Showing 1–40 of 40 results for author: Susner, M A

Non-local low energy neutral excitations in a strongly disordered triangular Mott magnet Cr$_3$Se$_2$Br$_5$

Authors: Wenhao Liu, Dechen Zhang, Yuanqi Lyu, Lebing Chen, Lifang Hu, Keith M. Teddei, Yuting Zhang, Steve Shelton, Moon Kim, Xiqu Wang, Michael A. Susner, James G. Analytis, Dung-Hai Lee, Lu Li, Bing Lv, Robert J. Birgeneau

Abstract: Understanding if low-energy excitations can remain itinerant in the presence of strong disorder remains a central challenge in frustrated quantum magnets, where disorder is generally expected to localize excitations through Anderson-like mechanisms. Here we report the emergence of charge-neutral itinerant excitations in a van der Waals compound Cr$_3$Se$_2$Br$_5$, a strongly disordered $S = 3/2$ M… ▽ More Understanding if low-energy excitations can remain itinerant in the presence of strong disorder remains a central challenge in frustrated quantum magnets, where disorder is generally expected to localize excitations through Anderson-like mechanisms. Here we report the emergence of charge-neutral itinerant excitations in a van der Waals compound Cr$_3$Se$_2$Br$_5$, a strongly disordered $S = 3/2$ Mott insulator with a frustrated triangular lattice. Structural analysis reveals substantial intrinsic disorder arising from Cr-site deficiency and Se/Br-site mixing, which appear to be fixed and cannot be readily tuned. No long-range magnetic order or conventional glassy behavior is observed. In addition to its highly insulating nature, the magnetic specific heat C_mag/T and thermal conductivity \k{appa}_xx/T both exhibit linear temperature dependencies with substantial finite intercepts. In particular, a sizeable field-independent residual term $κ/T \approx 0.03~\mathrm{W\,m^{-1}\,K^{-2}}$ is observed, providing compelling evidence of itinerant low-energy excitations that carry entropy without charge. These findings conceptually advance our understanding of quantum matter by demonstrating a rare regime where the interplay of disorder, frustration, and electronic correlations actively reshapes the nature of low-energy excitations, allowing itinerant neutral excitations to coexist with strong intrinsic disorder. △ Less

Submitted 24 May, 2026; originally announced May 2026.

Comments: 18 pages, 4 figures

Raman and Terahertz Spectroscopy of Low-Frequency Chiral Phonons in Amino Acids

Authors: Rahul Rao, Won Jin Choi, Joseph M. Slocik, Thuc T. Mai, Michael A. Susner, Kelsey A. Collins, Michael J. Newburger, Petr Bouř, Nicholas A. Kotov

Abstract: Chiral phonons are mirror-symmetric vibrations that correspond to twisting and rotational motions of atoms. In chiral biomolecules, they correspond to low-energy terahertz (THz)-range vibrations of the molecular segments involving dozens of atoms whose energies are sensitive to the chirality of the molecules and local atomic geometries. Here we present spectral signatures of chiral phonons in circ… ▽ More Chiral phonons are mirror-symmetric vibrations that correspond to twisting and rotational motions of atoms. In chiral biomolecules, they correspond to low-energy terahertz (THz)-range vibrations of the molecular segments involving dozens of atoms whose energies are sensitive to the chirality of the molecules and local atomic geometries. Here we present spectral signatures of chiral phonons in circularly polarized low-frequency Raman and Raman optical activity (ROA) spectra from crystals of several amino acids in different enantiomeric forms. Along with complementary THz circular dichroism (TCD) measurements, our ROA data reveal two sets of bisignate peaks in valine, alanine, tyrosine and proline between 1 and 4.5 THz that are more intense than the ROA peaks in the fingerprint region. Density functional theory (DFT) calculations on L-alanine attribute these modes to twisting and shearing molecular motions. The strong agreement between the ROA and TCD data demonstrates the power of these complementary vibrational spectroscopy techniques to identify chiral phonons in biomolecules, and offers new insights into their vibrational properties and interactions with circularly polarized light. △ Less

Submitted 29 March, 2026; originally announced March 2026.

Comments: 14 pages, 3 figures

Magnetism and magnetoelastic effect in 2D van der Waals multiferroic CuCrP2S6

Authors: Jiasen Guo, Ryan P. Siebenaller, Michael A. Susner, Jiaqiang Yan, Zachary Morgan, Feng Ye

Abstract: We report a magnetic and neutron diffraction study on the ground state magnetism and field evolution of single crystal van der Waals multiferroic CuCrP2S6. The ordered moments align along the b axis in the A-type antiferromagnetic configuration with a spin-flop transition along the same direction. Field application along a introduces a smooth transition to a fully-polarized ferromagnetic state via… ▽ More We report a magnetic and neutron diffraction study on the ground state magnetism and field evolution of single crystal van der Waals multiferroic CuCrP2S6. The ordered moments align along the b axis in the A-type antiferromagnetic configuration with a spin-flop transition along the same direction. Field application along a introduces a smooth transition to a fully-polarized ferromagnetic state via in-plane spin rotation. These findings resolve the ambiguity of the ground state magnetization direction in CuCrP2S6 and uncover its field responses, providing a firm basis for future magnetoelectric study. A magnetoelastic coupling effect connecting the interlayer spacing and the magnetic order was further revealed, highlighting the out-of-plane strain as an effective control knob for tuning magnetism both in this system and in related van der Waals magnets. △ Less

Submitted 26 March, 2026; originally announced March 2026.

Comments: 5 figures, 7 pages

Journal ref: Phys. Rev. B 2026

Highly Efficient Second/Third Harmonic Generation in van der Waals Layered Material AgScP2S6 with Anisotropic Polarization and Temperature Dependence

Authors: Mohamed Yaseen Noor, Ryan Siebenaller, Wenhao Liu, Zixin Zhai, Conrad Kuz, Simin Zhang, Mousumi Upadhyay Kahaly, Gergely Nagy, Aamir Mushtaq, Rahul Rao, Emmanuel Rowe, Benjamin S. Conner, Bing Lv, Michael A. Susner, Enam Chowdhury

Abstract: Single-crystal X-ray diffraction and nonlinear optical measurements, especially second- and third-harmonic generation (SHG/THG) are comprehensively investigated for the van der Waals layered material AgScP2S6 with a non-centrosymmetric P31c (159) space group. Linear optical constants are extracted using spectroscopic ellipsometry and applied in fitting the harmonic generation behavior. Polarizatio… ▽ More Single-crystal X-ray diffraction and nonlinear optical measurements, especially second- and third-harmonic generation (SHG/THG) are comprehensively investigated for the van der Waals layered material AgScP2S6 with a non-centrosymmetric P31c (159) space group. Linear optical constants are extracted using spectroscopic ellipsometry and applied in fitting the harmonic generation behavior. Polarization-resolved SHG and THG measurements exhibit pronounced anisotropy, with emission patterns well-described by theoretical models derived from the khi(2) and khi(3) tensor elements. The material demonstrates exceptionally high nonlinear susceptibilities, with khi(2) ~ 10^(-8) m/V and khi(3) ~ 10^(-17) m^2/V^2 which is a few orders of magnitude greater than comparable 2D materials reported in the literature. Temperature-dependent SHG and THG measurements from 300 K to 25 K reveal exponential decay in harmonic signal intensities, attributed to reduced carrier mobility, with no evidence of structural phase transitions, consistent with results from single crystal diffraction and heat capacity measurements. Polarization-resolved SHG and THG measurements also reveal distinct orientation and ellipticity trends, highlighting the anisotropic nonlinear tensor contributions and contrasting polarization selection rules in the material. These results establish AgScP2S6 as a high-performance, thermally stable, and highly anisotropic nonlinear candidate material suitable for compact photonic applications such as ultrafast optical modulators, polarization-sensitive detectors, and wavelength-tunable light sources. △ Less

Submitted 26 February, 2026; originally announced February 2026.

Lanthanide Ion Electronic Structure Controls Magnetic Excitations in Topological Quantum Ferrimagnets $LnMn_{6}Sn_{6}$ (Ln = Tb, Dy, Ho)

Authors: Kelsey A. Collins, Jacob Pfund, Michael R. Page, Menka Jain, Michael A. Susner, Michael J. Newburger

Abstract: The $LnMn_{6}Sn_{6}$ family of topological magnets is a promising platform for next-generation spintronic and magnonic technologies. However, the influence of the lanthanide ion ($Ln^{3+}$) on the excited-state spin dynamics, or magnons, remains a critical knowledge gap. Here, we present the first comparative study of the magnetic dynamics in $LnMn_{6}Sn_{6}$ materials (Ln = Tb, Dy, Ho) using Bril… ▽ More The $LnMn_{6}Sn_{6}$ family of topological magnets is a promising platform for next-generation spintronic and magnonic technologies. However, the influence of the lanthanide ion ($Ln^{3+}$) on the excited-state spin dynamics, or magnons, remains a critical knowledge gap. Here, we present the first comparative study of the magnetic dynamics in $LnMn_{6}Sn_{6}$ materials (Ln = Tb, Dy, Ho) using Brillouin light scattering. Our findings reveal a direct correlation between the lanthanide ion's intrinsic properties and the magnon behavior. We demonstrate that the magnon frequency in the absence of an applied magnetic field is primarily dictated by the strength of the lanthanide exchange coupling, as modeled by its relationship with the de Gennes factor. The response of the magnon to an applied field is influenced by material's gyromagnetic ratio and the overall anisotropy of the material, which are dictated by total angular momentum and the anisotropy of the lanthanide sublattice, respectively. These results establish that simple lanthanide substitution provides a powerful and predictable method for tuning magnon properties, enabling the rational design of materials for advanced technological applications. △ Less

Submitted 6 July, 2026; v1 submitted 19 December, 2025; originally announced December 2025.

