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Efficiently gate-tunable ferromagnetism in ferromagnetic semiconductor-Dirac semimetal p-n heterojunctions
Authors:
Emma Steinebronn,
Saurav Islam,
Abhinava Chatterjee,
Bimal Neupane,
Alex Grutter,
Christopher Jensen,
Julie A. Borchers,
Timothy Charlton,
Wilson J. Yanez-Parreno,
Juan Chamorro,
Tanya Berry,
Supriya Ghosh,
K. A. Nivedith,
K. Andre Mkhoyan,
Tyrel McQueen,
Yuanxi Wang,
Chaoxing Liu,
Nitin Samarth
Abstract:
We use molecular beam epitaxy to develop a gate tunable p-n heterojunction that interfaces a canonical Dirac semimetal, Cd$_3$As$_2$, and a ferromagnetic semiconductor, In$_{1-x}$Mn$_x$As, with perpendicular magnetic anisotropy. Measurements of the anomalous Hall effect in top-gated Cd$_3$As$_2$/In$_{1-x}$Mn$_x$As devices show that the ferromagnetic Curie temperature ($T_\mathrm{C}$) can be effici…
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We use molecular beam epitaxy to develop a gate tunable p-n heterojunction that interfaces a canonical Dirac semimetal, Cd$_3$As$_2$, and a ferromagnetic semiconductor, In$_{1-x}$Mn$_x$As, with perpendicular magnetic anisotropy. Measurements of the anomalous Hall effect in top-gated Cd$_3$As$_2$/In$_{1-x}$Mn$_x$As devices show that the ferromagnetic Curie temperature ($T_\mathrm{C}$) can be efficiently tuned using a modest gate voltage of $\sim 10$ V, corresponding to a sensitivity to electric field ($E$) of $ΔT_{\mathrm{C}}/ΔE \sim 10$ K/MV/cm). The voltage tuning of $T_\mathrm{C}$ saturates near the charge neutrality point of Cd$_3$As$_2$ and vanishes at positive gate voltage in appropriately designed heterostructures. This non-monotonic behavior cannot be explained solely by hole-mediated ferromagnetism in the In$_{1-x}$Mn$_x$As alone, suggesting an interaction between the Dirac semimetal and the ferromagnetic semiconductor. Our results identify Cd$_3$As$_2$/In$_{1-x}$Mn$_x$As heterojunctions as a potentially attractive platform for studying emergent phenomena arising from the interplay between broken symmetry, topology, and magnetism in a topological semimetal.
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Submitted 5 March, 2026;
originally announced March 2026.
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Bonding Interactions Can Drive Topological Phase Transitions in a Zintl Antiferromagnetic Insulator
Authors:
Tanya Berry,
Jaime M. Moya,
David Smiadak,
Scott B. Lee,
Sigalit Aharon,
Alexandra Zevalkink,
Tyrel M. McQueen,
Leslie M. Schoop
Abstract:
While $\sim$30% of materials are reported to be topological, topological insulators are rare. Magnetic topological insulators (MTI) are even harder to find. Identifying crystallographic features that can host the coexistence of a topological insulating phase with magnetic order is vital for finding intrinsic MTI materials. Thus far, most materials that are investigated for the determination of an…
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While $\sim$30% of materials are reported to be topological, topological insulators are rare. Magnetic topological insulators (MTI) are even harder to find. Identifying crystallographic features that can host the coexistence of a topological insulating phase with magnetic order is vital for finding intrinsic MTI materials. Thus far, most materials that are investigated for the determination of an MTI are some combination of known topological insulators with a magnetic ion such as MnBi$_2$Te$_4$. Motivated by the recent success of EuIn$_{2}$As$_{2}$, we investigate the role of chemical pressure on topologically trivial insulator, Eu$_5$In$_2$Sb$_6$ via Ga substitution. Eu$_5$Ga$_2$Sb$_6$ is predicted to be topological but is synthetically difficult to stabilize. We look into the intermediate compositions between Eu$_5$In$_2$Sb$_6$ and Eu$_5$Ga$_2$Sb$_6$ through theoretical works to explore a topological phase transition and band inversion mechanism. We attribute the band inversion mechanism to changes in Eu-Sb hybridization as Ga is substituted for In due to chemical pressure. We also synthesize Eu$_{5}$In$_{4/3}$Ga$_{2/3}$Sb$_{6}$, the highest Ga concentration in Eu$_{5}$In$_{2-x}$Ga$_{x}$Sb$_{6}$, and report the thermodynamic, magnetic, transport, and Hall properties. Overall, our work paints a picture of a possible MTI via band engineering and explains why Eu-based Zintl compounds are suitable for the co-existence of magnetism and topology.
