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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 1,171 records · Page 65
All sourcesNASA NTRSDOE OSTI
DOE OSTI2026

Observation of room-temperature charge density wave correlations via coherent phonon spectroscopy in the Sn-doped kagome superconductor CsV 3 ⁢Sb 5

In this study, we perform ultrafast time-resolved reflectivity measurements to track the evolution of charge density wave (CDW) correlations in Sn-doped Kagome superconductor CsV 3⁢ Sb 5−𝑥 ⁢Sn 𝑥 . By extracting the coherent phonon spectrum, we evidence robust signatures of CDW correlations at temperature and doping ranges far beyond the phase boundary of long-range CDW order. Remarkably, we show that short-range CDW correlations survive up to room temperature in 𝑥 = 0.32 Sn-doped CsV 3 ⁢Sb 5 , supported by synchrotron x-ray diffraction measurements. We point out that the introduction of quenched disorder by Sn doping can pin the CDW and form static short-range CDW, which can explain the observed persistent CDW signatures. Our results thus corroborate the ubiquity and robustness of CDW correlations in Sn-doped CsV 3⁢ Sb 5 and provide new insights on the role of disorders on the CDW correlations in AV 3 ⁢Sb 5 family.

Charge density waves↗
DOE OSTI2010

Highly entangled photons and rapidly responding polarization qubit phase gates in a room-temperature active Raman gain medium

We present a scheme for obtaining entangled photons and quantum phase gates in a room-temperature four-state tripod-type atomic system with two-mode active Raman gain (ARG). We analyze the linear and nonlinear optical responses of this ARG system and show that the scheme is fundamentally different from those based on electromagnetically induced transparency and hence can avoid significant probe-field absorption as well as a temperature-related Doppler effect. We demonstrate that highly entangled photon pairs can be produced and rapidly responding polarization qubit phase gates can be constructed based on the unique features of the enhanced cross-phase-modulation and superluminal probe-field propagation of the system.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2019

Long mean free paths of room-temperature THz acoustic phonons in a high thermal conductivity material

We report measurements of room-temperature mean free paths of long-lived THz acoustic phonons in wurtzite GaN. Longitudinal phonon wave packets are excited and probed by femtosecond laser pulses in two InGaN-GaN multiple quantum well structures separated by a GaN layer. By measuring the temperature dependence of the phonon attenuation in the range 80–300 K we isolate the intrinsic phonon mean free path at 300 K which is found to be 5.3 and 3.5 μm at 1.06 and 1.43 THz, respectively. The measurements are found to be in good agreement with ab initio calculations which show that the main channel of the acoustic phonon decay is a three-phonon scattering process involving the acoustic phonon and two high-frequency optical phonons. Here, our results indicate that the contribution of low-THz acoustic phonons to thermal transport in GaN is relatively smaller than in Si; thus finite size effects are expected to be less important in GaN.

36 MATERIALS SCIENCE↗
DOE OSTI2019

Polarization-resolved Raman spectroscopy of α – RuCl 3 and evidence of room-temperature two-dimensional magnetic scattering

Polarization-resolved Raman spectroscopy was performed and analyzed from large, high-quality, monodomain single crystal of α–RuCl 3 , a proximate Kitaev quantum spin liquid. Spectra were collected with laser polarizations parallel and perpendicular to the honeycomb plane. Pairs of nearly degenerate phonons were discovered and show either a fourfold or twofold polarization angle dependence in their Raman intensity, thereby providing evidence to definitively assign the bulk crystal point group as C 2h . The low-frequency continuum that is often attributed to scattering from pairs of Majorana fermions was also examined and found to disappear when the laser excitation and scattered photon polarizations were perpendicular to the honeycomb plane. This disappearance, along with the behavior of the phonon spectrum in the same polarization configuration, strongly suggests that the scattering continuum is two-dimensional. Here, we argue that this scattering continuum originates from the Kitaev magnetic interactions that survives up to room temperature, a scale larger than the bare Kitaev exchange energy of approximately 50 K.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2020