Comments: 12 pages main text, 41 total including supplementary info, 4 main figures, 40 supplementary figures

Report number: AFRL-2025-5776

Engineering the Magnetocaloric Effect in Nd$T_4$B

Authors: Kyle W. Fruhling, Enrique O. González Delgado, Siddharth Nandanwar, Xiaohan Yao, Zafer Turgut, Michael A. Susner, Fazel Tafti

Abstract: We present a comprehensive study of the magnetocaloric effect (MCE) in the Nd$T_4$B system where $T$ = Fe, Co, and Ni. These compounds are ferromagnetic kagome materials with tunable ordering temperatures, transition width, and magnetic moments depending on the choice of transition metal. Thus, they are good candidates for investigating the MCE. We characterize the MCE using standard metrics and c… ▽ More We present a comprehensive study of the magnetocaloric effect (MCE) in the Nd$T_4$B system where $T$ = Fe, Co, and Ni. These compounds are ferromagnetic kagome materials with tunable ordering temperatures, transition width, and magnetic moments depending on the choice of transition metal. Thus, they are good candidates for investigating the MCE. We characterize the MCE using standard metrics and construct ternary phase diagrams as functions of Fe, Co, and Ni concentrations. Using these phase diagrams, we engineer the composition NdFe$_{1.15}$Co$_{0.46}$Ni$_{2.39}$B to maximize the MCE. Interestingly, the Nd$T_4$B system shows a notable entropy change over a wide temperature range ($\sim$10 to 650 K), and particular compositions have notable MCEs spanning hundreds of Kelvin, making this a suitable system to study for technologies used in a wide range of temperatures. In a few cases, we observe a two-peak MCE. These two transitions, releasing comparable entropy, provide an interesting platform to study for applications in multi-stage cooling. △ Less

Submitted 22 November, 2025; originally announced November 2025.

Comments: Main Text: 7 pages, 3 figures 2 tables. Supplemental: 7 pages, 8 figures, 1 table

Journal ref: Journal of Applied Physics 139, 165001 (2026)

Adjudicating Conduction Mechanisms in High Performance Carbon Nanotube Fibers

Authors: John Bulmer, Chris Kovacs, Thomas Bullard, Charlie Ebbing, Timothy Haugan, Ganesh Pokharel, Stephen D. Wilson, Fedor F. Balakirev, Oscar A. Valenzuela, Michael A. Susner, David Turner, Pengyu Fu, Teresa Kulka, Jacek Majewski, Irina Lebedeva, Karolina Z. Milowska, Agnieszka Lekawa-Raus, Magdalena Marganska

Abstract: The performance of carbon nanotube (CNT) cables, a contender for copper-wire replacement, is tied to its metallic and semi-conducting-like conductivity responses with temperature; the origin of the semi-conducting-like response however is an underappreciated incongruity in literature. With controlled aspect-ratio and doping-degree, over 61 unique cryogenic experiments including anisotropy and Hall… ▽ More The performance of carbon nanotube (CNT) cables, a contender for copper-wire replacement, is tied to its metallic and semi-conducting-like conductivity responses with temperature; the origin of the semi-conducting-like response however is an underappreciated incongruity in literature. With controlled aspect-ratio and doping-degree, over 61 unique cryogenic experiments including anisotropy and Hall measurements, CNT cable performance is explored at extreme temperatures (65 mK) and magnetic fields (60 T). A semi-conducting-like conductivity response with temperature becomes temperature-independent approaching absolute-zero, uniquely demonstrating the necessity of heterogeneous fluctuation induced tunneling; complete de-doping leads to localized hopping, contrasting graphite's pure metallic-like response. High-field magneto-resistance (including +22% longitudinal magneto-resistance near room-temperature) is analyzed with hopping and classical two-band models, both similarly yielding a parameter useful for conductor development. Varying field-orientation angle uncovers two-and four-fold symmetries from Aharonov-Bohm-like corrections to curvature-induced bandgap. Tight-binding calculations using Green's Function formalism model large-scale, coherent transport in commensurate CNT bundles in magnetic field, revealing non-uniform transmission across bundle cross-sections with doping restoring uniformity; independent of doping, transport in bundle-junction-bundle systems are predominantly from CNTs adjacent to the other bundle. The final impact is predicting the ultimate conductivity of heterogeneous CNT cables using temperature and field-dependent transport, surpassing conductivity of traditional metals. △ Less

Submitted 5 September, 2025; v1 submitted 27 July, 2025; originally announced July 2025.

Journal ref: Carbon 248 (2026) 121162

Spin-lattice entanglement in $\mathbf{CoPS}_3$

Authors: Thuc T. Mai, Amber McCreary, K. F. Garrity, Rebecca L. Dally, Sambridhi Shah, Bryan C. Chakoumakos, Md Nasim Afroj Taj, Jeffrey W. Lynn, Michael A. McGuire, Benjamin S. Conner, Mona Zebarjadi, Janice L. Musfeldt, Angela R. Hight Walker, Rahul Rao, Michael A. Susner

Abstract: Complex chalcogenides in the $M$PS$_3$ family of materials ($M$ = Mn, Fe, Co, and Ni) display remarkably different phase progressions depending upon the metal center orbital filling, character of the P-P linkage, and size of the van der Waals gap. There is also a stacking pattern and spin state difference between the lighter and heavier transition metal-containing systems that places CoPS$_3$ at t… ▽ More Complex chalcogenides in the $M$PS$_3$ family of materials ($M$ = Mn, Fe, Co, and Ni) display remarkably different phase progressions depending upon the metal center orbital filling, character of the P-P linkage, and size of the van der Waals gap. There is also a stacking pattern and spin state difference between the lighter and heavier transition metal-containing systems that places CoPS$_3$ at the nexus of these activities. Despite these unique properties, this compound is under-explored. Here, we bring together Raman scattering spectroscopy and infrared absorption spectroscopy with X-ray techniques to identify a structural component to the 119 K magnetic ordering transition as well as a remarkable lower temperature set of magnon-phonon pairs that engage in avoided crossings along with a magnetic scattering continuum that correlates with phonon lifetime effects. These findings point to strong spin-phonon entanglement as well as opportunities to control these effects under external stimuli. △ Less

Submitted 4 July, 2025; v1 submitted 10 June, 2025; originally announced June 2025.

Comments: 12 pages, 5 figures

Observation of giant remnant polarization in ultrathin AlScN at cryogenic temperatures

Authors: Seunguk Song, Dhiren K. Pradhan, Zekun Hu, Yinuo Zhang, Rachael N. Keneipp, Michael A. Susner, Pijush Bhattacharya, Marija Drndić, Roy H. Olsson III, Deep Jariwala

Abstract: The discovery of wurtzite ferroelectrics opens new frontiers in polar materials, yet their behavior at cryogenic temperatures remains unexplored. Here, we reveal unprecedented ferroelectric properties in ultrathin (10 nm) Al$_{0.68}$Sc$_{0.32}$N (AlScN) at cryogenic temperatures where the properties are fundamentally distinct from those of conventional oxide ferroelectrics. At 12 K, we demonstrate… ▽ More The discovery of wurtzite ferroelectrics opens new frontiers in polar materials, yet their behavior at cryogenic temperatures remains unexplored. Here, we reveal unprecedented ferroelectric properties in ultrathin (10 nm) Al$_{0.68}$Sc$_{0.32}$N (AlScN) at cryogenic temperatures where the properties are fundamentally distinct from those of conventional oxide ferroelectrics. At 12 K, we demonstrate a giant remnant polarization exceeding 250 $μ$C/cm$^2$ -- more than twice that of any known ferroelectric -- driven by an enhanced c/a ratio in the wurtzite structure. Our devices sustain remarkably high electric fields (~13 MV/cm) while maintaining reliable switching, achieving over 104 polarization reversal cycles at 12 K. Critically, this breakdown field strength approaches that of passive dielectric materials while maintaining ferroelectric functionality. The extraordinary polarization enhancement and high-field stability at cryogenic temperatures contrasts sharply with oxide ferroelectrics, establishing wurtzite ferroelectrics as a distinct class of polar materials with implications spanning fundamental physics to cryogenic non-volatile memory and quantum technologies. △ Less

Submitted 25 March, 2025; originally announced March 2025.

