly driven by anharmonicity effects. The results call for reconsideration of the crystallographic phases of IrTe$_2$. △ Less

Submitted 18 September, 2016; originally announced September 2016.

Journal ref: Phys. Rev. B 89, 224301 (2014)

Interlayer electronic transport in CaMnBi$_{2}$ antiferromagnet

Authors: Aifeng Wang, D. Graf, Lijun Wu, Kefeng Wang, E. Bozin, Yimei Zhu, C. Petrovic

Abstract: We report interlayer electronic transport in CaMnBi$_{2}$ single crystals. Quantum oscillations and angular magnetoresistance suggest coherent electronic conduction and valley polarized conduction of Dirac states. Small cyclotron mass, large mobility of carriers and nontrivial Berry's phase are consistent with the presence of Dirac fermions on the side wall of the warped cylindrical Fermi surface.… ▽ More We report interlayer electronic transport in CaMnBi$_{2}$ single crystals. Quantum oscillations and angular magnetoresistance suggest coherent electronic conduction and valley polarized conduction of Dirac states. Small cyclotron mass, large mobility of carriers and nontrivial Berry's phase are consistent with the presence of Dirac fermions on the side wall of the warped cylindrical Fermi surface. Similar to SrMnBi$_{2}$ that features an anisotropic Dirac cone, our results suggest that magnetic field-induced changes in the interlayer conduction are also present in layered bismuth-based materials with zero-energy line in momentum space created by the staggered alkaline earth atoms. △ Less

Submitted 13 September, 2016; originally announced September 2016.

Comments: 6 pages, 5 figures

Journal ref: Phys. Rev. B 94, 125118 (2016)

Multiband electronic transport in $α$-Yb$_{1-x}$Sr$_{x}$AlB$_{4}$ [x = 0, 0.19(3)] single crystals

Authors: Hyejin Ryu, Milinda Abeykoon, Emil Bozin, Yosuke Matsumoto, S. Nakatsuji, C. Petrovic

Abstract: We report on the evidence for the multiband electronic transport in $α$-YbAlB$_{4}$ and $α$-Yb$_{0.81(2)}$Sr$_{0.19(3)}$AlB$_{4}$. Multiband transport reveals itself below 10 K in both compounds via Hall effect measurements, whereas anisotropic magnetic ground state sets in below 3 K in $α$-Yb$_{0.81(2)}$Sr$_{0.19(3)}$AlB$_{4}$. Our results show that Sr$^{2+}$ substitution enhances conductivity, b… ▽ More We report on the evidence for the multiband electronic transport in $α$-YbAlB$_{4}$ and $α$-Yb$_{0.81(2)}$Sr$_{0.19(3)}$AlB$_{4}$. Multiband transport reveals itself below 10 K in both compounds via Hall effect measurements, whereas anisotropic magnetic ground state sets in below 3 K in $α$-Yb$_{0.81(2)}$Sr$_{0.19(3)}$AlB$_{4}$. Our results show that Sr$^{2+}$ substitution enhances conductivity, but does not change the quasiparticle mass of bands induced by heavy fermion hybridization. △ Less

Submitted 25 August, 2016; originally announced August 2016.

Comments: 5 pages, 4 figures

Journal ref: J. Phys. Condens. Matter. 28, 425602 (2016)

Superconducting Order from Disorder in 2H-TaSe$_{2-x}$S$_{x}$ (0$\leq$x$\leq$2)

Authors: Lijun Li, Xiaoyu Deng, Zhen Wang, Yu Liu, A. M. Milinda Abeykoon, E. Dooryhee, A. Tomic, Yanan Huang, J. B. Warren, E. S. Bozin, S. J . L. Billinge, Y. P. Sun, Yimei Zhu, G. Kotliar, C. Petrovic

