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Results for “Exotic phase of matter”

Search indexed NASA NTRS and DOE OSTI research on propulsion, heat transfer, battery materials and energy systems. Follow report and document links to the original sources.

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At least 199 records · Page 11
All sourcesNASA NTRSDOE OSTI
DOE OSTI2022

Charge-neutral fermions and magnetic field-driven instability in insulating YbIr 3 Si 7

Kondo lattice materials, where localized magnetic moments couple to itinerant electrons, provide a very rich backdrop for strong electron correlations. They are known to realize many exotic phenomena, with a dramatic example being recent observations of quantum oscillations and metallic thermal conduction in insulators, implying the emergence of enigmatic charge-neutral fermions. Here, we show that thermal conductivity and specific heat measurements in insulating YbIr 3 Si 7 reveal emergent neutral excitations, whose properties are sensitively changed by a field-driven transition between two antiferromagnetic phases. In the low-field phase, a significant violation of the Wiedemann-Franz law demonstrates that YbIr 3 Si 7 is a charge insulator but a thermal metal. In the high-field phase, thermal conductivity exhibits a sharp drop below 300 mK, indicating a transition from a thermal metal into an insulator/semimetal driven by the magnetic transition. These results suggest that spin degrees of freedom directly couple to the neutral fermions, whose emergent Fermi surface undergoes a field-driven instability at low temperatures.

36 MATERIALS SCIENCE↗
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 OSTI2023

Emergent layer stacking arrangements in c-axis confined MoTe 2

The layer stacking order in 2D materials strongly affects functional properties and holds promise for next-generation electronic devices. In bulk, octahedral MoTe 2 possesses two stacking arrangements, the ferroelectric Weyl semimetal T d phase and the higher-order topological insulator 1T' phase. However, in thin flakes of MoTe 2 , it is unclear if the layer stacking follows the T d , 1T', or an alternative stacking sequence. Here, we use atomic-resolution scanning transmission electron microscopy to directly visualize the MoTe 2 layer stacking. In thin flakes, we observe highly disordered stacking, with nanoscale 1T' and T d domains, as well as alternative stacking arrangements not found in the bulk. We attribute these findings to intrinsic confinement effects on the MoTe 2 stacking-dependent free energy. Our results are important for the understanding of exotic physics displayed in MoTe 2 flakes. More broadly, this work suggests c-axis confinement as a method to influence layer stacking in other 2D materials.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2024

Paradigm for approaching the forbidden phase transition in the one-dimensional Ising model at fixed finite temperature: Single chain in a magnetic field

In a previous paper, the forbidden spontaneous phase transition in the one-dimensional Ising model was found to be approachable arbitrarily closely in decorated ladders by ultra-narrow phase crossover (UNPC) at a given finite temperature T 0 with the crossover width 2δT reduced exponentially, which resemble a genuine first-order transition with large latent heat. Here, I reveal that the forbidden phase transition can be approached at fixed T 0 as well in decorated single-chain Ising models in the presence of a magnetic field, in which T 0 is determined by the interactions involving only the decorated parts and the magnetic field, while 2δT is independently, exponentially reduced (δT = 0 means a genuine transition) by restoring the ferromagnetic interaction between the ordinary spins on the chain backbone-which was neglected in the previous studies of pseudo-transition-thus manifesting that this asymptoticity to the forbidden transition is essentially the buildup of coherence in preformed crossover of local states. Furthermore, I show that the UNPC can be realized even in the absence of the conventional geometric frustration because the magnetic field itself can induce previously unnoticed hidden spin frustration. In conclusion, these findings make the doors wide open to the engineering and utilization of UNPC as a new paradigm for exploring exotic phenomena and 1D device applications.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2024