Comments: 27 pages, 5 figures, 5 supplementary figures

Anomalous Raman scattering in layered AgCrP$_2$Se$_6$: Helical modes and excitation energy-dependent intensities

Authors: Rahul Rao, Jie Jiang, Ruth Pachter, Thuc T. Mai, Valentine Mohaugen, Maria F. Muñoz, Ryan Siebenaller, Emmanuel Rowe, Ryan Selhorst, Andrea N. Giordano, Angela R. Hight Walker, Michael A. Susner

Abstract: Structural anisotropy in layered two-dimensional materials can lead to highly anisotropic optical absorption which, in turn, can profoundly a^ect their phonon modes. These e^ects include lattice orientation-dependent and excitation energy-dependent mode intensities that can enable new phononic and optoelectronic applications. Here, we report anomalous Raman spectra in single-crystalline AgCrP$_2$S… ▽ More Structural anisotropy in layered two-dimensional materials can lead to highly anisotropic optical absorption which, in turn, can profoundly a^ect their phonon modes. These e^ects include lattice orientation-dependent and excitation energy-dependent mode intensities that can enable new phononic and optoelectronic applications. Here, we report anomalous Raman spectra in single-crystalline AgCrP$_2$Se$_6$, a layered antiferromagnetic material. Density functional theory calculations and experimental measurements reveal several unique features in the Raman spectra of bulk and exfoliated AgCrP$_2$Se$_6$ crystals including three helical vibrational modes. These modes exhibit large Raman optical activities (circular intensity di^erences) in bulk AgCrP$_2$Se$_6$, which progressively decrease with thickness. We also observe strong excitation energy dependent peak intensities as well as a decrease in anti-Stokes peak intensities at room temperature with increasing excitation energy, resulting in an apparent cooling by up to 220 K. All of these anomalies in bulk and exfoliated flakes are attributed to the unique ABC layer stacking structure of AgCrP$_2$Se$_6$ and to the smaller unit cell volume that causes hybridization between the Se and Ag/Cr electron densities, resulting in charge transfer and strongly a^ecting the electron-phonon coupling. This work thus positions AgCrP$_2$Se$_6$ as an exciting new 2D material for optical and phononic applications. △ Less

Submitted 29 January, 2025; originally announced January 2025.

Comments: 22 pages, 6 figures

Thermal and dimensional stability of photocatalytic material ZnPS$_3$ under extreme environmental conditions

Authors: Abhishek Mukherjee, Vivian J. Santamaría-García, Damian Wlodarczyk, Ajeesh K. Somakumar, Piotr Sybilski, Ryan Siebenaller, Emmanuel Rowe, Saranya Narayanan, Michael A. Susner, L. Marcelo Lozano-Sanchez, Andrzej Suchocki, Julio L. Palma, Svetlana V. Boriskina

Abstract: Zinc phosphorus trisulfide (ZnPS$_3$), a promising material for photocatalysis and energy storage, is shown in this study to exhibit remarkable stability under extreme conditions. We explore its optical and structural properties under high pressure and cryogenic temperatures using photoluminescence (PL) spectroscopy, Raman scattering, and density functional theory (DFT). Our results identify a pre… ▽ More Zinc phosphorus trisulfide (ZnPS$_3$), a promising material for photocatalysis and energy storage, is shown in this study to exhibit remarkable stability under extreme conditions. We explore its optical and structural properties under high pressure and cryogenic temperatures using photoluminescence (PL) spectroscopy, Raman scattering, and density functional theory (DFT). Our results identify a pressure-induced phase transition starting at 6.75 GPa and stabilizing by 12.5 GPa, after which ZnPS$_3$ demonstrates robust stability across a broad pressure range of 15 to 100 GPa. DFT calculations predict a semiconductor-to-semimetal transition at 100 GPa, while PL measurements reveal defect-assisted emissions that quench under pressure due to enhanced non-radiative recombination. At cryogenic temperatures, PL quenching intensifies as non-radiative processes dominate, driven by a rising Grüneisen parameter and reduced phonon population. Cryogenic X-ray diffraction (XRD) also reveals a high mean thermal expansion coefficient (TEC) of (4.369 $\pm$ 0.393) $\times$ 10$^{-5}$ K$^{-1}$, among the highest reported for 2D materials. This unique combination of tunable electronic properties under low pressure and high thermal sensitivity makes ZnPS$_3$ a strong candidate for sensing applications in extreme environments. △ Less

Submitted 24 January, 2025; originally announced January 2025.

Defect-Induced Strain-Tunable Photoluminescence in AgScP$_2$S$_6$

Authors: Abhishek Mukherjee, Damian Wlodarczyk, Ajeesh K. Somakumar, Piotr Sybilski, Ryan Siebenaller, Michael A. Susner, Andrzej Suchocki, Svetlana V. Boriskina

Abstract: Metal thiophosphates (MTPs) are a large family of 2D materials that exhibit large structural and chemical diversity. They also show promise for applications in energy harvesting and photodetection. Strain and defect engineering have previously been demonstrated as useful mechanisms to tune several properties of MTPs such as resistivity, magnetic state, and electronic band gap. However, the effect… ▽ More Metal thiophosphates (MTPs) are a large family of 2D materials that exhibit large structural and chemical diversity. They also show promise for applications in energy harvesting and photodetection. Strain and defect engineering have previously been demonstrated as useful mechanisms to tune several properties of MTPs such as resistivity, magnetic state, and electronic band gap. However, the effect of these stimuli on engineering tunable light emission in MTPs remains unexplored. Here, we show experimentally that structural defects in metal thiophosphate AgScP$_2$S$_6$ are prominent in exhibiting photoluminescence, which is likely driven by the defect-state-to-conduction-band transitions and can be further tuned by temperature-induced strain gradients. △ Less

Submitted 3 March, 2024; originally announced March 2024.

Comments: 7 pages, 4 figures, 1 table

Raman fingerprints of spin-phonon coupling and magnetic transition in an organic molecule intercalated Cr2Ge2Te6

Authors: Sudeshna Samanta, Hector Iturriaga, Thuc T. Mai, Adam J. Biacchi, Rajbul Islam, Angela R. Hight Walker, Mohamed Fathi Sanad, Charudatta Phatak, Ryan Siebenaller, Emmanuel Rowe Michael A. Susner, Fei Xue, Srinivasa R. Singamaneni

Abstract: The manipulation of spin-phonon coupling in both formations and explorations of magnetism in two-dimensional van der Waals ferromagnetic semiconductors facilitates unprecedented prospects for spintronics devices. The interlayer engineering tunes spin-phonon coupling significantly and holds the promise for controllable magnetism via organic cation intercalation. Here, we present spectroscopic evide… ▽ More The manipulation of spin-phonon coupling in both formations and explorations of magnetism in two-dimensional van der Waals ferromagnetic semiconductors facilitates unprecedented prospects for spintronics devices. The interlayer engineering tunes spin-phonon coupling significantly and holds the promise for controllable magnetism via organic cation intercalation. Here, we present spectroscopic evidence to reveal the intercalation effect on intrinsic magnetic and electronic transitions in quasi-two-dimensional Cr2Ge2Te6 using tetrabutyl ammonium as the intercalant. The temperature-evolution of Raman modes E_g^3 and A_g^1, along with the magnetization measurements, unambiguously captures the enhancement of the ferromagnetic Curie temperature in the intercalated heterostructure. Moreover, the E_g^4 mode highlighted the increased effect of spin-phonon interaction in magnetic order-induced lattice distortion. Combined with the first-principle calculations, we observed a substantial number of electrons transferred from TBA+ to Cr through the interface. These results provide the interplay between spin-phonon coupling and magnetic ordering in van der Waals magnets where Raman fingerprints would be highly beneficial for further understanding the manipulation of magnetism in layered heterostructures. △ Less

Submitted 2 December, 2023; originally announced December 2023.

Raman spectroscopy study of pressure-induced phase transitions in single crystal CuInP2S6

Authors: R. Rao, B. S. Conner, J. Jiang, R. Pachter, M. A. Susner

Abstract: Two dimensional ferroic materials exhibit a variety of functional properties that can be tuned by temperature and pressure. CuInP2S6 is a layered material that is ferrielectric at room temperature and whose properties are a result of the unique structural arrangement of ordered Cu and In cations within a P2S6 anion backbone. Here, we investigate the effect of hydrostatic pressure on the structure… ▽ More Two dimensional ferroic materials exhibit a variety of functional properties that can be tuned by temperature and pressure. CuInP2S6 is a layered material that is ferrielectric at room temperature and whose properties are a result of the unique structural arrangement of ordered Cu and In cations within a P2S6 anion backbone. Here, we investigate the effect of hydrostatic pressure on the structure of CuInP2S6 single crystals through a detailed Raman spectroscopy study. Analysis of the peak frequencies, intensities and widths reveals four high pressure regimes. At 5 GPa the material undergoes a monoclinic-trigonal phase transition. At higher pressures (5 - 12 GPa) we see Raman peak sharpening, indicative of a change in the electronic structure, followed by an incommensurate phase between 12 - 17 GPa. Above 17 GPa we see evidence for metallization in the material. The original state of the material is fully recovered upon decompression, showing that hydrostatic pressure could be used to tune the electronic and ferrielectric properties of CuInP2S6. △ Less

Submitted 13 June, 2023; v1 submitted 11 June, 2023; originally announced June 2023.