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Submitted 31 August, 2024;
originally announced September 2024.
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Anomalous electronic energy relaxation and soft phonons in the Dirac semimetal Cd$_3$As$_2$
Authors:
Rishi Bhandia,
David Barbalas,
Run Xiao,
Juan R. Chamorro,
Tanya Berry,
Tyrel M. McQueen,
Nitin Samarth,
N. P. Armitage
Abstract:
We have used a combination of linear response time-domain THz spectroscopy (TDTS) and high-field non-linear THz spectroscopy to separately probe the electronic momentum and energy relaxation rates respectively of the Dirac semimetal Cd$_3$As$_2$. We find, consistent with prior measurements, that Cd$_3$As$_2$ has an enormous nonlinearities in the THz frequency range. We extract the momentum relaxat…
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We have used a combination of linear response time-domain THz spectroscopy (TDTS) and high-field non-linear THz spectroscopy to separately probe the electronic momentum and energy relaxation rates respectively of the Dirac semimetal Cd$_3$As$_2$. We find, consistent with prior measurements, that Cd$_3$As$_2$ has an enormous nonlinearities in the THz frequency range. We extract the momentum relaxation rate of Cd$_3$As$_2$ using Drude fits to the optical conductivity. We also conduct THz range 2D coherent spectroscopy. The dominant response is a pump-probe signal, which allow us to separately extract the energy relaxation rate. We find that the rate of energy relaxation decreases down to the lowest measured temperatures. We connect this to Cd$_3$As$_2$ anomalous lattice dynamics, evidence for which is found in its low thermal conductivity and soft phonons in Raman scattering. The lack of a peak in the energy relaxation rate as a function of T can be connected to the linear in T dependence of the current relaxation e.g. the phonon scattering is elastic down to the lowest measured temperatures approximately 120 K.
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Submitted 30 July, 2024; v1 submitted 5 May, 2024;
originally announced May 2024.
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Non-collinear 2k antiferromagnetism in the Zintl semiconductor Eu$_5$In$_2$Sb$_6$
Authors:
Vincent C. Morano,
Jonathan Gaudet,
Nicodemos Varnava,
Tanya Berry,
Thomas Halloran,
Chris J. Lygouras,
Xiaoping Wang,
Christina M. Hoffman,
Guangyong Xu,
Jeffrey W. Lynn,
Tyrel M. McQueen,
David Vanderbilt,
Collin L. Broholm
Abstract:
Eu$_5$In$_2$Sb$_6$ is an orthorhombic non-symmorphic small band gap semiconductor with three distinct Eu$^{2+}$ sites and two low-temperature magnetic phase transitions. The material displays one of the greatest (negative) magnetoresistances of known stoichiometric antiferromagnets and belongs to a family of Zintl materials that may host an axion insulator. Using single crystal neutron diffraction…
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Eu$_5$In$_2$Sb$_6$ is an orthorhombic non-symmorphic small band gap semiconductor with three distinct Eu$^{2+}$ sites and two low-temperature magnetic phase transitions. The material displays one of the greatest (negative) magnetoresistances of known stoichiometric antiferromagnets and belongs to a family of Zintl materials that may host an axion insulator. Using single crystal neutron diffraction, we show that the $T_{\mathrm{N1}}=14\mathrm{~K}$ second-order phase transition is associated with long-range antiferromagnetic order within the chemical unit cell $\left( k_1 = (000) \right)$. Upon cooling below $T_{\mathrm{N1}}$, the relative sublattice magnetizations of this structure vary until a second-order phase transition at $T_{\mathrm{N2}}=7\mathrm{~K}$ that doubles the unit cell along the $\hat{c}$ axis $\left( k_2 = \left(00\frac{1}{2}\right) \right)$. We show the anisotropic susceptibility and our magnetic neutron diffraction data are consistent with magnetic structures described by the $Γ_3$ irreducible representation with the staggered magnetization of the $k_1$ and $k_2$ components polarized along the $\hat{b}$ and $\hat{a}$ axis, respectively. As the $k_2$ component develops, the amplitude of the $k_1$ component is reduced, which indicates a 2k non-collinear magnetic structure. Density functional theory is used to calculate the energies of these magnetic structures and to show the $k_1$ phase is a metal so $T_{\mathrm{N1}}$ is a rare example of a unit-cell-preserving second-order phase transition from a paramagnetic semiconductor to an antiferromagnetic metal. DFT indicates the transition at $T_{\mathrm{N2}}$ to a doubled unit cell reduces the carrier density of the metal, which is consistent with resistivity data.