Abstract: We report on the emergence of robust superconducting order in single crystal alloys of 2H-TaSe$_{2-x}$S$_{x}$ (0$\leq$x$\leq$2) . The critical temperature of the alloy is surprisingly higher than that of the two end compounds TaSe$_{2}$ and TaS$_{2}$. The evolution of superconducting critical temperature T$_{c} (x)$ correlates with the full width at half maximum of the Bragg peaks and with the lin… ▽ More We report on the emergence of robust superconducting order in single crystal alloys of 2H-TaSe$_{2-x}$S$_{x}$ (0$\leq$x$\leq$2) . The critical temperature of the alloy is surprisingly higher than that of the two end compounds TaSe$_{2}$ and TaS$_{2}$. The evolution of superconducting critical temperature T$_{c} (x)$ correlates with the full width at half maximum of the Bragg peaks and with the linear term of the high temperature resistivity. The conductivity of the crystals near the middle of the alloy series is higher or similar than that of either one of the end members 2H-TaSe$_{2}$ and/or 2H-TaS$_{2}$. It is known that in these materials superconductivity (SC) is in close competition with charge density wave (CDW) order. We interpret our experimental findings in a picture where disorder tilts this balance in favor of superconductivity by destroying the CDW order. △ Less

Submitted 7 December, 2016; v1 submitted 22 August, 2016; originally announced August 2016.

Comments: 24 pages, 4 figures, supplement. Nature Publishing Group Quantum Materials (in press)

Insulating and metallic spin glass in K$_{x}$Fe$_{2-δ-y}$Ni$_{y}$Se$_{2}$ (0.06 $\leq$ $y$ $\leq$ 1.44) single crystals

Authors: Hyejin Ryu, Milinda Abeykoon, Kefeng Wang, Hechang Lei, N. Lazarevic, J. B. Warren, E. S. Bozin, Z. V. Popovic, C. Petrovic

Abstract: We report electron doping effects by Ni in K$_{x}$Fe$_{2-δ-y}$Ni$_{y}$Se$_{2}$ (0.06 $\leq$ $y$ $\leq$ 1.44) single crystal alloys. A rich ground state phase diagram is observed. Small amount of Ni ($\sim$ 4\%) suppressed superconductivity below 1.8 K, inducing insulating spin glass magnetic ground state for higher Ni content. With further Ni substitution, metallic resistivity is restored. For hig… ▽ More We report electron doping effects by Ni in K$_{x}$Fe$_{2-δ-y}$Ni$_{y}$Se$_{2}$ (0.06 $\leq$ $y$ $\leq$ 1.44) single crystal alloys. A rich ground state phase diagram is observed. Small amount of Ni ($\sim$ 4\%) suppressed superconductivity below 1.8 K, inducing insulating spin glass magnetic ground state for higher Ni content. With further Ni substitution, metallic resistivity is restored. For high Ni concentration in the lattice the unit cell symmetry is high symmetry $I4/mmm$ with no phase separation whereas both $I4/m + I4/mmm$ space groups were detected in the phase separated crystals when concentration of Ni $<$ Fe. The absence of superconductivity coincides with the absence of crystalline Fe vacancy order. △ Less

Submitted 19 April, 2016; originally announced April 2016.

Comments: 6 pages, 4 figures

Journal ref: Phys. Rev. B 91, 184503 (2015)

Sustained phase separation and spin glass in Co-doped K$_{x}$Fe$_{2-y}$Se$_{2}$ single crystals

Authors: Hyejin Ryu, Kefeng Wang, M. Opacic, N. Lazarevic, J. B. Warren, Z. V. Popovic, E. S. Bozin, C. Petrovic

Abstract: We present Co substitution effects in K$_{x}$Fe$_{2-y-z}$Co$_{z}$Se$_{2}$ (0.06 $\leq$ $z$ $\leq$ 1.73) single crystal alloys. By 3.5\% of Co doping superconductivity is suppressed whereas phase separation of semiconducting K$_{2}$Fe$_{4}$Se$_{5}$ and superconducting/metallic K$_{x}$Fe$_{2}$Se$_{2}$ is still present. We show that the arrangement and distribution of superconducting phase (stripe ph… ▽ More We present Co substitution effects in K$_{x}$Fe$_{2-y-z}$Co$_{z}$Se$_{2}$ (0.06 $\leq$ $z$ $\leq$ 1.73) single crystal alloys. By 3.5\% of Co doping superconductivity is suppressed whereas phase separation of semiconducting K$_{2}$Fe$_{4}$Se$_{5}$ and superconducting/metallic K$_{x}$Fe$_{2}$Se$_{2}$ is still present. We show that the arrangement and distribution of superconducting phase (stripe phase) is connected with the arrangement of K, Fe and Co atoms. Semiconducting spin glass is found in proximity to superconducting state, persisting for large Co concentrations. At high Co concentrations ferromagnetic metallic state emerges above the spin glass. This is coincident with changes of the unit cell, arrangement and connectivity of stripe conducting phase. △ Less

Submitted 12 April, 2016; originally announced April 2016.