Comments: 10 pages, 4 figures

Investigating strain between phase-segregated domains in Cu-deficient CuInP2S6

Authors: Rahul Rao, Ryan Selhorst, Jie Jiang, Benjamin S. Conner, Ryan Siebenaller, Emmanuel Rowe, Andrea Giordano, Ruth Pachter, Michael A. Susner

Abstract: CuInP2S6 (CIPS) is an emerging layered ferroelectric material with a TC above room temperature. When synthesized with Cu deficiencies (i.e., Cu1-xIn1+x/3P2S6), the material segregates into CIPS and In4/3P2S6 (IPS) self-assembled heterostructures within the same single crystal. This segregation results in significant in-plane and out-of-plane strains between the CIPS and IPS phases as the volume fr… ▽ More CuInP2S6 (CIPS) is an emerging layered ferroelectric material with a TC above room temperature. When synthesized with Cu deficiencies (i.e., Cu1-xIn1+x/3P2S6), the material segregates into CIPS and In4/3P2S6 (IPS) self-assembled heterostructures within the same single crystal. This segregation results in significant in-plane and out-of-plane strains between the CIPS and IPS phases as the volume fraction of CIPS (IPS) domains shrink (grow) with decreasing Cu fraction. Here, we synthesized CIPS with varying amounts of Cu (x = 0, 0.2, 0.3, 0.4, 0.5, 0.7, 0.8 and 1) and measured the strains between the CIPS and IPS phases through the evolution of the respective Raman, infrared, and optical reflectance spectra. Density functional theory calculations revealed vibrational modes unique to the CIPS and IPS phases, which can be used to distinguish between the two phases through two-dimensional Raman mapping. A comparison of the composition-dependent frequencies and intensities of the CIPS and IPS Raman peaks showed interesting trends with decreasing CIPS phase fraction (i.e., Cu/In ratio). Our data reveal red- and blue-shifted Raman and infrared peak frequencies that we correlate to lattice strains arising from the segregation of the material into CIPS and IPS chemical domains. The strain is highest for a Cu/In ratio of 0.33 (Cu0.4In1.2P2S6), which we attribute to equal and opposite strains exerted by the CIPS and IPS phases on each other. In addition, bandgaps extracted from the optical reflectance spectra revealed a decrease in values, with the lowest value (~ 2.3 eV) for Cu0.4In1.2P2S6. △ Less

Submitted 24 May, 2023; originally announced May 2023.

Comments: 12 pages, 6 figures

Pressure-Induced Insulator-to-Metal Transition in van der Waals compound CoPS$_3$

Authors: Takahiro Matsuoka, Rahul Rao, Michael A. Susner, Benjamin S. Conner, Dongzhou Zhang, David Mandrus

Abstract: We have studied the insulator-to-metal transition and crystal structure evolution under high pressure in the van der Waals compound CoPS$_3$ through $\textit{in-situ}$ electrical resistance, Hall resistance, magnetoresistance, X-ray diffraction, and Raman scattering measurements. CoPS$_3$ exhibits a $C2/m$ $\rightarrow$ $P\overline{3}$ structural transformation at 7 GPa accompanied by a 2.9$\%$ re… ▽ More We have studied the insulator-to-metal transition and crystal structure evolution under high pressure in the van der Waals compound CoPS$_3$ through $\textit{in-situ}$ electrical resistance, Hall resistance, magnetoresistance, X-ray diffraction, and Raman scattering measurements. CoPS$_3$ exhibits a $C2/m$ $\rightarrow$ $P\overline{3}$ structural transformation at 7 GPa accompanied by a 2.9$\%$ reduction in the volume per formula unit. Concomitantly, the electrical resistance decreases significantly, and CoPS$_3$ becomes metallic. This metallic CoPS$_3$ is a hole-dominant conductor with multiple conduction bands. The linear magnetoresistance and the small volume collapse at the metallization suggest the incomplete high-spin $\rightarrow$ low-spin transition in the metallic phase. Thus, the metallic CoPS$_3$ possibly possesses an inhomogeneous magnetic moment distribution and short-range magnetic ordering. This report summarizes the comprehensive phase diagram of $M$PS$_3$ ($M$ = V, Mn, Fe, Co, Ni, and Cd) that metalize under pressures. △ Less

Submitted 2 April, 2023; v1 submitted 15 November, 2022; originally announced November 2022.

Comments: 14 pages, 6 figures, 1 table

Polarization-controlled volatile ferroelectric and capacitive switching in Sn$_2$P$_2$S$_6$

Authors: Sabine M. Neumayer, Anton V. Ievlev, Alexander Tselev, Sergey A. Basun, Benjamin S. Conner, Michael A. Susner, Petro Maksymovych

Abstract: Smart electronic circuits that support neuromorphic computing on the hardware level necessitate materials with memristive, memcapacitive, and neuromorphic-like functional properties; in short, the electronic response must depend on the voltage history, thus enabling learning algorithms. Here we demonstrate volatile ferroelectric switching of Sn$_2$P$_2$S$_6$ at room temperature and see that initia… ▽ More Smart electronic circuits that support neuromorphic computing on the hardware level necessitate materials with memristive, memcapacitive, and neuromorphic-like functional properties; in short, the electronic response must depend on the voltage history, thus enabling learning algorithms. Here we demonstrate volatile ferroelectric switching of Sn$_2$P$_2$S$_6$ at room temperature and see that initial polarization orientation strongly determines the properties of polarization switching. In particular, polarization switching hysteresis is strongly imprinted by the original polarization state, shifting the regions of non-linearity toward zero-bias. As a corollary, polarization switching also enables effective capacitive switching, approaching the sought-after regime of memcapacitance. Landau-Ginzburg-Devonshire simulations demonstrate that one mechanism by which polarization can control the shape of the hysteresis loop is the existence of charged domain walls decorating the periphery of the repolarization nucleus. These walls oppose the growth of the switched domain and favor back-switching, thus creating a scenario of controlled volatile ferroelectric switching. Although the measurements were carried out with single crystals, prospectively volatile polarization switching can be tuned by tailoring sample thickness, domain wall mobility and electric fields, paving way to non-linear dielectric properties for smart electronic circuits. △ Less

Submitted 26 August, 2022; originally announced August 2022.

Gapless spin-excitations in the superconducting state of a quasi-one-dimensional spin-triplet superconductor

Authors: Keith M. Taddei, Bing-Hua Lei, Michael A. Susner, Hui-Fei Zhai, Thomas J. Bullard, Liurukara D. Sanjeewa, Qiang Zheng, Athena S. Sefat, Songxue Chi, Clarina dela Cruz, David J. Singh, Bing Lv

Abstract: Majorana zero modes form as intrinsic defects in an odd-orbital one-dimensional superconductor thus motivating the search for such materials in the pursuit of Majorana physics. Here, we present combined experimental results and first principles calculations which suggest that quasi-one-dimensional K$_2$Cr$_3$As$_3$ may be such a superconductor. Using inelastic neutron scattering we probe the dynam… ▽ More Majorana zero modes form as intrinsic defects in an odd-orbital one-dimensional superconductor thus motivating the search for such materials in the pursuit of Majorana physics. Here, we present combined experimental results and first principles calculations which suggest that quasi-one-dimensional K$_2$Cr$_3$As$_3$ may be such a superconductor. Using inelastic neutron scattering we probe the dynamic spin-susceptibilities of K$_2$Cr$_3$As$_3$ and K$_2$Mo$_3$As$_3$ and show the presence of antiferromagnetic spin-fluctuations in both compounds. Below the superconducting transition, these fluctuations gap in K$_2$Mo$_3$As$_3$ but not in K$_2$Cr$_3$As$_3$. Using first principles calculations, we show that these fluctuations likely arise from nesting on one dimensional features of the Fermi surface. Considering these results we propose that while K$_2$Mo$_3$As$_3$ is a conventional superconductor, K$_2$Cr$_3$As$_3$ is likely a spin-triplet, and consequently, topological superconductor. △ Less

Submitted 23 June, 2022; originally announced June 2022.

Comments: 8 pages, 4 figures

Phonon anharmonicity in Cu-based layered thiophosphates

Authors: Rahul Rao, Michael A. Susner

Abstract: In this work, we performed temperature-dependent Raman spectroscopy studies on three layered metal thiophosphates: CuInP2S6, CuCrP2S6, and CuInP2S6-In4/3P2S6 heterostructures. These materials are emerging multiferroics, with CuInP2S6 and CuInP2S6-In4/3P2S6 exhibiting room temperature ferrielectric polarization while CuCrP2S6 is both anti-ferroelectric and antiferromagnetic at low temperatures (<14… ▽ More In this work, we performed temperature-dependent Raman spectroscopy studies on three layered metal thiophosphates: CuInP2S6, CuCrP2S6, and CuInP2S6-In4/3P2S6 heterostructures. These materials are emerging multiferroics, with CuInP2S6 and CuInP2S6-In4/3P2S6 exhibiting room temperature ferrielectric polarization while CuCrP2S6 is both anti-ferroelectric and antiferromagnetic at low temperatures (<145 and <30 K, respectively). We studied two prominent peaks in the Raman spectra of these compounds, namely their P-P and P-S stretching modes. First order linear fits to the mode softening yielded coefficients similar to other layered materials. Furthermore, analysis of the phonon decay revealed the dominant role of anharmonic effects rather than thermal expansion. In particular, the decay of the optical phonons was found to be governed by three-phonon processes in CuInP2S6 and CuCrP2S6 and by the four-phonon process in the CuInP2S6-In4/3P2S6 heterostructures. These phonon lifetimes were found to be less than 1 ps, indicating high scattering rates and showing the potential for these materials to be used in thermoelectric applications. △ Less

Submitted 28 December, 2022; v1 submitted 3 December, 2021; originally announced December 2021.