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Submitted 1 November, 2023;
originally announced November 2023.
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Fragile superconductivity in a Dirac metal
Authors:
Chris J. Lygouras,
Junyi Zhang,
Jonah Gautreau,
Mathew Pula,
Sudarshan Sharma,
Shiyuan Gao,
Tanya Berry,
Thomas Halloran,
Peter Orban,
Gael Grissonnanche,
Juan R. Chamorro,
Kagetora Mikuri,
Dilip K. Bhoi,
Maxime A. Siegler,
Kenneth K. Livi,
Yoshiya Uwatoko,
Satoru Nakatsuji,
B. J. Ramshaw,
Yi Li,
Graeme M. Luke,
Collin L. Broholm,
Tyrel M. McQueen
Abstract:
Studying superconductivity in Dirac semimetals is an important step in understanding quantum matter with topologically non-trivial order parameters. We report on the properties of the superconducting phase in single crystals of the Dirac material LaCuSb2 prepared by the self-flux method. We find that chemical and hydrostatic pressure drastically suppress the superconducting transition. Furthermore…
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Studying superconductivity in Dirac semimetals is an important step in understanding quantum matter with topologically non-trivial order parameters. We report on the properties of the superconducting phase in single crystals of the Dirac material LaCuSb2 prepared by the self-flux method. We find that chemical and hydrostatic pressure drastically suppress the superconducting transition. Furthermore, due to large Fermi surface anisotropy, magnetization and muon spin relaxation measurements reveal Type-II superconductivity for applied magnetic fields along the $a$-axis, and Type-I superconductivity for fields along the $c$-axis. Specific heat confirms the bulk nature of the transition, and its deviation from single-gap $s$-wave BCS theory suggests multigap superconductivity. Our tight-binding model points to an anisotropic gap function arising from the spin-orbital texture near the Dirac nodes, providing an explanation for the appearance of an anomaly in specific heat well below $T_c$. Given the existence of superconductivity in a material harboring Dirac fermions, LaCuSb2 proves an interesting material candidate in the search for topological superconductivity.
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Submitted 4 July, 2023;
originally announced July 2023.
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Formation of a simple cubic antiferromagnet through charge ordering in a double Dirac material
Authors:
T. Berry,
V. C. Morano,
T. Halloran,
X. Zhang,
T. J. Slade,
A. Sapkota,
S. L. Budko,
W. Xie,
D. H. Ryan,
Z. Xu,
Y. Zhao,
J. W. Lynn,
T. Fennell,
P. C. Canfield,
C. L. Broholm,
T. M. McQueen
Abstract:
The appearance of spontaneous charge order in chemical systems is often associated with the emergence of novel, and useful, properties. Here we show through single crystal diffraction that the Eu ions in the mixed valent metal EuPd$_3$S$_4$ undergo long-range charge ordering at $T_{\mathrm{CO}} = 340 \mathrm{~K}$ resulting in simple cubic lattices of Eu$^{2+}$ ($J = 7/2$) and Eu$^{3+}$ ($J = 0$) i…
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The appearance of spontaneous charge order in chemical systems is often associated with the emergence of novel, and useful, properties. Here we show through single crystal diffraction that the Eu ions in the mixed valent metal EuPd$_3$S$_4$ undergo long-range charge ordering at $T_{\mathrm{CO}} = 340 \mathrm{~K}$ resulting in simple cubic lattices of Eu$^{2+}$ ($J = 7/2$) and Eu$^{3+}$ ($J = 0$) ions. As only one of the two sublattices has a non-magnetic ground state, the charge order results in the emergence of remarkably simple G-type antiferromagnetic order at $T_{\mathrm{N}} = 2.85(6) \mathrm{~K}$, observed in magnetization, specific heat, and neutron diffraction. Application of a $0.3 \mathrm{~T}$ field is sufficient to induce a spin flop transition to a magnetically polarized, but still charge ordered, state. Density functional theory calculations show that this charge order also modifies the electronic degeneracies present in the material: without charge order, EuPd$_3$S$_4$ is an example of a double Dirac material containing 8-fold degenerate electronic states, greater than the maximum degeneracy of six possible in molecular systems. The symmetry reduction from charge order transmutes 8-fold double Dirac states into 4-fold Dirac states, a degeneracy that can be preserved even in the presence of the magnetic order. Our results show not only how charge order can be used to produce interesting magnetic lattices, but also how it can be used to engineer controlled degeneracies in electronic states.