Large spin Hall effect in Si at room temperature

Silicon's weak intrinsic spin-orbit coupling and centrosymmetric crystal structure are a critical bottleneck to the development of Si spintronics, because they lead to an insignificant spin Hall effect (spin current generation) and inverse spin Hall effect (spin current detection). In this work, we undertake current, magnetic field, crystallography dependent magnetoresistance, and magnetothermal transport measurements to study the spin transport behavior in freestanding Si thin films. We observe a large spin Hall magnetoresistance in both $\mathcal{p}$ – Si and $\mathcal{n}$ – Si at room temperature and it is an order of magnitude larger than that of Pt. One explanation of the unexpectedly large and efficient spin Hall effect is spin-phonon coupling instead of spin-orbit coupling. The macroscopic origin of the spin-phonon coupling can be large strain gradients that can exist in the freestanding Si films. This discovery in a light, earth abundant and centrosymmetric material opens a new path of strain engineering to achieve spin dependent properties in technologically highly developed materials.

36 MATERIALS SCIENCE↗
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

Itinerant ferromagnetism in van der Waals Fe 5 – x Ge Te 2 crystals above room temperature

Two-dimensional (2D) van der Waals (vdW) magnets have recently attracted increasing attention, as they provide a novel system for exploring 2D magnetism. However, intrinsic ferromagnetism in 2D systems has almost exclusively been observed at low temperatures, limiting their technological relevance. Fe N Ge Te 2 ( N = 3 , 4, and 5) systems are currently becoming the most attractive 2D vdW materials due to their relatively high Curie temperatures and large saturation magnetization. However, the nature of their complex yet intriguing magnetic behaviors is still unclear, in part due to the multiple inequivalent iron sites and iron vacancies. In this study, we show evolution of magnetic ordering transitions in Fe 5 – x Ge Te 2 with high Curie temperature and a strong saturation magnetization using photoemission electron microscopy and transport measurements. At 275 K, the ferromagnet transitions to a ferrimagnet, and below 110 K transitions to a state with glassy clusters. These are evidenced from temperature-dependent magnetic stripe domain evolution and anisotropic magnetoresistance measurements. Our findings show a clear magnetic ground state of Fe 5 – x Ge Te 2 at room temperature which signals that Fe 5 – x Ge Te 2 system is a very promising candidate for spintronic devices and provides a material design pathway to further increase the Curie temperature and saturation moments in vdW ferromagnets.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2021

Defect-engineered room-temperature ferromagnetism in quasi-two-dimensional nitrided CoTa 2 O 6

Thermal ammonolysis of quasi-two-dimensional (quasi-2D) CoTa 2 O 6 yields the O 2– /N 3– and anionic vacancy-ordered Co 2+ $Ta^{5+}_{2}$O 6–x N 2x/3 $\square$ x/3 (x $\leqslant$ 0.15), which exhibits a transition from antiferromagnetism to defect-engineered above-room-temperature ferromagnetism. First-principles calculations reveal that the origin of ferromagnetism is a particular CoO 5 N configuration with N located at Wyckoff position 8j, which breaks mirror symmetry about the ab plane. A pressure-induced electronic phase transition is also predicted at ~24.5 GPa, accompanied by insulator-to-metal transition and magnetic moment vanishing.

36 MATERIALS SCIENCE↗
DOE OSTI2021

Room-temperature large magnetoelectricity in a transition metal doped ferroelectric perovskite

There is increasing interest in novel magnetoelectric (ME) materials that exhibit robust ME coupling at room-temperature (RT) for advanced memory, energy, spintronics, and other multifunctional device applications, by making use of the ability to control polarization with a magnetic field and/or magnetization via an electric field. Obtaining ME materials with strong ME coupling, understanding the origin, and manipulating its processing along with composition to realize large ME coefficients at RT constitute an important step in multiferroic research. To address this, we have investigated the multiferroic and ME properties of Ni-doped Pb(Zr 0.20 Ti 0.80 )O 3 (PZT). We find that the ferroelectric (T C ~ 700 K) and weak ferromagnetic (~ 602 K) phase transitions of Ni-doped PZT are well above RT, leading to a strong ME coupling coefficient (α E,31 ) of 11.7 mVcm -1 Oe -1 (H ac = 1 Oe and f = 1 kHz). While X-ray diffraction suggests a single-phase material, high resolution transmission electron microscopy reveals regions with and without Ni present; thus magnetoelectric coupling between two phases is possible. First-principle calculations suggest the (Ni Pb ) × defect is likely to be responsible for the experimental observed magnetism and ME coupling in Ni-doped PZT. Furthermore, we demonstrate that Ni-doped PZT exhibits low loss tangent, low leakage current, large saturation polarization and weak ferromagnetism. Ultimately, our work demonstrates that Ni-doped PZT is a cost-effective RT multiferroic with strong ME coupling.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2021