Dual quantum spin Hall insulator by density-tuned correlations in TaIrTe 4

The convergence of topology and correlations represents a highly coveted realm in the pursuit of new quantum states of matter. Introducing electron correlations to a quantum spin Hall (QSH) insulator can lead to the emergence of a fractional topological insulator and other exotic time-reversal-symmetric topological order, not possible in quantum Hall and Chern insulator systems. Here, in this study, we report a new dual QSH insulator within the intrinsic monolayer crystal of TaIrTe 4 , arising from the interplay of its single-particle topology and density-tuned electron correlations. At charge neutrality, monolayer TaIrTe 4 demonstrates the QSH insulator, manifesting enhanced nonlocal transport and quantized helical edge conductance. After introducing electrons from charge neutrality, TaIrTe 4 shows metallic behaviour in only a small range of charge densities but quickly goes into a new insulating state, entirely unexpected on the basis of the single-particle band structure of TaIrTe 4 . This insulating state could arise from a strong electronic instability near the van Hove singularities, probably leading to a charge density wave (CDW). Remarkably, within this correlated insulating gap, we observe a resurgence of the QSH state. The observation of helical edge conduction in a CDW gap could bridge spin physics and charge orders. The discovery of a dual QSH insulator introduces a new method for creating topological flat minibands through CDW superlattices, which offer a promising platform for exploring time-reversal-symmetric fractional phases and electromagnetism.

electronic devices↗
DOE OSTI2022

Dynamic compression of water to conditions in ice giant interiors

Abstract Recent discoveries of water-rich Neptune-like exoplanets require a more detailed understanding of the phase diagram of H 2 O at pressure–temperature conditions relevant to their planetary interiors. The unusual non-dipolar magnetic fields of ice giant planets, produced by convecting liquid ionic water, are influenced by exotic high-pressure states of H 2 O—yet the structure of ice in this state is challenging to determine experimentally. Here we present X-ray diffraction evidence of a body-centered cubic (BCC) structured H 2 O ice at 200 GPa and ~ 5000 K, deemed ice XIX, using the X-ray Free Electron Laser of the Linac Coherent Light Source to probe the structure of the oxygen sub-lattice during dynamic compression. Although several cubic or orthorhombic structures have been predicted to be the stable structure at these conditions, we show this BCC ice phase is stable to multi-Mbar pressures and temperatures near the melt boundary. This suggests variable and increased electrical conductivity to greater depths in ice giant planets that may promote the generation of multipolar magnetic fields.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2021

Dynamical symmetry indicators for Floquet crystals

Various exotic topological phases of Floquet systems have been shown to arise from crystalline symmetries. Yet, a general theory for Floquet topology that is applicable to all crystalline symmetry groups is still in need. In this work, we propose such a theory for (effectively) non-interacting Floquet crystals. We first introduce quotient winding data to classify the dynamics of the Floquet crystals with equivalent symmetry data, and then construct dynamical symmetry indicators (DSIs) to sufficiently indicate the inherently dynamical Floquet crystals. The DSI and quotient winding data, as well as the symmetry data, are all computationally efficient since they only involve a small number of Bloch momenta. We demonstrate the high efficiency by computing all elementary DSI sets for all spinless and spinful plane groups using the mathematical theory of monoid, and find a large number of different nontrivial classifications, which contain both first-order and higher-order 2+1D anomalous Floquet topological phases. Using the framework, we further find a new 3+1D anomalous Floquet second-order topological insulator (AFSOTI) phase with anomalous chiral hinge modes.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2020

Ferroelectric Switching of Pure Spin Polarization in Two-Dimensional Electron Gas