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Submitted 1 November, 2023; v1 submitted 3 March, 2023;
originally announced March 2023.
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Charge density wave-templated spin cycloid in topological semimetal $NdSb_{x}Te_{2-x-δ}$
Authors:
Tyger H. Salters,
Fabio Orlandi,
Tanya Berry,
Jason F. Khoury,
Ethan Whittaker,
Pascal Manuel,
Leslie M. Schoop
Abstract:
Magnetic topological semimetals present open questions regarding the interplay of crystal symmetry, magnetism, band topology, and electron correlations. $LnSb_{x}Te_{2-x-δ}$ (Ln= lanthanide) is a family of square-net-derived topological semimetals that allows compositional control of band filling, and access to different topological states via an evolving charge density wave (CDW) distortion. Prev…
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Magnetic topological semimetals present open questions regarding the interplay of crystal symmetry, magnetism, band topology, and electron correlations. $LnSb_{x}Te_{2-x-δ}$ (Ln= lanthanide) is a family of square-net-derived topological semimetals that allows compositional control of band filling, and access to different topological states via an evolving charge density wave (CDW) distortion. Previously studied Gd and Ce members containing a CDW have shown complex magnetic phase diagrams, which implied that spins localized on Ln interact with the CDW, but to this date, no magnetic structures have been solved within the CDW regime of this family of compounds. Here, we report on the interplay of the CDW with magnetism in $NdSb_{x}Te_{2-x-δ}$, by comparing the undistorted square net member $NdSb_{0.94}Te_{0.92}$ with the CDW-distorted phase $NdSb_{0.48}Te_{1.37}$, via single-crystal x-ray diffraction, magnetometry, heat capacity, and neutron powder diffraction. $NdSb_{0.94}Te_{0.92}$ is a collinear antiferromagnet with $T_N$ $\sim$ 2.7 K, where spins align antiparallel to each other, but parallel to the square net of the nuclear structure. $NdSb_{0.48}Te_{1.37}$ exhibits a nearly five-fold modulated CDW ($q_{CDW}=0.18b^*$), isostructural to other $LnSb_{x}Te_{2-x-δ}$ at similar x. $NdSb_{0.48}Te_{1.37}$ displays more complex magnetism with $T_N = 2.3 K$, additional metamagnetic transitions, and an elliptical cycloid magnetic structure with $q_{mag}=-0.41b^*$.The magnitudes of $q_{CDW}$ and $q_{mag}$ exhibit a integer relationship, $1+2q_{mag}=q_{CDW}$, implying a coupling between the CDW and magnetic structure. Given that the CDW is localized within the nonmagnetic distorted square net, we propose that conduction electrons "template" the spin modulation via the Ruderman-Kittel-Kasuya-Yosida interaction.
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Submitted 30 January, 2023;
originally announced January 2023.
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Machine-guided Design of Oxidation Resistant Superconductors for Quantum Information Applications
Authors:
Carson Koppel,
Brandon Wilfong,
Allana Iwanicki,
Elizabeth Hedrick,
Tanya Berry,
Tyrel M. McQueen
Abstract:
Decoherence in superconducting qubits has long been attributed to two level systems arising from the surfaces and interfaces present in real devices. A recent significant step in reducing decoherence was the replacement of superconducting niobium by superconducting tantalum, resulting in a tripling of transmon qubit lifetimes (T1). One of these surface variables, the identity, thickness, and quali…
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Decoherence in superconducting qubits has long been attributed to two level systems arising from the surfaces and interfaces present in real devices. A recent significant step in reducing decoherence was the replacement of superconducting niobium by superconducting tantalum, resulting in a tripling of transmon qubit lifetimes (T1). One of these surface variables, the identity, thickness, and quality of the native surface oxide, is thought to play a major role as tantalum only has one oxide whereas niobium has several. Here we report the development of a thermodynamic metric to rank materials based on their potential to form a well-defined, thin, surface oxide. We first compute this metric for known binary and ternary metal alloys using data available from Materials Project, and experimentally validate the strengths and limits of this metric through preparation and controlled oxidation of 8 known metal alloys. Then we train a convolutional neural network to predict the value of this metric from atomic composition and atomic properties. This allows us to compute the metric for materials that are not present in materials project, including a large selection of known superconductors, and, when combined with Tc, allow us to identify new candidate superconductors for quantum information science (QISE) applications. We test the oxidation resistance of a pair of these predictions experimentally. Our results are expected to lay the foundation for tailored and rapid selection of improved superconductors for QISE.
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Submitted 27 January, 2023;
originally announced January 2023.