Comments: 7 pages, 6 figures

Journal ref: Phys. Rev. B 92, 174522 (2015)

Two-dimensional Dirac fermions in YbMnBi2 antiferromagnet

Authors: Aifeng Wang, I. Zaliznyak, Weijun Ren, Lijun Wu, D. Graf, V. O. Garlea, J. B. Warren, E. Bozin, Yimei Zhu, C. Petrovic

Abstract: We report two-dimensional quantum transport and Dirac fermions in YbMnBi2 single crystals. YbMnBi2 is a layered material with anisotropic conductivity and magnetic order below 290 K. Magnetotransport properties, nonzero Berry phase and small cyclotron mass indicate the presence of quasi two dimensional Dirac fermions. Quantum oscillations in Hall resistivity suggest the presence of both electron a… ▽ More We report two-dimensional quantum transport and Dirac fermions in YbMnBi2 single crystals. YbMnBi2 is a layered material with anisotropic conductivity and magnetic order below 290 K. Magnetotransport properties, nonzero Berry phase and small cyclotron mass indicate the presence of quasi two dimensional Dirac fermions. Quantum oscillations in Hall resistivity suggest the presence of both electron and hole parts of the Fermi surface whereas the Berry phase suggests spin-orbit coupling. △ Less

Submitted 4 April, 2016; originally announced April 2016.

Comments: 7 pages, 5 figures

Thermal evolution of antiferromagnetic correlations and tetrahedral bond angles in superconducting FeTe$_{1-x}$Se$_x$

Authors: Zhijun Xu, J. A. Schneeloch, Jinsheng Wen, E. S. Bozin, G. E. Granroth, B. L. Winn, M. Feygenson, R. J. Birgeneau, Genda Gu, I. A. Zaliznyak, J. M. Tranquada, Guangyong Xu

Abstract: It has recently been demonstrated that dynamical magnetic correlations measured by neutron scattering in iron chalcogenides can be described with models of short-range correlations characterized by particular {choices of four-spin plaquettes, where the appropriate choice changes as the} parent material is doped towards superconductivity. Here we apply such models to describe measured maps of magne… ▽ More It has recently been demonstrated that dynamical magnetic correlations measured by neutron scattering in iron chalcogenides can be described with models of short-range correlations characterized by particular {choices of four-spin plaquettes, where the appropriate choice changes as the} parent material is doped towards superconductivity. Here we apply such models to describe measured maps of magnetic scattering as a function of two-dimensional wave vectors obtained for optimally superconducting crystals of FeTe$_{1-x}$Se$_x$. We show that the characteristic antiferromagnetic wave vector evolves from that of the bicollinear structure found in underdoped chalcogenides (at high temperature) to that associated with the stripe structure of antiferromagnetic iron arsenides (at low temperature); {these can both be described with the same local plaquette, but with different inter-plaquette correlations}. While the magnitude of the low-energy magnetic spectral weight is substantial at all temperatures, it actually weakens somewhat at low temperature, where the charge carriers become more itinerant. The observed change in spin correlations is correlated with the dramatic drop in the electronic scattering rate and the growth of the bulk nematic response on cooling. Finally, we also present powder neutron diffraction results for lattice parameters in FeTe$_{1-x}$Se$_x$ indicating that the tetrahedral bond angle tends to increase towards the ideal value on cooling, in agreement with the increased screening of the crystal field by more itinerant electrons and the correspondingly smaller splitting of the Fe $3d$ orbitals. △ Less

Submitted 25 February, 2016; v1 submitted 25 December, 2015; originally announced December 2015.

