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At least 829 records · Page 46
All sourcesNASA NTRSDOE OSTI
DOE OSTI2020

Ab initio calculations of the rate of carrier trapping and release at dopant sites in NaI: Tl beyond the harmonic approximation

We present ab initio calculations of electron capture and release coefficients as a function of temperature for Tl dopants in the widely used scintillator NaI, which is a soft ionic crystal. The modelled capture and release events occur by transitions mediated by multiple phonon absorption and emission between states with significantly different local geometries around the trapping site. We demonstrate that such transitions are not well-described by the normal harmonic approximation to the nuclear dynamics. We avoid the harmonic approximation by numerically solving the vibrational Schrödinger equation for the motion along/of a single effective phonon coordinate. The localized trapped state is not correctly described by semilocal density functionals, so we employ a hybrid functional tuned to reproduce the band gap and lattice constant of the host material. Our calculations, which combine plane-wave and cluster electronic structure methods in a novel way, are in reasonable agreement with available experimental data for detrapping and predict an unusual temperature dependence for trapping rates.

Prange, Micah P.↗
DOE OSTI2020

Topological magnon bands in a room-temperature kagome magnet

Topological magnon is a vibrant research field gaining more and more attention in the past few years. Among many theoretical proposals and limited experimental studies, ferromagnetic kagome lattice emerges as one of the most elucidating systems. Here we report neutron scattering studies of YMn 6 Sn 6 , a metallic system consisting of ferromagnetic kagome planes. This system undergoes a commensurate-to-incommensurate antiferromagnetic phase transition upon cooling with the incommensurability along the out-of-plane direction. We observe magnon band gap opening at the symmetry-protected K points and ascribe this feature to the antisymmetric Dzyaloshinskii-Moriya (DM) interactions. Our observation supports the existence of topological Dirac magnons in both the commensurate collinear and incommensurate coplanar magnetic orders, which is further corroborated by symmetry analysis. This finding places YMn 6 Sn 6 as a promising candidate for room-temperature magnon spintronics applications.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2020

Phase diagrams of single-layer two-dimensional transition metal dichalcogenides: Landau theory

We report single layer (SL) two-dimensional transition metal dichalcogenides (TMDs), such as MoS 2 , ReS 2 , WSe 2 , and MoTe 2 have now become the focus of intensive fundamental and applied research due to their intriguing and tunable physical properties. These materials exhibit a broad range of structural phases that can be induced via elastic strain, chemical doping, and electrostatic field effect. These transformations, in turn, can open and close the band gap of SL-TMDs, leading to metal-insulator transitions, and lead to emergence of more complex quantum phenomena. These considerations necessitate detailed understanding of the mesoscopic mechanisms of these structural phase transitions. Here we develop a Landau-type thermodynamic description of SL-TMDs on a sample of the SL-(MoS 2 ) 1-x -(ReS 2 ) x system and analyze the free energy surfaces, phase diagrams, and order parameter behavior. Our results predict the existence of multiple structural phases with two-, six- and 12-fold degenerated energy minima for in-plane and out-of-plane order parameters. This analysis suggests that out-of-plane ferroelectricity can exist in many of these phases, with the switchable polarization being proportional to the out-of-plane order parameter. We further predict that the domain walls in SL-(MoS 2 ) 1-x -(ReS 2 ) x should become conductive above a certain strain threshold.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2020

First-principles coupled cluster theory of the electronic spectrum of transition metal dichalcogenides

The electronic properties of two-dimensional transition metal dichalcogenides (2D TMDs) have attracted much attention during the last decade. Here we show how a diagrammatic ab initio coupled cluster singles and doubles (CCSD) treatment paired with a careful thermodynamic limit extrapolation in two dimensions can be used to obtain converged band gaps for monolayer materials in the MoS 2 family. We find CCSD gaps to lie in the upper range of the spread of $\textit{GW}$ approximation based on density functional theory (DFT) simulations, and also find slightly higher effective hole masses compared to previous reports. We also investigate the ability of CCSD to describe trion states, finding a reasonable qualitative structure, but poor excitation energies due to the lack of screening of three-particle excitations in the effective Hamiltonian. Our study provides an independent high-level benchmark of the role of many-body effects in 2D TMDs and showcases the potential strengths and weaknesses of diagrammatic coupled cluster approaches for realistic materials.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2020