Room-temperature superconductivity in boron- and nitrogen-doped lanthanum superhydride

Recent theoretical and experimental studies of hydrogen-rich materials at megabar pressures (i.e., >100 GPa) have led to the discovery of very high-temperature superconductivity in these materials. Lanthanum superhydride LaH10 has been of particular focus as the first material to exhibit a superconducting critical temperature (T c ) near room temperature. Experiments indicate that the use of ammonia borane as the hydrogen source can increase the conductivity onset temperatures of lanthanum superhydride to as high as 290 K. In this work, we examine the doping effects of B and N atoms on the superconductivity of LaH 10 in its fcc ($Fm\bar{3}m$) clathrate structure at megabar pressures. Doping at H atomic positions strengthens the H 32 cages of the structure to give higher phonon frequencies that enhance the Debye frequency and thus the calculated T c . The predicted T c can reach 288 K in LaH 9.985 N 0.015 within the average high-symmetry structure at 240 GPa. The remarkably large shift in T c to higher values produced by small degrees of chemical doping opens the prospect for the creation of still higher-temperature superconductivity in superhydrides potentially at even lower-pressure conditions.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2023

Refined room-temperature equation of state of Bi up to 260 GPa

At room temperature, bismuth undergoes several structural transitions with increasing pressure before taking on a body-centered cubic (bcc) phase at approximately 8 GPa. The bcc structure is stable to the highest measured pressure and its simplicity, along with its high compressibility and atomic number, makes it an enticing choice as a pressure calibrant. Here, we present three data sets on the compression of bismuth in a diamond anvil cell in a neon pressure medium, up to a maximum pressure of about 260 GPa. The use of a soft pressure medium reduces deviatoric stress when compared to previous work. With an expanded pressure range, a higher point density, and a decreased uniaxial stress component, we are able to provide more reliable equation of state parameters. We also conduct density functional theory electronic-structure calculations that confirm that the bcc phase is energetically favored at high pressure.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2011

Oxygen Reordering Near Room temperature in YBa2Cu3O6+x: A Thermodynamic Model

We propose a thermodynamic model to explain an unusual phase transformation occurring near room temperature in YBa{sub 2}Cu{sub 3}O{sub 6+x} that greatly affects properties of the superconductor. Based on our model, the material's thermodynamic response functions, specific heat, thermal-expansion coefficient, and elastic compliances are deduced at the critical temperature of the phase transformation. We discuss the change of critical temperature with stress, and analyze the anomaly of specific heat in critical temperature of the phase transformation.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2011

Room-temperature phonon boundary scattering below the Casimir limit

Thermal conductivity data for rough surface silicon nanowires suggest the breakdown of the Casimir limit which assumes completely diffuse phonon boundary scattering. We show that coherent effects in phonon transport at room temperature indeed lead to such breakdown. Correlated multiple scattering of phonons off the rough surface lead to a reduced thermal conductivity that is dependent not only on the roughness amplitude but more importantly on the roughness correlation length. A correlation length less than the diameter of the wire is typically necessary for lowering the thermal conductivity below the Casimir limit. Our model explains seeming anomalies in data reported for electrolessly etched and electron beam lithography defined nanowires.