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Single Crystal Growth Tricks and Treats
Authors:
Tanya Berry,
Nicholas Ng,
Tyrel M. McQueen
Abstract:
Single crystal growth is a widely explored method of synthesizing materials in the solid state. The last few decades have seen significant improvements in the techniques used to synthesize single crystals, but there has been comparatively little discussion on ways to disseminate this knowledge. We aim to change that. Here we describe the principles of known single crystal growth techniques as well…
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Single crystal growth is a widely explored method of synthesizing materials in the solid state. The last few decades have seen significant improvements in the techniques used to synthesize single crystals, but there has been comparatively little discussion on ways to disseminate this knowledge. We aim to change that. Here we describe the principles of known single crystal growth techniques as well as lesser-known variations that have assisted in the optimization of defect control in known materials. We offer a perspective on how to think about these synthesis methods in a grand scheme. We consider the temperature interdependence with the reaction time as well as ways to carry out synthesis to scale up and address some outstanding synthesis challenges. We hope our descriptions will aid in technological advancements as well as further developments to gain even better control over synthesis.
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Submitted 19 September, 2022;
originally announced September 2022.
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Magnetic phase crossover in strongly correlated EuMn2P2
Authors:
Tanya Berry,
Nicodemos Varnava,
Dominic Ryan,
Veronica Stewart,
Riho Rästa,
Ivo Heinmaa,
Nitesh Kumar,
Walter Schnelle,
Rishi Bhandia,
Christopher Pasco,
N. P. Armitage,
Raivo Stern,
Claudia Felser,
David Vanderbilt,
Tyrel M. McQueen
Abstract:
Strong electron correlations underlie a plethora of electronic and magnetic components and devices and are often used to identify and probe novel ground states in quantum materials. Herein we report a magnetic phase crossover in EuMn2P2, an insulator which shows Eu antiferromagnetism at TN=17K, but no phase transition attributed to Mn magnetism. The absence of a Mn magnetic phase transition contra…
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Strong electron correlations underlie a plethora of electronic and magnetic components and devices and are often used to identify and probe novel ground states in quantum materials. Herein we report a magnetic phase crossover in EuMn2P2, an insulator which shows Eu antiferromagnetism at TN=17K, but no phase transition attributed to Mn magnetism. The absence of a Mn magnetic phase transition contrasts with the formation of long-range Mn order at T=130K in isoelectronic EuMn2Sb2 and EuMn2As2. Temperature-dependent specific heat and 31P NMR measurements provide evidence for the development of Mn magnetic correlations from T=250-100 K. Density functional theory calculations demonstrate an unusual sensitivity of the band structure to the details of the imposed Mn and Eu magnetic order, with antiferromagnetic Mn order required to recapitulate an insulating state. Our results imply a picture in which long range Mn magnetic order is suppressed by chemical pressure, but that magnetic correlations persist, narrowing bands and producing an insulating state.
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Submitted 4 September, 2022;
originally announced September 2022.
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Bonding and Electronic Nature of the Anionic Framework in LaPd$_3$S$_4$
Authors:
Tanya Berry,
Michael Nicklas,
Qun Yang,
Walter Schnelle,
Rafał Wawrzyńczak,
Tobias Förster,
Johannes Gooth,
Claudia Felser,
Tyrel M. McQueen
Abstract:
Double Dirac materials are a topological phase of matter in which a non-symmorphic symmetry enforces greater electronic degeneracy than normally expected up to eightfold. The cubic palladium bronzes NaPd$_3$O$_4$ and LaPd$_3$S$_4$ are built of Pd$_3$X$_4$ (X = O, S) anionic frameworks that are ionically bonded to A cations (A = Na, La). These materials were recently identified computationally as h…
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Double Dirac materials are a topological phase of matter in which a non-symmorphic symmetry enforces greater electronic degeneracy than normally expected up to eightfold. The cubic palladium bronzes NaPd$_3$O$_4$ and LaPd$_3$S$_4$ are built of Pd$_3$X$_4$ (X = O, S) anionic frameworks that are ionically bonded to A cations (A = Na, La). These materials were recently identified computationally as harboring eightfold fermions. Here we report the preparation of single crystals and electronic properties of LaPd$_3$S$_4$. Measurements down to T = 0.45 K and in magnetic fields up to $μ$0H = 65 T are consistent with normal Fermi liquid physics of a Dirac metal in the presence of dilute magnetic impurities. This interpretation is further confirmed by analysis of specific heat, magnetization measurements and comparison to density functional theory (DFT) calculations. Through a bonding analysis of the DFT electronic structure of NaPd$_3$O$_4$ and LaPd$_3$S$_4$, we identify the origin of the stability of the anionic Pd$_3$X$_4$ framework at higher electron counts for X = S than X = O, and propose chemical tuning strategies to enable shifting the 8-fold fermion points to the Fermi level.