Thermal effects on the electronic properties of sodium electride under high pressures

Sodium is a simple metal at ambient conditions, while it transits to an electride phase at pressures above ~160 GPa along the room temperature isotherm. Here we explore the thermal effects on the electronic properties of the $hP4$ phase of sodium along the $ρ$ = 5.872 g/cm 3 isochore. We quantitatively classify this phase as an insulator based on the criterion of nearsightedness of the one-particle density matrix. Ab initio calculations suggest that the band gap of this insulator decreases with increasing temperature along the isochore, primarily because of ionic distortions; culminating in an insulator-to-metal transition upon melting at $T_m \approx 2100$ $\hbox{K}$. This transition is accompanied by residual electronic localization (in d orbitals) in the form of dynamic electron bubbles and a change in hybridization form $p–d$ to $s–p$ upon melting. This transition is explored by tracking the electronic and electro-optical properties along the isochore under consideration.

36 MATERIALS SCIENCE↗
DOE OSTI2020

Two good metals make a semiconductor: A potassium-nickel compound under pressure

We predict a potassium-nickel intermetallic compound K 2 Ni at high pressure and identify it as the long-sought structure of the only known K-Ni compound to date. Although both constituent elements are metallic, K 2 Ni exhibits a semiconducting ground state with an indirect band gap of 0.65 eV. Electron instability due to the degeneracy at the Fermi level arises from the particular motif of the structure, which in turn induces symmetry-breaking Peierls distortion and a nonmetallic ground state. Overall, the results indicate that the chemical properties of elements can change dramatically under extreme conditions and have significant implications for the postulation that potassium is incorporated in Earth's core.

36 MATERIALS SCIENCE↗
DOE OSTI2021

Metastable piezoelectric group-IV monochalcogenide monolayers with a buckled honeycomb structure

Multiple two-dimensional materials are being naïvely termed stable on the grounds of displaying phonon dispersions with no negative frequencies and of not collapsing on molecular dynamics calculations at fixed volume. But, if these phases do not possess the smallest possible structural energy, how does one understand and establish their actual meta stability? To answer this question, twelve two-dimensional group-IV monochalcogenide monolayers (SiS, SiSe, SiTe, GeS, GeSe, GeTe, SnS, SnSe, SnTe, PbS, PbSe, and PbTe) with a buckled honeycomb atomistic structure—belonging to symmetry group P3m1—displaying an out-of-plane intrinsic electric polarization are shown to be metastable by three independent methods. First, we uncover a coordination-preserving structural transformation from the low-buckled honeycomb structure onto the lower-energy Pnm2 1 (or Pmmn for PbS, PbSe, and PbTe) phase to estimate energy barriers E B that must be overcome during such structural transformation. Using the curvature of the local minima and E B as inputs to Kramers escape formula, large escape times are found, implying the structural metastability of the buckled honeycomb phase (with the exception of PbS and PbSe, these phases display escape times ranging from 700 years to multiple times the age of the universe and can be considered “stable” for practical purposes in that relative sense). The second demonstration is provided by phonon dispersion relations that include the effect of long-range Coulomb forces and display no negative vibrational modes. The third and final demonstration of structural metastability is furnished by room-temperature ab initio molecular dynamics for selected compounds. Here, the magnitude of the electronic band gap evolves with chemical composition. Different from other binary two-dimensional compounds such as transition metal dichalcogenide monolayers and hexagonal boron nitride monolayers which only develop an in-plane piezoelectric response, the twelve group-IV monochalcogenide monolayers with a buckled honeycomb structure also display out-of-plane piezoelectric properties.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2021

