Engineering topics

Aryal, Niraj

Publications and source records attributed to Aryal, Niraj.

11 records

DOE OSTI2024

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

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

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗

DOE OSTI2022

Fluctuating Ru trimer precursor to a two-stage electronic transition in RuP

Superconductivity in binary ruthenium pnictides occurs proximal to and upon suppression of a non-magnetic ground state, preceded by a pseudogap phase associated with Fermi surface instability, and its critical temperature, T c , is maximized around the pseudogap quantum critical point. By analogy with isoelectronic iron based counterparts, antiferromagnetic fluctuations became ''usual suspects" as putative mediators of superconducting pairing. Here we report on a high temperature local symmetry breaking in RuP, nonsuperconducting parent of the maximum- T c branch of these novel superconductors, revealed by combined nanostructure-sensitive powder and single crystal X-ray total scattering analyses. Large local distortions of Ru chains associated with orbital-charge trimerization, further assembled into hexamers, exist above the two-stage electronic transition in RuP. In the pseudogap regime the precursors order with distortions retaining their strength, whereas they acquire spin-singlet characteristics which dramatically enhances the distortion as the nonmagnetic ground state establishes. The precursors enable the nonmagnetic ground state and presumed complex oligomerization, and the relevance of pseudogap fluctuations for superconductivity emerges as a distinct prospect. As a transition metal system in which partial d-manifold filling combined with high crystal symmetry promotes electronic instabilities, this represents a further example of local electronic precursors underpinning the macroscopic collective behavior of quantum materials.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗

DOE OSTI2022

Topological antiferromagnetic semimetal for spintronics: A case study of a layered square-net system EuZnSb 2

Here, we use the first-principles and effective Hamiltonian methods to study the electronic structure and magnetic properties of a recently synthesized layered antiferromagnetic square-net topological semimetal EuZnSb2 [Phys. Rev. Res. 2, 033462 (2020)]. The main message of the paper is that effects of small changes in the band structure produced by the magnetic ordering and changes in the orientation of the Néel vector are amplified in such transport properties as the spin Hall conductivity. We predict that the effects of the broken symmetry introduced by the ordering of the Néel vector, being very weak in the bulk, are pronounced in the surface electronic dispersion, suggesting that surface probes may be more suited to measure them. The coexistence of the magnetism with many other competing phases make this material interesting and possibly useful for quantum spintronics applications.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗

DOE OSTI2022

Colossal Nernst power factor in topological semimetal NbSb 2

Today solid-state cooling technologies below liquid nitrogen boiling temperature (77 K), crucial to quantum information technology and probing quantum state of matter, are greatly limited due to the lack of good thermoelectric and/or thermomagnetic materials. Here, we report the discovery of colossal Nernst power factor of 3800 × 10 -4 W m -1 K -2 under 5 T at 25 K and high Nernst figure-of-merit of 71 × 10 -4 K -1 under 5 T at 20 K in topological semimetal NbSb 2 single crystals. The observed high thermomagnetic performance is attributed to large Nernst thermopower and longitudinal electrical conductivity, and relatively low transverse thermal conductivity. The large and unsaturated Nernst thermopower is the result of the combination of highly desirable electronic structures of NbSb 2 having compensated high mobility electrons and holes near Fermi level and strong phonon-drag effect. This discovery opens an avenue for exploring material option for the solid-state heat pumping below liquid nitrogen temperature.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗

DOE OSTI2022

Thermal transport properties of IrSbSe

Here we report a thermal transport study of IrSbSe, which crystallizes in a noncentrosymmetric cubic structure with the P2 1 3 space group and shows a narrow-gap semiconducting behavior. The large discrepancy between the activation energy for conductivity [E ρ = 128(2) meV] and for thermopower [E S = 17.7 (9) meV] from 200 to 300 K indicates a polaronic transport mechanism. The electrical resistivity varies as exp (T 0 /T) 1/4 and thermopower varies as T 1/2 at low temperatures, indicating that it evolves into Mott variable-range hopping dominant conduction with decreasing temperature. IrSbSe shows a relatively low value of thermal conductivity (~1.65 W/Km) and thermopower of about 0.24 mV/K around 100 K, yet poor electrical conductivity. On the other hand, a high vacancy defect concentration on both Ir and Sb atomic sites of up to 15% suggests a high defect tolerance and points to the possibility of a future improvement of carrier density by chemical substitution or defect optimization.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗

DOE OSTI2022

Anomalous Hall effect at the Lifshitz transition in ZrTe 5

Zirconium pentatelluride ZrTe 5 is a topological semimetal. The presence of a temperature-induced Lifshitz transition, in which the Fermi level goes from the conduction band to the valence band with increasing temperature, provides unique opportunities to study the interplay between Fermi-surface topology, dynamics of Dirac fermions, and Berry curvature in one system. Here we present a combined experimental and theoretical study and show that a low-energy model can be used to understand the complicated Hall response and large anomalous Hall effect observed in ZrTe 5 over a wide range of temperature and magnetic field. We found that the anomalous Hall contribution dominates the Hall response in a narrow temperature window around the Lifshitz transition, away from which the orbital contribution dominates. Moreover, our results indicate that a topological phase transition coexists with the Lifshitz transition. To conclude, our model provides a unifying framework to understand the Hall effect in semimetals with large Zeeman splitting and nontrivial topology.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗

DOE OSTI2021

Photoinduced anisotropic lattice dynamic response and domain formation in thermoelectric SnSe

Identifying and understanding the mechanisms behind strong phonon–phonon scattering in condensed matter systems is critical to maximizing the efficiency of thermoelectric devices. To date, the leading method to address this has been to meticulously survey the full phonon dispersion of the material in order to isolate modes with anomalously large linewidth and temperature-dependence. Here we combine quantitative MeV ultrafast electron diffraction (UED) analysis with Monte Carlo based dynamic diffraction simulation and first-principles calculations to directly unveil the soft, anharmonic lattice distortions of model thermoelectric material SnSe. A small single-crystal sample is photoexcited with ultrafast optical pulses and the soft, anharmonic lattice distortions are isolated using MeV-UED as those associated with long relaxation time and large displacements. We reveal that these modes have interlayer shear strain character, induced mainly by c -axis atomic displacements, resulting in domain formation in the transient state. These findings provide an innovative approach to identify mechanisms for ultralow and anisotropic thermal conductivity and a promising route to optimizing thermoelectric devices.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗

DOE OSTI2021

Topological Phase Transition and Phonon-Space Dirac Topology Surfaces in ZrTe5

We use first-principles methods to demonstrate that, in ZrTe 5 , a layered van der Waals material like graphite, atomic displacements corresponding to five of the six zone-center A g (symmetry-preserving) phonon modes can drive a topological transition from a strong to a weak topological insulator with a Dirac semimetal state emerging at the transition, giving rise to a Dirac topology surface in the multidimensional space formed by the A g phonon modes. This implies that the topological transition in ZrTe 5 , can be realized with many different settings of external stimuli capable of penetrating through the phonon-space Dirac surface without breaking the crystallographic symmetry. Furthermore, we predict that domains with effective mass of opposite signs can be created by laser pumping and will host Weyl modes of opposite chirality propagating along the domain boundaries. Studying phonon-space topology surfaces provides a new route to understanding and utilizing the exotic physical properties of ZrTe 5 , and related quantum materials.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗

DOE OSTI2020

Multiple Dirac nodes and symmetry protected Dirac nodal line in orthorhombic α-RhSi

Exotic multifold topological excitations have been predicted and were recently observed in transition metal silicides like β-RhSi. Herein, we report that interesting topological features of RhSi are also observed in its orthorhombic α phase, which displays multiple types of Dirac nodes very close to the Fermi level ε F . Here, we discuss the symmetry analysis, band connectivity along high-symmetry lines using group representations, band structure, the nature of the Dirac points and of a nodal line occurring near ε F which is protected by the crystalline symmetry. The de Haas–van Alphen effect indicates a Fermi surface in agreement with the calculations. We find an elliptically shaped nodal line very close to ε F around and near the S point on the k y -k z plane that results from the intersection of two upside-down Dirac cones. Both Dirac points of the participating Kramers degenerate bands are only 5 meV apart; hence, an accessible magnetic field might induce a crossing between the spin-up partner of the upper Dirac cone and the spin-down partner of the lower Dirac cone, possibly explaining the anomalies observed in the magnetic torque. α-RhSi is a unique system since all bands crossing ε F emerge from Dirac nodes.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗

DOE OSTI2020

Bulk Fermi surfaces of the Dirac type-II semimetallic candidate NiTe 2

Here, we present a study on the Fermi surface of the Dirac type-II semimetallic candidate NiTe 2 via the temperature and angular dependence of the de Haas–van Alphen (dHvA) effect measured in single crystals grown through Te flux. In contrast to its isostructural compounds like PtSe 2 , band-structure calculations predict NiTe 2 to display a tilted Dirac node very close to its Fermi level that is located along the Γ to A high-symmetry direction within its first Brillouin zone. The angular dependence of the dHvA frequencies is found to be in agreement with the first-principles calculations when the electronic bands are slightly shifted with respect to the Fermi level (ε F ), and therefore provide support for the existence of a Dirac type-II node in NiTe 2 . Nevertheless, we observed mild disagreements between experimental observations and density functional theory (DFT) calculations as, for example, nearly isotropic and light experimental effective masses. This indicates that the dispersion of the bands is not well captured by DFT. Despite the coexistence of Dirac-like fermions with topologically trivial carriers, samples of the highest quality display an anomalous and large either linear or sublinear magnetoresistivity. Finally, this suggests that Lorentz invariance breaking Dirac-like quasiparticles dominate the carrier transport in this compound.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗

DOE OSTI2020

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

We carried out a comprehensive study of magnetic critical behavior in single crystals of ternary chalcogenide Fe Cr 2 Te 4 that undergoes a ferrimagnetic transition below T c ~ 123 K. Detailed critical behavior analysis and scaled magnetic entropy change indicate a second-order ferrimagnetic transition. Critical exponents β = 0.30 ( 1 ) with T c = 122.4 ( 5 ) K , γ = 1.22 ( 1 ) with T c = 122.8 ( 1 ) K , and δ = 4.24 ( 2 ) at T c ~ 123 K suggest that the spins approach the three-dimensional Ising ( β = 0.325 , γ = 1.24 , and δ = 4.82 ) model coupled with attractive long-range interactions between spins that decay as J ( r ) ≈ r - 4.88 . Our results suggest that the ferrimagnetism in Fe Cr 2 Te 4 is due to itinerant ferromagnetism among the antiferromagnetically coupled Cr-Fe-Cr trimers.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