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Submitted 7 July, 2022;
originally announced July 2022.
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Dipolar Magnetic Interactions and A-type Antiferromagnetic Order in the Zintl Phase Insulator EuZn2P2
Authors:
Tanya Berry,
Veronica J. Stewart,
Benjamin W. Y. Redemann,
Chris Lygouras,
Nicodemos Varnava,
David Vanderbilt,
Tyrel M. McQueen
Abstract:
Zintl phases, containing strongly covalently bonded frameworks with separate ionically bonded ions, have emerged as a critical materials family in which to couple magnetism and strong spin-orbit coupling to drive diverse topological phases of matter. Here we report the single-crystal synthesis, magnetic, thermodynamic, transport, and theoretical properties of the Zintl compound EuZn2P2 that crysta…
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Zintl phases, containing strongly covalently bonded frameworks with separate ionically bonded ions, have emerged as a critical materials family in which to couple magnetism and strong spin-orbit coupling to drive diverse topological phases of matter. Here we report the single-crystal synthesis, magnetic, thermodynamic, transport, and theoretical properties of the Zintl compound EuZn2P2 that crystallizes in the anti-La2O3 P-3m1 structure, containing triangular layers of Eu2+ ions. In-plane resistivity measurements reveal insulating behavior with an estimated bandgap of Eg=0.11eV. Comparing Eu magnetic ordering temperatures across trigonal EuM2X2 (M=divalent metal, X=pnictide) shows that EuZn2P2 exhibits the highest ordering temperature, with variations in TN correlating with changes in expected dipolar interaction strengths within and between layers and independent of the magnitude of electrical conductivity. These results provide experimental validation of the cytochemical intuition that the cation Eu2+ layers and the anionic (M2X2)2- framework can be treated as electronically distinct subunits, enabling further predictive materials design.
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Submitted 23 March, 2022;
originally announced March 2022.
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Engineering magnetic topological insulators in Eu$_5M_2X_6$ Zintls
Authors:
Nicodemos Varnava,
Tanya Berry,
Tyrel M. McQueen,
David Vanderbilt
Abstract:
Magnetic topological insulator provide a prominent material platform for quantum anomalous Hall physics and axion electrodynamics. However, the lack of material realizations with cleanly gapped surfaces hinders technological utilization of these exotic quantum phenomena. Here, using the Zintl concept and the properties of non-symmorphic space groups, we computationally engineer magnetic topologica…
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Magnetic topological insulator provide a prominent material platform for quantum anomalous Hall physics and axion electrodynamics. However, the lack of material realizations with cleanly gapped surfaces hinders technological utilization of these exotic quantum phenomena. Here, using the Zintl concept and the properties of non-symmorphic space groups, we computationally engineer magnetic topological insulators. Specifically, we explore Eu$_5M_2X_6$ ($M$=metal, $X$=pnictide) Zintl compounds and find that Eu$_5$Ga$_2$Sb$_6$, Eu$_5$Tl$_2$Sb$_6$ and Eu$_5$In$_2$Bi$_6$ form stable structures with non-trivial $\mathbb{Z}_2$ indices. We also show that epitaxial and uniaxial strain can be used to control the $\mathbb{Z}_2$ index and the bulk energy gap. Finally, we discuss experimental progress towards the synthesis of the proposed candidates and provide insights that can be used in the search for robust magnetic topological insulators in Zintl compounds.
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Submitted 15 March, 2022; v1 submitted 11 March, 2022;
originally announced March 2022.
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Laser floating zone growth of SrVO$_3$ single crystals
Authors:
Tanya Berry,
Shannon Bernier,
Gudrun Auffermann,
Tyrel M. McQueen,
W. Adam Phelan
Abstract:
The perovskite SrVO$_3$ is of interest for a variety of applications due to its simple metallic character and stability in reducing environments. Here we report the preparation of single-crystal SrVO$_3$ using the laser floating zone technique. Laue diffraction implies single domains ca. 30 mm in length. The stoichiometry of optimized crystals was found to be Sr$_{0.985}$VO$_{2.91}$ using inductiv…
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The perovskite SrVO$_3$ is of interest for a variety of applications due to its simple metallic character and stability in reducing environments. Here we report the preparation of single-crystal SrVO$_3$ using the laser floating zone technique. Laue diffraction implies single domains ca. 30 mm in length. The stoichiometry of optimized crystals was found to be Sr$_{0.985}$VO$_{2.91}$ using inductively coupled plasma optical emission spectrometry and neutron powder diffraction analysis, with compositions adjustable depending on the crystal pulling rate. Heat capacity measurements from 2 to 300 K show variations with composition, attributable to a combination of impurity scattering and changes in phonon dynamics.Our results demonstrate the utility of the laser floating zone technique in preparing a range of materials, and our advances with SrVO$_3$ may help lead to applications including catalysis, transparent conducting oxides, thermionic emitters, and other electronic devices.