Stability of the peroxide group in BaO 2 under high pressure

Alkaline earth metal peroxides are typical examples of ionic compounds containing polyanions. We herein report a stable BaO 2 phase at high pressure up to 130 GPa found via a first-principles computational structure searches and high-pressure experimental investigations. The identified monoclinic structure (space group C2/m) can be derived by sublattice distortions of Ba atoms and peroxide groups associated with the phonon mode softening of the lower-pressure Cmmm structure. Contrary to the previous expectation of polymerization of the peroxide group at elevated pressure, the new phase retains the peroxide group and interestingly exhibits an insulating behavior demonstrating an increase of the band gap under compression. Our synchrotron XRD measurements could not distinguish between Cmmm and C2/m BaO 2 definitively because the difference in XRD patterns is very subtle. However, our data do not show any sign of polymerization transition up to 120 GPa. Raman spectra of the O-O peroxide vibration show a small anomaly in frequency at 110 GPa, which is qualitatively similar to that predicted theoretically due to the Cmmm to C2/m phase transition thus supporting the predicted transformation.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2021

Higher Chern numbers in multilayer Lieb lattices ( N ≥ 2 ): Topological transitions and quadratic band crossing lines

In this work, we consider a hitherto unexplored setting of a stacked multilayer (N) Lieb lattice which undergoes an unusual topological transition in the presence of intralayer spin-orbit coupling (SOC). The specific stacking configuration induces an effective nonsymmorphic two-dimensional lattice structure, even though the constituent monolayer Lieb lattice is characterized by a symmorphic space group. This emergent nonsymmorphicity leads to multiple doubly degenerate bands extending over the edge of the Brillouin zone (i.e., quadratic band crossing lines). In the presence of intralayer SOC, these doubly degenerate bands typically form three N-band subspaces, mutually separated by two band gaps. We analyze the topological properties of these multiband subspaces, using specially devised Wilson loop operators to compute non-Abelian Berry phases in order to show that they carry a higher Chern number N.

36 MATERIALS SCIENCE↗
DOE OSTI2021

Absorption properties of graphene quantum dots under ultrashort optical pulses

Here, we study the interaction of graphene quantum dots (GQDs) with ultrashort and strong optical pulses theoretically. An important characteristic of such interaction is the energy accumulated by GQDs after the pulse. We show that the GQD absorbance has a highly nonlinear dependence on the field amplitude. At small-field amplitudes, the absorbance strongly depends on the frequency of the pulse and the band gap of a QD, i.e., its size. At large-field amplitudes, ~1 V/Å, the absorbance has a weak nonmonotonic dependence on the size of the dot with its maximum value of ≈ 4.5 % realized for GQD consisting of ≈ 60 atoms. As a function of the field amplitude, the absorbance also has a maximum, the position of which depends on the size of the dot.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2021

First-principles modeling of conductivity at the (001), (110), and (111) SrTiO 3 /LaAlO 3 heterointerfaces

Here, the complex polar oxide heterojunction of SrTiO 3 /LaAlO 3 (STO/LAO) is of great interest due to the emergent physical phenomena observed at the interface. STO and LAO separately are wide band-gap insulators. However, upon joining them at the 001, 110, and 111 crystallographic planes, the interface undergoes a transition to a conductive state. Although first-principles modeling of the 001 plane interface has been widely studied, there is a lack of reports regarding the 110 and 111. This paper expands the theoretical model of the STO/LAO heterointerface to the three crystallographic planes (001, 110, and 111) where the conductivity has been experimentally reported. The calculations showed that whereas at the 001 interface the conductivity appears at a critical thickness of 4 monolayers of LAO, the 110 and 111 planes have no clear critical thickness; these two interfaces were always conductive. Nevertheless, the number of conductive electrons per unit cell increases with the thickness of the LAO layer in the 110 and 111 interfaces. This is related to the energy levels downshifting due to the electrostatic potential buildup (which was in the opposite direction respect to the 001 interface), increasing the number of conductive sates below the Fermi level. Given the absence of a critical thickness and the fact that chemical intermixing and oxygen vacancies at the interface were not considered, the main mechanism responsible for the conductivity in the 110 and 111 planes was attributed to the large structure reconstruction that locally changes the energy levels at the interface causing charge transfer and accumulation at the layers close to the interface.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2021

$A$-type antiferromagnetic order and magnetic phase diagram of the trigonal Eu spin-$\frac{7}{2}$ triangular-lattice compound $\mathrm{EuSn_2As_2}$