Two-dimensional electron gas (2DEG) created at compound interfaces can exhibit a broad range of exotic physical phenomena, including quantum Hall phase, emergent ferromagnetism, and superconductivity. Although electron spin plays key roles in these phenomena, the fundamental understanding and application prospects of such emergent interfacial states have been largely impeded by the lack of purely spin-polarized 2DEG. In this paper, by first-principles calculations of the multiferroic superlattice GeTe/MnTe, we find the ferroelectric polarization of GeTe is concurrent with the half-metallic 2DEG at interfaces. Remarkably, the pure spin polarization of the 2DEG can be created and annihilated by polarizing and depolarizing the ferroelectrics and can be switched (between pure spin-up and pure spin-down) by flipping the ferroelectric polarization. Given the electric-field amplification effect of ferroelectric electronics, we envision multiferroic superlattices could open up new opportunities for low-power, high-efficiency spintronic devices such as spin field-effect transistors.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2024

Probing the van Hove singularity of the kagome metal YV 6 Sn 6 through quantum oscillations

Kagome metals with the Fermi energy tuned near the van Hove singularities (vHss) have shown to host exotic phases including unconventional superconductivity and a chiral flux phase arising from a charge density wave. However, most quantum oscillations studies of the electronic structure of kagome metals focus on compounds which electronically or magnetically order, obscuring the unperturbed vHs. Here we present quantum oscillation measurements of YV 6 ⁢Sn 6 which contains a pristine kagome lattice free from long-range order. We discovered quantum oscillations corresponding to a large orbit (≈70% of the Brillouin Zone area) with the heaviest mass ever observed in vanadium-based kagome metals (≈3.3⁢m e ), consistent with a Fermi pocket whose Fermi level is near the vHs. Comparing with first-principle calculations suggests that the effective mass of this pocket is highly sensitive to the position of Fermi level. Finally, our study establishes the enhanced density of states associated with a vHs in a kagome metal, allowing further insight into a potential driving mechanism for the unconventional electronic orderings in this class of materials.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2024

Dynamical decoding of the competition between charge density waves in a kagome superconductor

The kagome superconductor CsV 3 Sb 5 hosts a variety of charge density wave (CDW) phases, which play a fundamental role in the formation of other exotic electronic instabilities. However, identifying the precise structure of these CDW phases and their intricate relationships remain the subject of intense debate, due to the lack of static probes that can distinguish the CDW phases with identical spatial periodicity. Here, we unveil the competition between two coexisting 2×2×2 CDWs in CsV 3 Sb 5 harnessing time-resolved X-ray diffraction. By analyzing the light-induced changes in the intensity of CDW superlattice peaks, we demonstrate the presence of both phases, each displaying a significantly different amount of melting upon excitation. The anomalous light-induced sharpening of peak width further shows that the phase that is more resistant to photo-excitation exhibits an increase in domain size at the expense of the other, thereby showcasing a hallmark of phase competition. Our results not only shed light on the interplay between the multiple CDW phases in CsV 3 Sb 5 , but also establish a non-equilibrium framework for comprehending complex phase relationships that are challenging to disentangle using static techniques.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2020

Thermally superactive artificial kagome spin ice structures obtained with the interfacial Dzyaloshinskii-Moriya interaction

Artificial kagome spin ice exhibits exotic magnetic correlations driven by a combination of geometric frustration and dipolar interactions that, at low-enough temperature, can result in ordered phases. This order, whether it is the ground state of several kagome rings, or the theoretically predicted long-range order of an extended array, has yet to be experimentally observed. By introducing an interfacial Dzyaloshinskii-Moriya interaction, we are able to reduce the blocking temperature of the individual nanomagnets, allowing a system of 30 kagome nanomagnets to explore its vast manifold of microstates and find its ground state. Furthermore, the extracted magnetic correlations in an extended artificial kagome spin ice are found to exhibit quantitative signatures of long-range charge order, providing evidence of the theoretically predicted continuous phase transition to the charge-ordered state. The significant lowering of the blocking temperature in nanomagnets is important for the exploitation of superparamagnetism in artificial spin systems and devices.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2020

Direct Evidence of the Competing Nature between Electronic and Lattice Breathing Order in Rare-Earth Nickelates