Journal ref: Phys. Rev. B 93, 104517 (2016)

Charge-screening role of $c$-axis atomic displacements in YBa$_2$Cu$_3$O$_{6+x}$ and related superconductors

Authors: E. S. Bozin, A. Huq, Bing Shen, H. Claus, W. K. Kwok, J. M. Tranquada

Abstract: The importance of charge reservoir layers for supplying holes to the CuO$_2$ planes of cuprate superconductors has long been recognized. Less attention has been paid to the screening of the charge transfer by the intervening ionic layers. We address this issue in the case of YBa$_2$Cu$_3$O$_{6+x}$, where CuO chains supply the holes for the planes. We present a simple dielectric-screening model tha… ▽ More The importance of charge reservoir layers for supplying holes to the CuO$_2$ planes of cuprate superconductors has long been recognized. Less attention has been paid to the screening of the charge transfer by the intervening ionic layers. We address this issue in the case of YBa$_2$Cu$_3$O$_{6+x}$, where CuO chains supply the holes for the planes. We present a simple dielectric-screening model that gives a linear correlation between the relative displacements of ions along the $c$ axis, determined by neutron powder diffraction, and the hole density of the planes. Applying this model to the temperature dependent shifts of ions along the $c$ axis, we infer a charge transfer of 5-10% of the hole density from the planes to the chains on warming from the superconducting transition to room temperature. Given the significant coupling of $c$-axis displacements to the average charge density, we point out the relevance of local displacements for screening charge modulations and note recent evidence for dynamic screening of in-plane quasiparticles. This line of argument leads us to a simple model for atomic displacements and charge modulation that is consistent with images from scanning-tunneling microscopy for underdoped Bi$_2$Sr$_2$CaCu$_2$O$_{8+δ}$. △ Less

Submitted 15 February, 2016; v1 submitted 25 November, 2015; originally announced November 2015.

Comments: 12 pages, 10 figures; final version, accepted for publication in Phys. Rev. B

Journal ref: Phys. Rev. B 93, 054523 (2016)

Non percolative nature of the metal-insulator transition and persistence of local Jahn-Teller distortions in the rhombohedral regime of La$_{1-x}$Ca$_{x}$MnO$_{3}$

Authors: Mouath Shatnawi, Emil S. Bozin, J. F. Mitchell, Simon J. L. Billinge

Abstract: Evolution of the average and local crystal structure of Ca-doped LaMnO$_{3}$ has been studied across the metal to insulator (MI) and the orthorhombic to rhombohedral (OR) structural phase transitions over a broad temperature range for two Ca concentrations ($x = 0.18; 0.22$). Combined Rietveld and high real space resolution atomic pair distribution function (PDF) analysis of neutron total scatteri… ▽ More Evolution of the average and local crystal structure of Ca-doped LaMnO$_{3}$ has been studied across the metal to insulator (MI) and the orthorhombic to rhombohedral (OR) structural phase transitions over a broad temperature range for two Ca concentrations ($x = 0.18; 0.22$). Combined Rietveld and high real space resolution atomic pair distribution function (PDF) analysis of neutron total scattering data was carried out with aims of exploring the possibility of nanoscale phase separation (PS) in relation to MI transition, and charting the evolution of local Jahn-Teller (JT) distortion of MnO$_{6}$ octahedra across the OR transition at T$_{S}$ $\sim$ 720 K. The study utilized explicit two-phase PDF structural modeling, revealing that away from TMI there is no evidence for nanoscale phase coexistence. The local JT-distortions disappear abruptly upon crossing into the metallic regime both with doping and temperature, with only small temperature-independent signature of quenched disorder being observable at low temperature as compared to CaMnO$_{3}$. The results hence do not support the percolative scenario for the MI transition in La$_{1-x}$Ca$_{x}$MnO$_{3}$ based on PS, and question its ubiquity in the manganites. In contrast to LaMnO3 that exhibits long range orbital correlations and sizeable octahedral distortions at low temperature, the doped samples with compositions straddling the MI boundary exhibit correlations (in the insulating regime) limited to only $\sim$ 1 nm with observably smaller distortions. In $x = 0.22$ sample local JT-distortions are found to persist across the OR transition and deep into the R-phase (up to $\sim$ 1050 K) where they are crystallographically prohibited. Their magnitude and subnanometer spatial extent remain unchanged. △ Less

Submitted 25 November, 2015; originally announced November 2015.