Comments: 4 figures

Ferrielectric-paraelectric phase transitions in layered CuInP2S6 and CuInP2S6- In4/3P2S6 heterostructures: A Raman spectroscopy and X-ray diffraction study

Authors: Rahul Rao, Ryan Selhorst, Benjamin S. Conner, Michael A. Susner

Abstract: CuInP2S6 (CIPS) is an emerging layered material that exhibits ferrielectric ordering well above room temperature (Curie temperature TC ~ 315 K). When synthesized with Cu deficiencies, CIPS spontaneously segregates into CuInP2S6 and In4/3P2S6 domains (CIPS-IPS), which form self-assembled heterostructures within the individual lamellae. This re-structuring and resultant chemical pressure raises the… ▽ More CuInP2S6 (CIPS) is an emerging layered material that exhibits ferrielectric ordering well above room temperature (Curie temperature TC ~ 315 K). When synthesized with Cu deficiencies, CIPS spontaneously segregates into CuInP2S6 and In4/3P2S6 domains (CIPS-IPS), which form self-assembled heterostructures within the individual lamellae. This re-structuring and resultant chemical pressure raises the Curie temperature and, depending on the Cu concentration, can be up to ~340 K for the highest Cu deficiency. In both CIPS and CIPS-IPS, the loss of polarization through the ferrielectric-paraelectric transition is driven by the movement of Cu ions within the lattice. Here we uncover the microscopic origins underpinning the phase transitions in pure CIPS and CIPS-IPS (Cu0.4In1.2P2S6) by performing a temperature-dependent Raman and XRD study. We measured the frequencies and linewidths of various cation and anion phonon modes and compared them to the extracted atomic positions from the refinement of XRD data. Our analysis shows that in addition to the Cu cation movement, the anion octahedral cages experience significant strains as they deform to accommodate the redistribution of Cu ions upon heating. This results in several discontinuities in peak frequencies and linewidths close to 315 K in CIPS. In the CIPS-IPS heterostructure, this process begins around 315 K and ends around 330 K. △ Less

Submitted 31 October, 2021; originally announced November 2021.

Comments: 4 figures

Pressure-driven phase transformations and phase segregation in ferrielectric CuInP$_2$S$_6$-In$_{4/3}$P$_2$S$_6$ self-assembled heterostructures

Authors: Rahul Rao, Benjamin S. Conner, Ryan Selhorst, Michael A. Susner

Abstract: Layered multi-ferroic materials exhibit a variety of functional properties that can be tuned by varying the temperature and pressure. As-synthesized CuInP$_2$S$_6$ is a layered material that displays ferrielectric behavior at room temperature. When synthesized with Cu deficiencies, CuInP$_2$S$_6$ spontaneously phase segregates to form ferrielectric CuInP$_2$S$_6$ (CIPS) and paraelectric In… ▽ More Layered multi-ferroic materials exhibit a variety of functional properties that can be tuned by varying the temperature and pressure. As-synthesized CuInP$_2$S$_6$ is a layered material that displays ferrielectric behavior at room temperature. When synthesized with Cu deficiencies, CuInP$_2$S$_6$ spontaneously phase segregates to form ferrielectric CuInP$_2$S$_6$ (CIPS) and paraelectric In$_{4/3}$P$_2$S$_6$ (IPS) domains in a two-dimensional self-assembled heterostructure. Here, we study the effect of hydrostatic pressure on the structure of Cu-deficient CuInP$_2$S$_6$ by Raman spectroscopy measurements up to 20 GPa. Detailed analysis of the frequencies, intensities, and linewidths of the Raman peaks reveals four discontinuities in the spectra around 2, 10, 13 and 17 GPa. At ~2 GPa, we observe a structural transition initiated by the diffusion of IPS domains, which culminates in a drastic reduction of the number of peaks around 10 GPa. We attribute this to a possible monoclinic-trigonal phase transition at 10 GPa. At higher pressures (~ 13 GPa), significant increases in peak intensities and sharpening of the Raman peaks suggest a bandgap-lowering and an isostructural electronic transition, with a possible onset of metallization at pressures above 17 GPa. When the pressure is released, the structure again phase-separates into two distinct chemical domains within the same single crystalline framework -- however, these domains are much smaller in size than the as-synthesized material resulting in suppression of ferroelectricity through nanoconfinement. Hydrostatic pressure can thus be used to tune the electronic and ferrielectric properties of Cu-deficient layered CuInP$_2$S$_6$. △ Less

Submitted 30 August, 2021; originally announced August 2021.

Comments: 7 Figures, 1 Table

Fast Scanning Probe Microscopy via Machine Learning: Non-rectangular scans with compressed sensing and Gaussian process optimization

Authors: Kyle P. Kelley, Maxim Ziatdinov, Liam Collins, Michael A. Susner, Rama K. Vasudevan, Nina Balke, Sergei V. Kalinin, Stephen Jesse

Abstract: Fast scanning probe microscopy enabled via machine learning allows for a broad range of nanoscale, temporally resolved physics to be uncovered. However, such examples for functional imaging are few in number. Here, using piezoresponse force microscopy (PFM) as a model application, we demonstrate a factor of 5.8 improvement in imaging rate using a combination of sparse spiral scanning with compress… ▽ More Fast scanning probe microscopy enabled via machine learning allows for a broad range of nanoscale, temporally resolved physics to be uncovered. However, such examples for functional imaging are few in number. Here, using piezoresponse force microscopy (PFM) as a model application, we demonstrate a factor of 5.8 improvement in imaging rate using a combination of sparse spiral scanning with compressive sensing and Gaussian processing reconstruction. It is found that even extremely sparse scans offer strong reconstructions with less than 6 % error for Gaussian processing reconstructions. Further, we analyze the error associated with each reconstructive technique per reconstruction iteration finding the error is similar past approximately 15 iterations, while at initial iterations Gaussian processing outperforms compressive sensing. This study highlights the capabilities of reconstruction techniques when applied to sparse data, particularly sparse spiral PFM scans, with broad applications in scanning probe and electron microscopies. △ Less

Submitted 23 April, 2020; originally announced April 2020.

Prediction of Antiferromagnetism in Barium Chromium Phosphide Confirmed after Synthesis

Authors: R. A. Jishi, J. P. Rodriguez, T. J. Haugan, M. A. Susner

Abstract: We have carried out density-functional theory (DFT) calculations for the chromium pnictide BaCr2P2, which is structurally analogous to BaFe2As2, a parent compound for iron-pnictide superconductors. Evolutionary methods combined with DFT predict that the chromium analog has the same crystal structure as the latter. DFT also predicts Neel antiferromagnetic order on the chromium sites. Comparison wit… ▽ More We have carried out density-functional theory (DFT) calculations for the chromium pnictide BaCr2P2, which is structurally analogous to BaFe2As2, a parent compound for iron-pnictide superconductors. Evolutionary methods combined with DFT predict that the chromium analog has the same crystal structure as the latter. DFT also predicts Neel antiferromagnetic order on the chromium sites. Comparison with a simple electron-hopping model over a square lattice of chromium atoms suggests that it is due to residual nesting of the Fermi surfaces. We have confirmed the DFT predictions directly after the successful synthesis of polycrystalline samples of BaCr2P2. X-ray diffraction recovers the predicted crystal structure to high accuracy, while magnetic susceptibility and specific-heat measurements are consistent with a transition to an antiferromagnetically ordered state below T_N ~ 60 K. △ Less

Submitted 20 October, 2019; v1 submitted 9 July, 2018; originally announced July 2018.

Comments: 26 pages, 14 figures

Journal ref: Journal of Physics: Condensed Matter 32, 025502 (2020)

Giant negative electrostriction and dielectric tunability in a van der Waals layered ferroelectric

Authors: Sabine M. Neumayer, Eugene A. Eliseev, Michael A. Susner, Alexander Tselev, Brian J. Rodriguez, John A. Brehm, Sokrates T. Pantelides, Ganesh Panchapakesan, Stephen Jesse, Sergei V. Kalinin, Michael A. McGuire, Anna N. Morozovska, Petro Maksymovych, Nina Balke

Abstract: The interest in ferroelectric van der Waals crystals arises from the potential to realize ultrathin ferroic systems owing to the reduced surface energy of these materials and the layered structure that allows for exfoliation. Here, we quantitatively unravel giant negative electrostriction of van der Waals layered copper indium thiophosphate (CIPS), which exhibits an electrostrictive coefficient Q3… ▽ More The interest in ferroelectric van der Waals crystals arises from the potential to realize ultrathin ferroic systems owing to the reduced surface energy of these materials and the layered structure that allows for exfoliation. Here, we quantitatively unravel giant negative electrostriction of van der Waals layered copper indium thiophosphate (CIPS), which exhibits an electrostrictive coefficient Q33 as high as -3.2 m4/C2 and a resulting bulk piezoelectric coefficient d33 up to -85 pm/V. As a result, the electromechanical response of CIPS is comparable in magnitude to established perovskite ferroelectrics despite possessing a much smaller spontaneous polarization of only a few uC/cm2. In the paraelectric state, readily accessible owing to low transition temperatures, CIPS exhibits large dielectric tunability, similar to widely-used barium strontium titanate, and as a result both giant and continuously tunable electromechanical response. The persistence of electrostrictive and tunable responses in the paraelectric state indicates that even few layer films or nanoparticles will sustain significant electromechanical functionality, offsetting the inevitable suppression of ferroelectric properties in the nanoscale limit. These findings can likely be extended to other ferroelectric transition metal thiophosphates and (quasi-) two-dimensional materials and might facilitate the quest towards novel ultrathin functional devices incorporating electromechanical response. △ Less

Submitted 1 February, 2019; v1 submitted 21 March, 2018; originally announced March 2018.