Sadhu, J↗
DOE OSTI2016

Possibility of realizing quantum spin Hall effect at room temperature in stanene / A l 2 O 3 ( 0001 )

Two-dimensional quantum spin Hall (QSH) insulators with reasonably wide band gaps are imperative for the development of various innovative technologies. Through systematic density functional calculations and tight-binding simulations, we found that stanene on an α -alumina surface may possess a sizeable topologically nontrivial band gap ( ~ 0.25 eV ) at the Γ point. Furthermore, stanene is atomically bonded to but electronically decoupled from the substrate, providing high structural stability and isolated QSH states to a large extent. We conclude the underlying physical mechanism is rather general, and this finding may lead to the opening of a new vista for the exploration of QSH insulators for room temperature device applications.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2015

Determination of the α -decay half-life of Po 210 based on film and slice bismuth samples at room temperature

For this research, the α decay rate of 210 Po was measured in a film sample ( 210 Po@Bi 2 O 3 ) and a slice sample ( 210 Po@Bi metal), respectively. The former was used as a reference sample. The half-lives of 210 Po@Bi 2 O 3 and 210 Po@Bi metal environments were observed to be (138.40 ± 0.21 d) and (138.87 ± 0.87 d) at room temperature, respectively. It was found that the half-life of 210 Po is consistent with international recommendations within the uncertainty limits, and we did not find any deviation of the α decay rate of 210 Po between film sample ( 210 Po@Bi 2 O 3 ) and slice sample ( 210 Po@Bi metal).

73 NUCLEAR PHYSICS AND RADIATION PHYSICS↗
DOE OSTI2012

Translation-rotation decoupling and nonexponentiality in room temperature ionic liquids

Using a combination of light scattering techniques and broadband dielectric spectroscopy, we have measured the temperature dependence of structural relaxation time and self diffusion in three imidazolium-based room temperature ionic liquids: [bmim][NTf2], [bmim][PF6], and [bmim][TFA]. A detailed analysis of the results demonstrates that self diffusion decouples from structural relaxation in these systems as the temperature is decreased toward Tg. The degree to which the dynamics are decoupled, however, is shown to be surprisingly weak when compared to other supercooled liquids of similar fragility. In addition to the weak decoupling, we demonstrate that the temperature dependence of the structural relaxation time in all three liquids can be well described by a single Vogel-Fulcher-Tamann function over 13 decades in time from 10 11 s up to 102 s. Furthermore, the stretching of the structural relaxation is shown to be temperature independent over the same range of time scales, i.e., time temperature superposition is valid for these ionic liquids from far above the melting point down to the glass transition temperature.We suggest that these phenomena are interconnected and all result from the same underlying mechanism strong and directional intermolecular interactions.

Griffin, Phillip↗
DOE OSTI2010

Room-Temperature Ferromagnetism of Cu-Doped ZnO Films Probed by Soft X-Ray Magnetic Circular Dichroism

We report direct evidence of room-temperature ferromagnetic ordering in O-deficient ZnO:Cu films by using soft x-ray magnetic circular dichroism and x-ray absorption. Our measurements have revealed unambiguously two distinct features of Cu atoms associated with (i) magnetically ordered Cu ions present only in the oxygen-deficient samples and (ii) magnetically disordered regular Cu{sup 2+} ions present in all the samples. We find that a sufficient amount of both oxygen vacancies (V{sub O}) and Cu impurities is essential to the observed ferromagnetism, and a non-negligible portion of Cu impurities is uninvolved in the magnetic order. Based on first-principles calculations, we propose a microscopic 'indirect double-exchange' model, in which alignments of localized large moments of Cu in the vicinity of the V{sub O} are mediated by the large-sized vacancy orbitals.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2015

Giant Spin-Driven Ferroelectric Polarization in BiFeO 3 at Room Temperature

Although BiFeO 3 is the most extensively investigated multiferroic material, its magnetoelectic couplings are barely understood. Here we report a thorough study of the magentoelectric (ME) couplings in spin-cycloidal buk BiFeO 3 using first-principles calculations and microscopic spin-wave models compared with neutron-scattering measurements. We find that huge exchange-striction (ES) polarizations, i.e. the electric response of the magnetic exchange through ferroelectric and antiferrodistortive distortions, is giant enough to dominate over all other ME couplings. We show that BiFeO 3 has a hidden record-high spin-driven polarization ( 3 C/cm 2 ) at room-temperature. The huge ES polarizations can be tuned by coupling to the antiferrodistortive rotations.

36 MATERIALS SCIENCE↗