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Submitted 16 August, 2021;
originally announced August 2021.
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Antiferro- and Meta-magnetism in the S=7/2 Hollandite Analog EuGa2Sb2
Authors:
Tanya Berry,
Sean R. Parkin,
Tyrel M. McQueen
Abstract:
Recent work analyzing the impact of non-symmorphic symmetries on electronic states has given rise to the discovery of multiple types of topological matter. Here we report the single-crystal synthesis and magnetic properties of EuGa2Sb2, an Eu-based antiferromagnet structurally consisting of pseudo-1D chains of Eu ions related by a non-symmorphic glide plane. We find the onset of antiferromagnetic…
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Recent work analyzing the impact of non-symmorphic symmetries on electronic states has given rise to the discovery of multiple types of topological matter. Here we report the single-crystal synthesis and magnetic properties of EuGa2Sb2, an Eu-based antiferromagnet structurally consisting of pseudo-1D chains of Eu ions related by a non-symmorphic glide plane. We find the onset of antiferromagnetic order at TN = 8 K. Above TN the magnetic susceptibility is isotropic. Curie-Weiss analysis suggests competing ferromagnetic and antiferromagnetic interactions, with peff = 8.1 muB as expected for 4f7 J = S = 7/2 Eu^2+ ions. Below TN and at low applied magnetic fields, an anisotropy develops linearly, reaching chi perpendicular over chi parallel =6 at T = 2 K. There is concomitant metamagnetic behavior along with chi parallel, with a magnetic field of mu0 H=0.5 T sufficient to suppress the anisotropy. Independent of crystal orientation, there is a continuous evolution to a field polarized paramagnetic state with M=7 muB/Eu^2+ cation at mu0 H=2 T as T approaches 0 K. Specific heat measurements show a recovered magnetic entropy of dSmag=16.4 J/mol.K from T near 0 K to T = TN, close to the expected value of Rln(8) for an S = 7/2 ion, indicating negligible low dimensional spin fluctuations above TN. We find no evidence of unusual behaviors arising either from the dimensionality or the presence of the non-symmorphic symmetries.
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Submitted 12 August, 2021;
originally announced August 2021.
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Thermodynamic and transport properties of semiconducting two-dimensional metal-organic kagomé lattices with disorder
Authors:
Tanya Berry,
Jennifer R. Morey,
Kathryn E. Arpino,
Jin-Hu Dou,
Claudia Felser,
Mircea Dincă,
Tyrel M. McQueen
Abstract:
The kagomé lattice is a fruitful source of novel physical states of matter, including the quantum spin liquid and Dirac fermions. Here we report a structural, thermodynamic, and transport study of the two-dimensional kagomé metal-organic frameworks Ni_3(HIB)v2 and Cu_3(HIB)_2 (HIB = hexaiminobenzene). Magnetization measurements yield Curie constants of 1.12 and 0.352 emu K mol f.u.-1 Oe-1 respecti…
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The kagomé lattice is a fruitful source of novel physical states of matter, including the quantum spin liquid and Dirac fermions. Here we report a structural, thermodynamic, and transport study of the two-dimensional kagomé metal-organic frameworks Ni_3(HIB)v2 and Cu_3(HIB)_2 (HIB = hexaiminobenzene). Magnetization measurements yield Curie constants of 1.12 and 0.352 emu K mol f.u.-1 Oe-1 respectively, close to the values expected for ideal S=1 and S=1/2 moments. Weiss temperatures of -20.3 K and -6.52 K, respectively, indicate moderate to weak magnetic interactions. Electrical transport measurements reveal that both materials are semiconducting, with gaps of Eg = 22.2 and 103 meV, respectively. Specific heat measurements reveal a large T-linear contribution of γ = 148(4) mJ mol-f.u.-1 K-2 in Ni_3(HIB)_2 with only a gradual upturn below T ~ 5 K and no evidence of a phase transition to an ordered state down to T = 0.1 K. Cu_3(HIB)_2 also lacks evidence of a phase transition above T = 0.1 K, with a substantial, field-dependent, magnetic contribution below T ~ 5 K. Despite being superficially in agreement with expectations of magnetic frustration and spin liquid physics, we are able to explain these observations as arising due to known stacking disorder in these materials. Our results further state the art of kagomé lattice physics, especially in the rarely explored regime of semiconducting but not metallic behavior.