Comments: Published version in Phys. Rev. Materials https://journals.aps.org/prmaterials/abstract/10.1103/PhysRevMaterials.3.024401

Journal ref: Phys. Rev. Materials 3, 024401 (2019)

Flux growth in a horizontal configuration: an analogue to vapor transport growth

Authors: J. -Q. Yan, B. C. Sales, M. A. Susner, M. A. McGuire

Abstract: Flux growth of single crystals is normally performed in a vertical configuration with an upright refractory container holding the flux melt. At high temperatures, flux dissolves the charge forming a homogeneous solution before nucleation and growth of crystals take place under proper supersaturation generated by cooling or evaporating the flux. In this work, we report flux growth in a horizontal c… ▽ More Flux growth of single crystals is normally performed in a vertical configuration with an upright refractory container holding the flux melt. At high temperatures, flux dissolves the charge forming a homogeneous solution before nucleation and growth of crystals take place under proper supersaturation generated by cooling or evaporating the flux. In this work, we report flux growth in a horizontal configuration with a temperature gradient along the horizontal axis: a liquid transport growth analogous to the vapor transport technique. In a typical liquid transport growth, the charge is kept at the hot end of the refractory container and the flux melt dissolves the charge and transfers it to the cold end. Once the concentration of charge is above the solubility limit at the cold end, the thermodynamically stable phase nucleates and grows. Compared to the vertical flux growth, the liquid transport growth can provide a large quantity of crystals in a single growth since the charge/flux ratio is not limited by the solubility limit at the growth temperature. This technique is complementary to the vertical flux growth and can be considered when a large amount of crystals are needed but the yield from the conventional vertical flux growth is limited. We applied this technique to the growth of IrSb$_3$, Mo$_3$Sb$_7$, MnBi from self flux, and the growth of FeSe, CrTe$_3$, NiPSe$_3$, FePSe$_3$, and InCuP$_2$S$_6$ from a halide flux. △ Less

Submitted 7 June, 2017; originally announced June 2017.

Journal ref: Phys. Rev. Materials 1, 023402 (2017)

Effect of Chemical Pressure on High Temperature Ferrimagnetic Double Perovskites Sr2CrOsO6 and Ca2CrOsO6

Authors: Ryan Morrow, Jennifer R. Soliz, Adam J. Hauser, James C. Gallagher, Michael A. Susner, Michael D. Sumption, Adam A. Aczel, Jiaqiang Yan, Fengyuan Yang, Patrick M. Woodward

Abstract: The ordered double perovskites Sr2CrOsO6 and Ca2CrOsO6 have been synthesized and characterized with neutron powder diffraction, electrical transport measurements, and high field magnetization experiments. Sr2CrOsO6 and Ca2CrOsO6 crystallize with R-3 and P21/n space group symmetry, respectively. Both materials are found to be ferrimagnetic insulators with saturation magnetizations near 0.2 μB. Sr2C… ▽ More The ordered double perovskites Sr2CrOsO6 and Ca2CrOsO6 have been synthesized and characterized with neutron powder diffraction, electrical transport measurements, and high field magnetization experiments. Sr2CrOsO6 and Ca2CrOsO6 crystallize with R-3 and P21/n space group symmetry, respectively. Both materials are found to be ferrimagnetic insulators with saturation magnetizations near 0.2 μB. Sr2CrOsO6 orders at 660 K, showing non-monotonic magnetization temperature dependence, while Ca2CrOsO6 orders at 490 K and does not show non-monotonic behavior. Evidence for a theoretically predicted canted magnetic structure in Sr2CrOsO6 is sought and not found. △ Less

Submitted 2 September, 2015; originally announced September 2015.

Comments: 18 pages and 8 figures

Journal ref: J Solid State Chem 238, 46-52 (2016)

Growth and Characterization of Ce- Substituted Nd2Fe14B Single Crystals

Authors: M. A. Susner, B. S. Conner, B. I. Saparov, M. A. McGuire, E. J. Crumlin, G. M. Veith, H. B. Cao, K. V. Shanavas, D. S. Parker, B. C. Chakoumakos, B. C. Sales

Abstract: Single crystals of (Nd1-xCex)2Fe14B are grown out of Fe-(Nd,Ce) flux. Chemical and structural analysis of the crystals indicates that (Nd1-xCex)2Fe14B forms a solid solution until at least x = 0.38 with a Vegard-like variation of the lattice constants with x. Refinements of single crystal neutron diffraction data indicate that Ce has a slight site preference (7:3) for the 4g rare earth site over t… ▽ More Single crystals of (Nd1-xCex)2Fe14B are grown out of Fe-(Nd,Ce) flux. Chemical and structural analysis of the crystals indicates that (Nd1-xCex)2Fe14B forms a solid solution until at least x = 0.38 with a Vegard-like variation of the lattice constants with x. Refinements of single crystal neutron diffraction data indicate that Ce has a slight site preference (7:3) for the 4g rare earth site over the 4f site. Magnetization measurements show that for x = 0.38 the saturation magnetization at 400 K, a temperature important to applications, falls from 29.8 for the parent Nd2Fe14B to 27.6 (mu)B/f.u., the anisotropy field decreases from 5.5 T to 4.7 T, and the Curie temperature decreases from 586 to 543 K. First principles calculations carried out within density functional theory are used to explain the decrease in magnetic properties due to Ce substitution. Though the presence of the lower-cost and more abundant Ce slightly affects these important magnetic characteristics, this decrease is not large enough to affect a multitude of applications. Ce-substituted Nd2Fe14B is therefore a potential high-performance permanent magnet material with substantially reduced Nd content. △ Less

Submitted 31 August, 2015; originally announced August 2015.

Comments: 11 Pages, 8 figures, 5 tables

Itinerant Antiferromagnetism in FeMnP0.8Si0.2 Single Crystals

Authors: B. C. Sales, M. A. Susner, B. S. Conner, J. Q. Yan, A. F. May

Abstract: Compounds based on the Fe2P structure have continued to attract interest because of the interplay between itinerant and localized magnetism in a non-centrosymmetric crystal structure, and because of the recent developments of these materials for magnetocaloric applications. Here we report the growth and characterization of mm size single crystals of FeMnP0.8Si0.2. Single crystal x-ray diffraction,… ▽ More Compounds based on the Fe2P structure have continued to attract interest because of the interplay between itinerant and localized magnetism in a non-centrosymmetric crystal structure, and because of the recent developments of these materials for magnetocaloric applications. Here we report the growth and characterization of mm size single crystals of FeMnP0.8Si0.2. Single crystal x-ray diffraction, magnetization, resistivity, Hall and heat capacity data are reported. Surprisingly, the crystals exhibit itinerant antiferromagnetic order below 158 K with no hint of ferromagnetic behavior in the magnetization curves and with the spins ordered primarily in the ab plane. The room temperature resistivity is close to the Ioffe-Regel limit for a metal. Single crystal x-ray diffraction indicates a strong preference for Mn to occupy the larger pyramidal 3g site. The cation site preference in the as-grown crystals and the antiferromagnetism are not changed after high temperature anneals and a rapid quench to room temperature. △ Less

Submitted 1 July, 2015; originally announced July 2015.

Comments: 9 Pages, 7 Figures, 2 Tables

Journal ref: Phys. Rev. B 92, 104429 (2015)

Magnetic Structure of the Quasi-One-Dimensional La3OsO7 as Determined by Neutron Powder Diffraction

Authors: Ryan Morrow, Michael A. Susner, Michael D. Sumption, Patrick M. Woodward

Abstract: Insulating 5d3 La3OsO7 and hole doped La2.8Ca0.2OsO7 materials featuring well separated pseudo-one-dimensional zig-zag chains of corner-sharing OsO6 octahedra have been synthesized and their magnetic and electrical transport properties characterized. Long range magnetic order between the antiferromagnetic chains is determined with a propagation vector k = 1/2, 1/2, 0 and TN = 45 and 53 K for the p… ▽ More Insulating 5d3 La3OsO7 and hole doped La2.8Ca0.2OsO7 materials featuring well separated pseudo-one-dimensional zig-zag chains of corner-sharing OsO6 octahedra have been synthesized and their magnetic and electrical transport properties characterized. Long range magnetic order between the antiferromagnetic chains is determined with a propagation vector k = 1/2, 1/2, 0 and TN = 45 and 53 K for the parent and doped materials. An Os5+ moment of 1.7(1) μB for La3OsO7 and 1.2(2) μB for La2.8Ca0.2OsO7 is refined. The long range magnetic structure is compared to the few currently known for isostructural Ln3MO7 compounds. △ Less

Submitted 11 June, 2015; v1 submitted 5 June, 2015; originally announced June 2015.