Correlated electrons give rise to both exotic electronic and magnetic properties in rare-earth nickelates. IN this research, we introduce evidence of the interfacial coupling between two nickelate systems, EuNiO 3 (ENO) and LaNiO 3 (LNO), with different electronic and magnetic properties but with compatible structural registry giving rise to an electrostructural transition, unobserved in each constituent. Nominally, LNO remains in a paramagnetic-metallic R 3 ¯ c phase while orthorhombic ENO undergoes antiferromagnetic and insulating transitions. However, the ENO/LNO heterostructure displays a uniform rotational symmetry set by an entwined interface. This leads to an anomalous reduction of bond disproportionation in the ENO layer through the metal to insulator transition and concomitantly charge disproportionation opens the gap accompanied by antiferromagnetic ordering. Our findings resolve a long-standing question in the physics of rare-earth nickelates, herein demonstrating that charge and bond disproportionation are competing mechanisms for the charge localization process in the rare-earth nickelate system.

36 MATERIALS SCIENCE↗
DOE OSTI2020

Exotic bonding interactions and coexistence of chemically distinct periodic lattice distortions in the charge density wave compound TaTe2

The superstructure of TaTe 2 is examined by total high-energy x-ray scattering and large-scale structure modeling over a broad temperature range. At room temperature, it features double zigzag chains of Ta atoms exhibiting metallic bonding. The chains are sandwiched between planes of weakly interacting Te atoms. Below 170 K, the chains appear broken to “butterflylike” clusters and single zigzag chains of covalently bonded Te atoms emerge while Ta-Te interactions remain largely ionic. This is a rare example of a charge density wave compound stabilized by bonding that is a mix of covalent, ionic, and metallic character giving rise to chemically distinct periodic lattice distortions. We argue that the formation of periodic lattice distortions in the Te sublattice in addition to those in the Ta sublattice favors charge delocalization, which may explain the unusually diminished resistivity and increased magnetic susceptibility exhibited by the low-temperature charge density wave phase of TaTe 2 . The effect may be common for 5d transition metal chalcogenides containing Te because of the extended electronic orbitals of the constituent atoms.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2022

A room temperature polar magnetic metal

The emergence of long-range magnetic order in noncentrosymmetric compounds has stimulated interest in the possibility of exotic spin transport phenomena and topologically protected spin textures for applications in next-generation spintronics. Polar magnets, with broken symmetries of spatial inversion and time reversal, usually host chiral spin textures. This work reports on a wurtzite-structure polar magnetic metal, identified as AA'-stacked (Fe 0.5 Co 0.5 ) 5 GeTe 2 , which exhibits a Néel-type skyrmion lattice as well as a Rashba-Edelstein effect at room temperature. Atomic resolution imaging of the structure reveals a structural transition as a function of Co-substitution, leading to the emergence of the polar phase at 50% Co. We report this discovery reveals an unprecedented layered polar magnetic system for investigating intriguing spin topologies, and it ushers in a promising new framework for spintronics.

36 MATERIALS SCIENCE↗
DOE OSTI2022

Quantum spin liquid with emergent chiral order in the triangular-lattice Hubbard model

The interplay between spin frustration and charge fluctuation gives rise to an exotic quantum state in the intermediate-interaction regime of the half-filled triangular-lattice Hubbard model, while the nature of the state is under debate. Using the density matrix renormalization group with SU(2) spin ⓍU(1) charge symmetries implemented, we study the triangular-lattice Hubbard model defined on the long cylinder geometry up to circumference W=6. A gapped quantum spin liquid, with on-site interaction 9≲U/t≲10.75, is identified between the metallic and the antiferromagnetic Mott insulating phases. In particular, we find that this spin liquid develops a robust long-range spin scalar-chiral correlation as the system length L increases, which unambiguously unveils the spontaneous time-reversal symmetry breaking. In addition, the degeneracy of the entanglement spectrum supports symmetry fractionalization and spinon edge modes in the obtained ground state. The possible origin of chiral order in this intermediate spin liquid and its relation to the rotonlike excitations have also been discussed.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2020