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Submitted 30 June, 2021;
originally announced July 2021.
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Twisting of 2D kagomé sheets in layered intermetallics
Authors:
Mekhola Sinha,
Hector K. Vivanco,
Cheng Wan,
Maxime A. Siegler,
Veronica J. Stewart,
Elizabeth A. Pogue,
Lucas A. Pressley,
Tanya Berry,
Ziqian Wang,
Isaac Johnson,
Mingwei Chen,
Thao T. Tran,
W. Adam Phelan,
Tyrel M. McQueen
Abstract:
Chemical bonding in 2D layered materials and van der Waals solids is central to understanding and harnessing their unique electronic, magnetic, optical, thermal and superconducting properties. Here we report the discovery of spontaneous, bidirectional, bilayer twisting (twist angle ~ 4.5°) in the metallic kagomé MgCo6Ge6 at T = 100(2) K via X-ray diffraction measure-ments, enabled by the preparati…
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Chemical bonding in 2D layered materials and van der Waals solids is central to understanding and harnessing their unique electronic, magnetic, optical, thermal and superconducting properties. Here we report the discovery of spontaneous, bidirectional, bilayer twisting (twist angle ~ 4.5°) in the metallic kagomé MgCo6Ge6 at T = 100(2) K via X-ray diffraction measure-ments, enabled by the preparation of single crystals by the Laser Bridgman method. Despite the appearance of static twisting on cooling from T ~ 300 K to 100 K, no evidence for a phase transition was found in physical properties measurements. Combined with the presence of an Einstein phonon mode contribution in the specific heat, this implies that the twisting exists at all temperatures but is thermally fluctuating at room temperature. Crystal Orbital Hamilton Population analysis demonstrates that the cooperative twisting between layers stabilizes the Co-kagomé network when coupled to strongly bonded and rigid (Ge2) dimers that connect adjacent layers. Further modelling of the displacive disorder in the crystal structure shows the presence of second, Mg-deficient, stacking sequence. This alternative stacking sequence also exhibits inter-layer twisting, but with a different pattern, consistent with the change in electron count due to removal of Mg. Magnetization, resistivity, and low-temperature specific heat measurements are all consistent with a Pauli paramagnetic, strongly correlated metal. Our results provide crucial insight into how chemical concepts lead to interesting electronic structures and behaviors in layered materials.
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Submitted 6 August, 2021; v1 submitted 6 February, 2021;
originally announced February 2021.
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Improving thermoelectric performance of TiNiSn by mixing MnNiSb in the half-Heusler structure
Authors:
T. Berry,
S. Ouardi,
G. H. Fecher,
B. Balke,
G. Kreiner,
G. Auffermann,
W. Schnelle,
C. Felser
Abstract:
The thermoelectric properties of n type semiconductor, TiNiSn is optimized by partial substitution with metallic, MnNiSb in the half Heusler structure. Herein, we study the transport properties and intrinsic phase separation in the system. The Ti1-xMnxNiSn1-xSbx alloys were prepared by arc-melting and were annealed at temperatures obtained from differential thermal analysis and differential scanni…
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The thermoelectric properties of n type semiconductor, TiNiSn is optimized by partial substitution with metallic, MnNiSb in the half Heusler structure. Herein, we study the transport properties and intrinsic phase separation in the system. The Ti1-xMnxNiSn1-xSbx alloys were prepared by arc-melting and were annealed at temperatures obtained from differential thermal analysis and differential scanning calorimetry results. The phases were characterized using powder X-ray diffraction patterns, energy dispersive X-ray spectroscopy, and differential scanning calorimetry. After annealing the majority phase was TiNiSn with some Ni rich sites and the minority phases was majorly Ti6Sn5, Sn, and MnSn2. Ni rich sites were caused by Frenkel defects, this led to a metal-like behavior of the semiconducting specimens at low temperature. For x up to 0.05 the samples showed an activated conduction, whereas for x>0.05 they showed metallic character. The figure of merit for x=0.05 was increased by 61% (ZT=0.45) in comparison to the pure TiNiSn.
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Submitted 9 October, 2016;
originally announced October 2016.