Comments: 10 pages, 11 figures

Journal ref: Phys. Rev. B 93, 165138 (2016)

Reconciliation of local and long range tilt correlations in underdoped La2-xBaxCuO4

Authors: Emil S. Bozin, Ruidan Zhong, Kevin R. Knox, Genda Gu, John P. Hill, John M. Tranquada, Simon J. L. Billinge

Abstract: A long standing puzzle regarding the disparity of local and long range CuO6 octahedral tilt correlations in the underdoped regime of La2-xBaxCuO4 is addressed by utilizing complementary neutron powder diffraction and inelastic neutron scattering (INS) approaches. Long-range and static CuO6 tilt order with orthogonally inequivalent Cu-O bonds in the CuO2 planes in the low temperature tetragonal (LT… ▽ More A long standing puzzle regarding the disparity of local and long range CuO6 octahedral tilt correlations in the underdoped regime of La2-xBaxCuO4 is addressed by utilizing complementary neutron powder diffraction and inelastic neutron scattering (INS) approaches. Long-range and static CuO6 tilt order with orthogonally inequivalent Cu-O bonds in the CuO2 planes in the low temperature tetragonal (LTT) phase is succeeded on warming through the low-temperature transition by one with orthogonally equivalent bonds in the low temperature orthorhombic (LTO) phase. In contrast, the signatures of LTT-type tilts in the instantaneous local atomic structure persist on heating throughout the LTO crystallographic phase on the nanoscale, although becoming weaker as temperature increases. Analysis of the INS spectra for the x = 1/8 composition reveals the dynamic nature of the LTT-like tilt fluctuations within the LTO phase and their 3D character. The doping dependence of relevant structural parameters indicates that the magnitude of the Cu-O bond anisotropy has a maximum at x = 1/8 doping where bulk superconductivity is most strongly suppressed, suggesting that the structural anisotropy might be influenced by electron-phonon coupling and the particular stability of the stripe-ordered phase at this composition. The bond-length modulation that pins stripe order is found to be remarkably subtle, with no anomalous bond length disorder at low temperature, placing an upper limit on any in-plane Cu-O bondlength anisotropy. The results further reveal that although appreciable octahedral tilts persist through the high-temperature transition and into the high temperature tetragonal (HTT) phase, there is no significant preference between different tilt directions in the HTT regime. △ Less

Submitted 19 December, 2014; originally announced December 2014.

Comments: 14 pages, 19 figures, submitted to Physical Review B

Journal ref: Phys. Rev. B 91, 054521 (2015)

Intra-unit-cell nematic charge order in the titanium-oxypnictide family of superconductors

Authors: Benjamin A. Frandsen, Emil S. Bozin, Hefei Hu, Yimei Zhu, Yasumasa Nozaki, Hiroshi Kageyama, Yasutomo J. Uemura, Wei-Guo Yin, Simon J. L. Billinge

Abstract: Understanding the role played by broken symmetry states such as charge, spin, and orbital orders in the mechanism of emergent properties such as high-temperature superconductivity (HTSC) is a major current topic in materials research. That the order may be within one unit cell, such as nematic, was only recently considered theoretically, but its observation in the iron-pnictide and doped cuprate s… ▽ More Understanding the role played by broken symmetry states such as charge, spin, and orbital orders in the mechanism of emergent properties such as high-temperature superconductivity (HTSC) is a major current topic in materials research. That the order may be within one unit cell, such as nematic, was only recently considered theoretically, but its observation in the iron-pnictide and doped cuprate superconductors places it at the forefront of current research. Here we show that the recently discovered BaTi$_2$Sb$_2$O superconductor and its "parent" compound BaTi$_2$As$_2$O form a symmetry-breaking nematic ground state that can be naturally explained as an intra-unit-cell charge order with $d$-wave symmetry, pointing to the ubiquity of the phenomenon. These findings, together with the key structural features in these materials being intermediate between the cuprate and iron-pnictide HTSC materials, render the titanium oxypnictides an important new material system to understand the nature of nematic order and its relationship to superconductivity. △ Less

Submitted 9 December, 2014; v1 submitted 13 May, 2014; originally announced May 2014.

Comments: 7 pages, 5 figures

Journal ref: Nature Communications 5, 5761 (2014)