Comments: 15 pages, 6 figures

Journal ref: Phys. Rev. B 92, 134402 (2015)

Pressure-induced electronic phase separation of magnetism and superconductivity in CrAs

Authors: R. Khasanov, Z. Guguchia, I. Eremin, H. Luetkens, A. Amato, P. K. Biswas, Ch. Rüegg, M. A. Susner, A. S. Sefat, N. D. Zhigadlo, E. Morenzoni

Abstract: The recent discovery of pressure induced superconductivity in the binary helimagnet CrAs has attracted much attention. How superconductivity emerges from the magnetic state and what is the mechanism of the superconducting pairing are two important issues which need to be resolved. In the present work, the suppression of magnetism and the occurrence of superconductivity in CrAs as a function of pre… ▽ More The recent discovery of pressure induced superconductivity in the binary helimagnet CrAs has attracted much attention. How superconductivity emerges from the magnetic state and what is the mechanism of the superconducting pairing are two important issues which need to be resolved. In the present work, the suppression of magnetism and the occurrence of superconductivity in CrAs as a function of pressure ($p$) were studied by means of muon spin rotation. The magnetism remains bulk up to $p\simeq3.5$~kbar while its volume fraction gradually decreases with increasing pressure until it vanishes at $p\simeq$7~kbar. At 3.5 kbar superconductivity abruptly appears with its maximum $T_c \simeq 1.2$~K which decreases upon increasing the pressure. In the intermediate pressure region ($3.5\lesssim p\lesssim 7$~kbar) the superconducting and the magnetic volume fractions are spatially phase separated and compete for phase volume. Our results indicate that the less conductive magnetic phase provides additional carriers (doping) to the superconducting parts of the CrAs sample thus leading to an increase of the transition temperature ($T_c$) and of the superfluid density ($ρ_s$). A scaling of $ρ_s$ with $T_c^{3.2}$ as well as the phase separation between magnetism and superconductivity point to a conventional mechanism of the Cooper-pairing in CrAs. △ Less

Submitted 26 February, 2015; originally announced February 2015.

Comments: 9 pages, 8 figures

Journal ref: Scientific Reports 5, 13788 (2015)

Pressure dependence of the magnetic order in CrAs: a neutron diffraction investigation

Authors: L. Keller, J. S. White, M. Frontzek, P. Babkevich, M. A. Susner, Z. C. Sims, A. S. Sefat, H. M. Ronnow, Ch. Rüegg

Abstract: The suppression of magnetic order with pressure concomitant with the appearance of pressure- induced superconductivity was recently discovered in CrAs. Here we present a neutron diffraction study of the pressure evolution of the helimagnetic ground-state towards and in the vicinity of the superconducting phase. Neutron diffraction on polycrystalline CrAs was employed from zero pressure to 0.65 GPa… ▽ More The suppression of magnetic order with pressure concomitant with the appearance of pressure- induced superconductivity was recently discovered in CrAs. Here we present a neutron diffraction study of the pressure evolution of the helimagnetic ground-state towards and in the vicinity of the superconducting phase. Neutron diffraction on polycrystalline CrAs was employed from zero pressure to 0.65 GPa and at various temperatures. The helimagnetic long-range order is sustained under pressure and the magnetic propagation vector does not show any considerable change. The average ordered magnetic moment is reduced from 1.73(2) μB at ambient pressure to 0.4(1) μB close to the critical pressure Pc=0.7 GPa, at which magnetic order is completely suppressed. The width of the magnetic Bragg peaks strongly depends on temperature and pressure, showing a maximum in the region of the onset of superconductivity. We interpret this as associated with competing ground-states in the vicinity of the superconducting phase. △ Less

Submitted 19 September, 2014; originally announced September 2014.

Effects of carbon concentration and filament number on advanced internal-Mg-infiltration-processed MgB2 Strands

Authors: G Z Li, M D Sumption, J B Zwayer, M A Susner, M A Rindfleisch, C J Thong, M J Tomsic, E W Collings

Abstract: An advanced internal Mg infiltration method (AIMI) in this paper has been shown to be effective in producing superconducting wires containing dense MgB2 layers with high critical current densities. In this study, the in-field critical current densities of a series of AIMI-fabricated MgB2 strands were investigated in terms of C doping levels, heat treatment (HT) time and filament numbers. The highe… ▽ More An advanced internal Mg infiltration method (AIMI) in this paper has been shown to be effective in producing superconducting wires containing dense MgB2 layers with high critical current densities. In this study, the in-field critical current densities of a series of AIMI-fabricated MgB2 strands were investigated in terms of C doping levels, heat treatment (HT) time and filament numbers. The highest layer Jc for our monofilamentary AIMI strands is 1.5 x 10^5 A/cm2 at 10 T, 4.2 K, when the C concentration was 3 mol% and the strand was heat-treated at 675 °C for 4 hours. Transport critical currents were also measured at 4.2 K on short samples and one-meter segments of eighteen-filament C-doped AIMI strands. The layer Jcs reached 4.3 x 10^5 A/cm2 at 5 T and 7.1 x 10^4 A/cm2 at 10 T, twice as high as those of the best PIT strands. The analysis of these results indicates that the AIMI strands, possessing both high layer Jcs and engineering Jes after further optimization, have strong potential for commercial applications. △ Less

Submitted 3 July, 2013; originally announced July 2013.

Comments: 15 pages, 2 tables, 8 figures (to be published in Supercond. Sci. Technol.)

The critical current density of advanced internal-Mg-diffusion-processed MgB2 wires

Authors: G. Z. Li, M. D. Sumption, M. A. Susner, Y. Yang, K. M. Reddy, M. A. Rindfleisch, M. J. Tomsic, C. J. Thong, E. W. Collings

Abstract: Recent advances in MgB2 conductors are leading to a new level of performance. Based on the use of proper powders, proper chemistry, and an architecture which incorporates internal Mg diffusion (IMD), a dense MgB2 structure with not only a high critical current density Jc, but also a high engineering critical current density, Je, can be obtained. In this paper, a series of these advanced (or second… ▽ More Recent advances in MgB2 conductors are leading to a new level of performance. Based on the use of proper powders, proper chemistry, and an architecture which incorporates internal Mg diffusion (IMD), a dense MgB2 structure with not only a high critical current density Jc, but also a high engineering critical current density, Je, can be obtained. In this paper, a series of these advanced (or second - generation, "2G") conductors has been prepared. Scanning electron microscopy and associated energy dispersive X-ray spectroscopy were applied to characterize the microstructures and compositions of the wires, and a dense MgB2 layer structure was observed. The best layer Jc for our sample is 1.07x105 A/cm2 at 10 T, 4.2 K, and our best Je is seen to be 1.67x104 A/cm2 at 10 T, 4.2 K. Optimization of the transport properties of these advanced wires is discussed in terms of B-powder choice, area fraction, and the MgB2 layer growth mechanism. △ Less

Submitted 10 October, 2012; v1 submitted 15 June, 2012; originally announced June 2012.

Comments: 13 pages, 3 tables, 7 figures (or 8 pp in published version)

Journal ref: published in Supercond. Sci. Technol. 25 (2012) 115023 (8pp)

Drawing induced texture and the evolution of superconductive properties with heat treatment time in powder-in-tube in-situ processed MgB2 strands

Authors: M. A. Susner, T. W. Daniels, M. D. Sumption, E. W. Collings

Abstract: Monocore powder-in-tube MgB2 strands were cold-drawn and heat-treated at 600C and 700C for times of up to 71 hours and structure-property relationships examined. Drawing-induced elongation of the Mg particles led, after HT, to a textured macrostructure consisting of elongated polycrystalline MgB2 fibers separated by elongated pores. The superconducting Tc, Jc and Fp were correlated with the macros… ▽ More Monocore powder-in-tube MgB2 strands were cold-drawn and heat-treated at 600C and 700C for times of up to 71 hours and structure-property relationships examined. Drawing-induced elongation of the Mg particles led, after HT, to a textured macrostructure consisting of elongated polycrystalline MgB2 fibers separated by elongated pores. The superconducting Tc, Jc and Fp were correlated with the macrostructure and grain size. Grain size increased with HT time at both 600C and 700C. Jc and hence Fp decreased monotonically but not linearly with grain size. Overall, it was observed that at 700C, the MgB2 reaction was more or less complete after as little as 30 min; at 600C, full reaction completion did not occur until 71 h. into the HT. Transport, Jct(B) was measured in a perpendicular applied field, and the magnetic critical current densities, Jcm\bot(B) and Jcmφ(B), were measured in perpendicular and parallel (axial) applied fields, respectively. Particularly noticeable was the premature dropoff of Jcm\bot(B) at fields well below the irreversibility field of Jct(B). This effect is attributed to the fibrous macrostructure and its accompanying anisotropic connectivity. Magnetic measurements with the field directed along the strand axis yielded a critical density, Jcm\bot(B), for current flowing transversely to the strand axis that was less than and dropped off more rapidly than Jct(B). In the conventional magnetic measurement, the loop currents that support the magnetization are restricted by the lower of Jct(B) and Jcmφ (B). In the present case the latter, leading to the premature dropoff of the measured Jcm(B) compared to Jct(B) with increasing field. This result is supported by Kramer plots of the Jcmφ (B) and Jct(B) data which lead to an irreversibility field for transverse current that is very much less than the usual transport-measured longitudinal one, Birr,t. △ Less

Submitted 21 September, 2011; originally announced September 2011.