Quantum anomalous Hall effect in two-dimensional magnetic insulator heterojunctions

Abstract Recent years have witnessed tremendous success in the discovery of topological states of matter. Particularly, sophisticated theoretical methods in time-reversal-invariant topological phases have been developed, leading to the comprehensive search of crystal database and the prediction of thousands of topological materials. In contrast, the discovery of magnetic topological phases that break time reversal is still limited to several exemplary materials because the coexistence of magnetism and topological electronic band structure is rare in a single compound. To overcome this challenge, we propose an alternative approach to realize the quantum anomalous Hall (QAH) effect, a typical example of magnetic topological phase, via engineering two-dimensional (2D) magnetic van der Waals heterojunctions. Instead of a single magnetic topological material, we search for the combinations of two 2D (typically trivial) magnetic insulator compounds with specific band alignment so that they can together form a type-III broken-gap heterojunction with topologically non-trivial band structure. By combining the data-driven materials search, first-principles calculations, and the symmetry-based analytical models, we identify eight type-III broken-gap heterojunctions consisting of 2D ferromagnetic insulators in the MXY compound family as a set of candidates for the QAH effect. In particular, we directly calculate the topological invariant (Chern number) and chiral edge states in the MnNF/MnNCl heterojunction with ferromagnetic stacking. This work illustrates how data-driven material science can be combined with symmetry-based physical principles to guide the search for heterojunction-based quantum materials hosting the QAH effect and other exotic quantum states in general.

36 MATERIALS SCIENCE↗
DOE OSTI2025

Higher Vortexability: Zero-Field Realization of Higher Landau Levels

The rise of moiré materials has led to experimental realizations of integer and FCIs in small or vanishing magnetic fields. At the same time, a set of minimal conditions sufficient to guarantee an Abelian fractional state in a flat band were identified, namely “ideal" or “vortexable" quantum geometry. Such vortexable bands share essential features with the LLL, while excluding the need for more fine-tuned aspects such as flat Berry curvature. A natural and important generalization is to ask if such conditions can be extended to capture the quantum geometry of higher Landau levels, particularly the first (1LL), where non-Abelian states at ν = 1/2, 2/5 are known to be competitive. The possibility of realizing these states at zero magnetic field, and perhaps even more exotic ones, could become a reality if we could identify the essential structure of the 1LL in Chern bands. In this work, we introduce a precise definition of 1LL quantum geometry, along with a figure of merit that measures how closely a given band approaches the 1LL. Furthermore, periodically strained Bernal graphene is shown to realize such a 1LL structure even in zero magnetic field.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2022

Evidence for a robust sign-changing s-wave order parameter in monolayer films of superconducting Fe (Se,Te)/Bi 2 Te 3

The Fe-based superconductor Fe (Se,Te) combines non-trivial topology with unconventional superconductivity and may be an ideal platform to realize exotic states such as high-order topological corner modes and Majorana modes. Thin films of Fe (Se,Te) are particularly important for device fabrication and phase sensitive transport measurements. While bulk Fe (Se,Te) has been extensively studied, the nature of the superconducting order parameter in the monolayer limit has not yet been explored. In this work, we study monolayer films of Fe (Se,Te) on Bi 2 Te 3 with scanning tunneling spectroscopy. Monolayer Fe (Se,Te)/Bi 2 Te 3 heterostructures host a multigap superconducting state that strongly resembles the bulk. Analysis of the phase-referenced quasiparticle interference signal reveals a sign-changing s-wave order parameter similar to the bulk as well as a unique pattern of sign changes which have not been observed in the bulk. Our work establishes monolayer Fe (Se,Te)/Bi 2 Te 3 as a robust multi-band unconventional superconductor and sets the stage for explorations of non-trivial topology in this highly-tunable system.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