The trigonal compound EuSn 2 As 2 was recently discovered to host Dirac surface states within the bulk band gap and orders antiferromagnetically below the N´eel temperature T N = 23.5(2) K from neutron-diffraction measurements. Here the magnetic ground state of single-crystal EuSn 2 As 2 and the evolution of its properties versus temperature T and applied magnetic field H are reported. Included are the zero-field single-crystal neutron diffraction measurements versus T, magnetization M(H, T), magnetic susceptibility χ(H, T) = M(H,T)/H, heat capacity C p (H, T), and electrical resistivity ρ(H, T) measurements. The neutron-diffraction and χ(T) measurements both indicate a collinear A-type antiferromagnetic (AFM) structure below T N , where the Eu 2+ spins S = 7/2 in a triangular ab-plane layer (hexagonal unit cell) are aligned ferromagnetically in the ab plane whereas the spins in adjacent Eu planes along the c axis are aligned antiferromagnetically. The χ(H ab , T) and χ(Hc, T) data together indicate a smooth crossover between the collinear AFM alignment and an unknown magnetic structure at H ≈ 0.12 T. Dynamic spin fluctuations up to 60 K are evident in the χ(T), Cp(T) and ρ(H, T) measurements, a temperature that is more than twice T N . The ρ(H, T) is consistent with a low-carrier-density metal with strong magnetic scattering and does not reflect a contribution of the topological state of the material as reported earlier by ARPES measurements. This observation is consistent with previous ones for other topological insulators where the chemical potential is above the Dirac point so that ARPES readily detects the surface states, whereas resistivity measurements do not. Finally, the magnetic phase diagrams for both H ∥ c and H ∥ ab in the H-T plane are constructed from the T N (H), χ(H, T), Cp(H, T), and ρ(H, T) data.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2021

Direct driving of electronic and phononic degrees of freedom in a honeycomb bilayer with infrared light

Here, we study theoretically AB-stacked honeycomb bilayers driven by light in resonance with an infrared phonon within a tight-binding description. We characterize the phonon properties of honeycomb bilayers with group theory and construct an electronic time-dependent tight-binding model for the system following photoexcitation in resonance with an infrared phonon. We adopt an atomically adiabatic approximation [V. Mohanty and E. J. Heller, Proc. Natl. Acad. Sci. 116, 18316 (2019)] to describe classically vibrating nuclei, but obtain the Floquet quasienergy spectrum associated with the time-dependent model exactly. We introduce a general scheme to disentangle the complex low-frequency Floquet spectrum to elucidate the relevant Floquet bands. As a prototypical example, we consider bilayer graphene. We find that light in the low-frequency regime can induce a band gap in the quasienergy spectrum in the vicinity of the $K$ points even if it is linearly polarized, in contrast with the expectations within the Born-Oppenheimer approximation and the high-frequency regime. Finally, we analyze the diabaticity of the driven electron and driven phonon processes and find contrasting effects on the autocorrelation functions at the same driving frequency: Driven phonons preserve the character of the initial state while driven electrons exhibit strong deviations within a few drive cycles. The procedure outlined here can be applied to other materials to describe the combined effects of low-frequency light on phonons and electrons.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2021

Quasi-two-dimensional ferromagnetism and anisotropic interlayer couplings in the magnetic topological insulator MnBi 2 Te 4

MnBi 2 Te 4 (MBT) is a promising van der Waals layered antiferromagnetic (AFM) topological insulator that combines a topologically nontrivial inverted Bi-Te band gap with ferromagnetic (FM) layers of Mn ions. Here, the inelastic neutron scattering on single crystals reported here describes rather complex magnetism in MBT. The magnetic anisotropy that controls the bulk and surface magnetic field response of MBT is found to have contributions from both single-ion and interlayer two-ion terms. A description of the quasi-two-dimensional intralayer FM spin waves requires long-range, competing FM and AFM interactions and anomalous damping. While this might suggest carrier-mediated magnetic coupling, abinitio calculations in insulating MBT also find long-range interactions, and classical spin dynamics simulations suggest that magnetic vacancies are at least partially responsible for observations of anomalous damping near the zone boundary.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2022

Single-molecule magnet Mn 12 on GaAs-supported graphene: Gate field effects from first principles