Comments: 41 pages, 14 figures

Critical Current Density and n-values of MgB2 Strands over a Wide Range of Temperatures and Fields

Authors: G Z Li, Y Yang, M A Susner, M D Sumption, E W Collings

Abstract: Transport measurements of critical current density, Jct, in monocore powder-in-tube MgB2 strands have been carried out at temperatures, T, of from 4.2 K to 40 K, and in transverse fields, B, of up to 14 T. Processing methods used were conventional continuous-tube-forming-filling (CTFF) and internal-magnesium-diffusion (IMD). Strands with several powder compositions were measured, including binary… ▽ More Transport measurements of critical current density, Jct, in monocore powder-in-tube MgB2 strands have been carried out at temperatures, T, of from 4.2 K to 40 K, and in transverse fields, B, of up to 14 T. Processing methods used were conventional continuous-tube-forming-filling (CTFF) and internal-magnesium-diffusion (IMD). Strands with several powder compositions were measured, including binary (undoped) MgB2, 2% carbon doped MgB2, and 3% carbon doped MgB2. Magnetization loops (M-B) were also measured, and magnetic critical current density, Jcm, values extracted from them. The transport, Jct(B) and magnetic, Jcm(B), critical current densities were compared. Also studied was the influence of doping on the resistively measured irreversibility field, Birr and upper critical field Bc2. Critical current densities, Jct, and n-values were extracted from transport measurements and were found to be universally related (for all B and T) according to n \propto Jctm in which m = 0.52 \pm 0.11. Likewise n was found to be related to B according to n \propto B-p with a T-dependent p in the range of about 0.08~0.21. Further analysis of the field (B) and temperature (T) dependencies of n-value resulted in an expression that enabled n(B,T), for all B and T, to be estimated for a given strand based on the results of transport Jct(B) measurements made at one arbitrarily chosen temperature. △ Less

Submitted 24 November, 2011; v1 submitted 29 August, 2011; originally announced August 2011.

Comments: 34 pages, 7 figures

Journal ref: Supercond. Sci. Technol. 25 (2012) 025001

Doping Effect and Flux Pinning Mechanism of Nano-SiC Additions in MgB2 Strands

Authors: Z. X. Shi, M. A. Susner, M. D. Sumption, E. W. Collings

Abstract: Superconducting MgB2 strands with nanometer-scale SiC additions have been investigated systematically using transport and magnetic measurements. A comparative study of MgB2 strands with different nano-SiC addition levels has shown C-doping-enhanced critical current density Jc through enhancements in the upper critical field, Hc2, and decreased anisotropy. The critical current density and flux pinn… ▽ More Superconducting MgB2 strands with nanometer-scale SiC additions have been investigated systematically using transport and magnetic measurements. A comparative study of MgB2 strands with different nano-SiC addition levels has shown C-doping-enhanced critical current density Jc through enhancements in the upper critical field, Hc2, and decreased anisotropy. The critical current density and flux pinning force density obtained from magnetic measurements were found to greatly differ from the values obtained through transport measurements, particularly with regards to magnetic field dependence. The differences in magnetic and transport results are largely attributed to connectivity related effects. On the other hand, based on the scaling behavior of flux pinning force, there may be other effective pinning centers in MgB2 strands in addition to grain boundary pinning. △ Less

Submitted 27 February, 2011; originally announced February 2011.

Comments: 22 pages, 9 figures, 1 table

Carbon Doping of MgB2 by Toluene and Malic-Acid-in-Toluene

Authors: S. D. Bohnenstiehl, M. A. Susner, Y. Yang, E. W. Collings, M. D. Sumption, M. A. Rindfleisch, R. Boone

Abstract: The decomposition of malic acid in the presence of Mg and B was studied using Differential Scanning Calorimetry (DSC) and Thermogravimetric Analysis (TGA) which revealed that malic acid reacted with Mg but not B. Also, the addition of toluene to dissolve malic acid followed by subsequent drying resulted in no reaction with Mg, indicating that the malic acid had decomposed during the dissolution/dr… ▽ More The decomposition of malic acid in the presence of Mg and B was studied using Differential Scanning Calorimetry (DSC) and Thermogravimetric Analysis (TGA) which revealed that malic acid reacted with Mg but not B. Also, the addition of toluene to dissolve malic acid followed by subsequent drying resulted in no reaction with Mg, indicating that the malic acid had decomposed during the dissolution/drying stage. The total carbon contributed by toluene versus a toluene/5 wt% malic acid mixture was measured using a LECO CS600 carbon analyzer. The toluene sample contained ~0.4 wt% C while the toluene/malic acid mixture had ~1.5 wt% C, demonstrating that the toluene contributed a significant amount of carbon to the final product. Resistivity measurements on powder-in-tube MgB2 monofilamentary wires established that the toluene/malic acid doped sample had the highest Bc2. However, the toluene-only sample had the highest transport Jc over most of the magnetic field range (0-9 T), equaled only by that of toluene/malic acid sample in fields above 9 T. △ Less

Submitted 5 October, 2010; originally announced October 2010.

Comments: 17 pages, 6 figures, 1 table

Enhanced superconducting properties of pre-doped B powder type MgB2 strands

Authors: M. A. Susner, Y. Yang, M. D. Sumption, M. A. Rindfleisch, M. J. Tomsic, J. V. Marzik, E. W. Collings

Abstract: Conventional doping methods that directly add C or a C-bearing species to Mg+B powder have the disadvantage of adding C inhomogeneously, yielding either under-reacted regions or blocking phases. Pre-doped B powder provides a more homogeneous distribution of the C dopant in MgB2. Powders containing varying amounts of C were used to produce in-situ MgB2 strands which showed high values of transport… ▽ More Conventional doping methods that directly add C or a C-bearing species to Mg+B powder have the disadvantage of adding C inhomogeneously, yielding either under-reacted regions or blocking phases. Pre-doped B powder provides a more homogeneous distribution of the C dopant in MgB2. Powders containing varying amounts of C were used to produce in-situ MgB2 strands which showed high values of transport Jc (104 A/cm2 at 13.3T). Compared to SiC-added and malic acid-treated strands the pre-doped MgB2 showed both higher values of Birr and transport Jc, indicating that the pre-doping of B leads to more efficient C substitution into the B-sublattice. △ Less

Submitted 18 August, 2010; originally announced August 2010.

Comments: 12 pages, 3 figures submitted to Applied Physics Letters

Anisotropic Connectivity and its Influence on Critical Current Densities, Irreversibility Fields, and Flux Creep in In-Situ-Processed MgB2 Strands

Authors: Z. X. Shi, M. A. Susner, M Majoros, M. D. Sumption, X. Peng, M. Rindfleisch, M. J. Tomsic, E. W. Collings

Abstract: The anisotropy of the critical current density (Jc) and its influence on measurement of irreversibility field (Birr) has been investigated for high quality, in-situ MgB2 strands. Comparison of transport and magnetization measurements has revealed the onset of a regime where large differences exist between transport and magnetically measured values of the critical current density and Birr. These… ▽ More The anisotropy of the critical current density (Jc) and its influence on measurement of irreversibility field (Birr) has been investigated for high quality, in-situ MgB2 strands. Comparison of transport and magnetization measurements has revealed the onset of a regime where large differences exist between transport and magnetically measured values of the critical current density and Birr. These effects, initially unexpected due to the lack of crystalline texture in these in-situ processed strands, appear to be due to a fibrous microstructure, connected with the details of the wire fabrication and MgB2 formation reactions. Scanning electron micrographs of in-situ-processed MgB2 monocore strands have revealed a fibrous microstructure. Grains (~100 nm) are randomly oriented, and there is no apparent local texture of the grains. However, this randomly oriented polycrystalline material has a fibrous texture at a larger length scale, with stringers of MgB2 (~ 60 μm long and ~5 μm in diameter) partially separated by elongated pores -- the spaces previously occupied by stringers of elemental Mg. This leads to an interpretation of the differences observed in transport and magnetically determined critical currents, in particular a large deviation between the two at higher fields, in terms of different transverse and longitudinal connectivities within the strand. The different values of connectivity also lead to different resistive transition widths, and thus irreversibility field values, as measured by transport and magnetic techniques. Finally, these considerations are seen to influence estimated pinning potentials for the strands. △ Less

Submitted 7 January, 2010; originally announced January 2010.

Comments: 43 Pages, 11 Figures, accepted by Supercon. Sci. Tech

Journal ref: Supercond. Sci. Technol. 23 (2010) 045018

Prospects for Improving the Intrinsic and Extrinsic Properties of Magnesium Diboride Superconducting Strands

Authors: E. W. Collings, M. D. Sumption, M. Bhatia, M. A. Susner, S. D. Bohnenstiehl

Abstract: The magnetic and transport properties of magnesium diboride films represent performance goals yet to be attained by powder-processed bulk samples and conductors. Such performance limits are still out of the reach of even the best magnesium diboride magnet wire. In discussing the present status and prospects for improving the performance of powder-based wire we focus attention on (1) the intrinsi… ▽ More The magnetic and transport properties of magnesium diboride films represent performance goals yet to be attained by powder-processed bulk samples and conductors. Such performance limits are still out of the reach of even the best magnesium diboride magnet wire. In discussing the present status and prospects for improving the performance of powder-based wire we focus attention on (1) the intrinsic (intragrain) superconducting properties of magnesium diboride, Hc2 and flux pinning, (2) factors that control the efficiency with which current is transported from grain-to-grain in the conductor, an extrinsic (intergrain) property. With regard to Item-(1), the role of dopants in Hc2 enhancement is discussed and examples presented. On the other hand their roles in increasing Jc, both via Hc2 enhancement as well as direct fluxoid/pining-center interaction, are discussed and a comprehensive survey of Hc2 dopants and flux-pinning additives is presented. Current transport through the powder-processed wire (an extrinsic property) is partially blocked by the inherent granularity of the material itself and the chemical or other properties of the intergrain surfaces. These and other such results indicate that in many cases less than 15% of the conductor's cross sectional area is able to carry transport current. It is pointed out that densification in association with the elimination of grain-boundary blocking phases would yield five-to ten-fold increases in Jc in relevant regimes, enabling the performance of magnesium diboride in selected applications to compete with that of Nb-Sn. △ Less

Submitted 9 May, 2008; originally announced May 2008.