In this work, we study gate field effects on the Mn 12 O 12 (COOH) 16 (H 2 O) 4 | graphene | GaAs heterostructure via first-principles calculations. We find that under moderate doping levels electrons can be added to but not taken from the single-molecule magnet Mn 12 O 12 (COOH) 16 (H 2 O) 4 (Mn 12 ). The magnetic anisotropy energy (MAE) of Mn 12 decreases as the electron doping level increases, due to electron transfer from graphene to Mn 12 and change in the band alignment between Mn 12 and graphene. At an electron doping level of -5.00×10 13 cm -2 , the MAE decreases by about 18% compared with zero doping. The band alignment between graphene and GaAs is more sensitive to electron doping than to hole doping, since the valence band of GaAs is close to the Fermi level. The GaAs substrate induces a small band gap in the supported graphene under zero gate field and a nearly strain-free configuration. Finally, we propose a vertical tunnel junction for probing the gate dependence of MAE via electron transport measurements.

77 NANOSCIENCE AND NANOTECHNOLOGY↗
DOE OSTI2022

Insulating antiferromagnetism in VTe

Here, we report a detailed theoretical and experimental study on the vanadium monotelluride VTe, which crystallizes in the NiAs hexagonal structure. First-principles calculations reveal a complex hierarchy of magnetic interactions and energy scales, with the ground state theoretically determined as an ($\frac{1}{2}$, 0, $\frac{1}{2}$) antiferromagnetic ordering with insulating character and a band gap of 0.5 eV. Experimental synthesis and characterization efforts find a substantially off-stoichiometric orthorhombic structure (a defect NiAs structure) with composition V 0.85 Te, and an apparent Néel point of some 45 K. First-principles calculations find good agreement with the observed Néel point. We also give an extended examination of the effects of off-stoichiometry on the calculated energetics, finding significant volume-related effects. Our first-principles calculations find the stoichiometric phase VTe to have a negative vanadium defect formation energy of over 1 eV, thus explaining the formation of the off-stoichiometric phase. Finally, we provide a structural explanation for the formation of defect structures in this and numerous other NiAs-structure materials.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2022

First-principles study of bilayer polymeric manganese phthalocyanine

Here we study bilayer manganese phthalocyanine (MnPc) molecules and MnPc polymeric sheets using first-principles simulations with a focus on the magnetic interactions between Mn atoms. We find that the most stable position of the upper layer with respect to the lower layer is shifted about 1/8 of a lattice vector from the center of the bottom layer along the direction toward a nearest-neighbor N atom. The magnetic ground state is the Néel antiferromagnetic (AFM) configuration within a layer and ferromagnetic (FM) between Mn atoms in adjacent layers. In this state, the system becomes a semiconductor with an indirect band gap of 11meV. The strongest interaction is the interlayer coupling between the closest Mn atoms. A maximally localized Wannier analysis suggests that the dominant coupling pathway is Mn-(N,C)-Mn rather than a direct Mn-Mn coupling. The maximum calculated magnetic anisotropy energy is found to be 1.0meV per Mn atom. We also find that the bilayer molecule shows a significant stacking angle change from FM to AFM configurations accompanied by a change of orbital filling ordering.

2-dimensional systems↗
DOE OSTI2022

High-temperature fractional quantum Hall state in the Floquet kagome flat band

A fractional quantum Hall effect (FQHE) has been predicted in a topological flat band (FB) by a single-particle band structure combined with phenomenological theory or solution of a many-body lattice Hamiltonian with fuzzy parameters. A long-standing roadblock toward the realization of a FB-FQHE is lacking the many-body solution of specific materials under realistic conditions. Here, we demonstrate a combined study of single-particle Floquet band theory with exact diagonalization (ED) of a many-body Hamiltonian. We show that a time-periodic circularly polarized laser inverts the sign of second-nearest-neighbor hopping in a kagome lattice and enhances spin-orbit coupling in one spin channel to produce a Floquet FB with a high flatness ratio of bandwidth over band gap, as exemplified in monolayer Pt 3 C 36 S 12 H 12 . The ED of the resultant Floquet-kagome lattice Hamiltonian gives a one-third-filling ground state with a laser-dependent excitation gap of a FQH state, up to an estimated temperature above 70 K. Our findings pave the way for exploring the alluding high-temperature FB-FQHE.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