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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 55 records · Page 3
All sourcesNASA NTRSDOE OSTI
DOE OSTI2015

Observation of a d -wave gap in electron-doped Sr 2 IrO 4

High-temperature superconductivity in cuprates emerges out of a highly enigmatic 'pseudogap' metal phase. The mechanism of high-temperature superconductivity is probably encrypted in the elusive relationship between the two phases, which spectroscopically is manifested as Fermi arcs—disconnected segments of zero-energy states—collapsing into d -wave point nodes upon entering the superconducting phase. Here, we reproduce this distinct cuprate phenomenology in the 5 d transition-metal oxide Sr 2 IrO 4 . Using angle-resolved photoemission, we show that the clean, low-temperature phase of 6-8% electron-doped Sr 2 IrO 4 has gapless excitations only at four isolated points in the Brillouin zone, with a predominant d -wave symmetry of the gap. Our work thus establishes a connection between the low-temperature d -wave instability and the previously reported high-temperature Fermi arcs in electron-doped Sr 2 IrO 4 (ref.). Although the physical origin of the d-wave gap remains to be understood, Sr 2 IrO 4 is the first non-cuprate material to spectroscopically reproduce the complete phenomenology of the cuprates, thus offering a new material platform to investigate the relationship between the pseudogap and the d -wave gap.

Kim, Y. K.↗
DOE OSTI2018

Anisotropy of the Hall Effect in a Quasi-Two-Dimensional Electron-Doped Nd{sub 2 –} {sub x}Ce{sub x}CuO{sub 4 + δ} Superconductor

Abstract—The temperature dependences of the Hall effect of an electron-doped Nd{sub 2 –} {sub x}Ce{sub x}CuO{sub 4 + δ} superconductor have been studied at the antiferromagnet–superconductor quantum phase transition boundary (0.135 ≤ x ≤ 0.15) in conducting CuO{sub 2} planes and in the direction perpendicular to the CuO{sub 2} plane. The hall coefficient between the conducting planes has been found be two orders higher than that in the conducting planes over entire temperature range, which is due to the incoherent character of the charge carrier transfer in the direction of axis c.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2019

Quantized Electronic Doping towards Atomically Controlled “Charge-Engineered” Semiconductor Nanocrystals

“Charge engineering” of semiconductor nanocrystals (NCs) through so-called electronic impurity doping is a long-standing challenge in colloidal chemistry and holds promise for ground-breaking advancements in many optoelectronic, photonic, and spin-based nanotechnologies. To date, our knowledge is limited to a few paradigmatic studies on a small number of model compounds and doping conditions, with important electronic dopants still unexplored in nanoscale systems. Equally importantly, fine-tuning of charge engineered NCs is hampered by the statistical limitations of traditional approaches. The resulting intrinsic doping inhomogeneity restricts fundamental studies to statistically averaged behaviors and complicates the realization of advanced device concepts based on their advantageous functionalities. We aim to address these issues by realizing the first example of II–VI NCs electronically doped with an exact number of heterovalent gold atoms, a known p-type acceptor impurity in bulk chalcogenides. Single-dopant accuracy across entire NC ensembles is obtained through a novel non-injection synthesis employing ligand-exchanged gold clusters as “quantized” dopant sources to seed the nucleation of CdSe NCs in organic media. Structural, spectroscopic, and magneto-optical investigations trace a comprehensive picture of the physical processes resulting from the exact doping level of the NCs. Gold atoms, doped here for the first time into II–VI NCs, are found to incorporate as nonmagnetic Au + species activating intense size-tunable intragap photoluminescence and artificially offsetting the hole occupancy of valence band states. Fundamentally, the transient conversion of Au + to paramagnetic Au 2+ (5d 9 configuration) under optical excitation results in strong photoinduced magnetism and diluted magnetic semiconductor behavior revealing the contribution of individual paramagnetic impurities to the macroscopic magnetism of the NCs. Altogether, our results demonstrate a new chemical approach toward NCs with physical functionalities tailored to the single impurity level and offer a versatile platform for future investigations and device exploitation of individual and collective impurity processes in quantum confined structures.

36 MATERIALS SCIENCE↗
DOE OSTI2024

Evolution of the Magnetic Excitations in Electron-Doped La 2 - x Ce x CuO 4

Here we investigated the high energy spin excitations in electron-doped La 2-x Ce x CuO 4 , a cuprate superconductor, by resonant inelastic x-ray scattering (RIXS) measurements. Efforts were paid to disentangle the paramagnon signal from non-spin-flip spectral weight mixing in the RIXS spectrum at Q ∥ =(0.6π,0) and (0.9π,0) along the (1 0) direction. Our results show that, for doping level x from 0.07 to 0.185, the variation of the paramagnon excitation energy is marginal. We discuss the implication of our results in connection with the evolution of the electron correlation strength in this system.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2018

Three-dimensional collective charge excitations in electron-doped copper oxide superconductors

High-temperature copper oxide superconductors consist of stacked CuO 2 planes, with electronic band structures and magnetic excitations that are primarily two-dimensional1,2, but with superconducting coherence that is three-dimensional. This dichotomy highlights the importance of out-of-plane charge dynamics, which has been found to be incoherent in the normal state within the limited range of momenta accessible by optics. Here in this study, we use resonant inelastic X-ray scattering to explore the charge dynamics across all three dimensions of the Brillouin zone. Polarization analysis of recently discovered collective excitations (modes) in electron-doped copper oxides reveals their charge origin, that is, without mixing with magnetic components. The excitations disperse along both the in-plane and out-of-plane directions, revealing its three-dimensional nature. The periodicity of the out-of-plane dispersion corresponds to the distance between neighbouring CuO 2 planes rather than to the crystallographic c-axis lattice constant, suggesting that the interplane Coulomb interaction is responsible for the coherent out-of-plane charge dynamics. The observed properties are hallmarks of the long-sought ‘acoustic plasmon’, which is a branch of distinct charge collective modes predicted for layered systems and argued to play a substantial part in mediating high-temperature superconductivity.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2014

Electron-doping of graphene-based devices by hydrazine

A facile and effective technique to tune the electronic properties of graphene is essential to facilitate the flexibility of graphene-based device performances. Here, the use of hydrazine as a solution-processable and effective n-type dopant for graphene is described. By dropping hydrazine solutions at different concentrations on a graphene surface, the Dirac point of graphene can be remarkably tuned. The transport behavior of graphene can be changed from p-type to n-type accordingly, demonstrating the controllable and adjustable doping effect of the hydrazine solutions. Accompanying the Dirac point shift is an enhanced hysteretic behavior of the graphene conductance, indicating an increasing trap state density induced by the hydrazine adsorbates. The electron-doping of graphene by the hydrazine solutions can be additionally confirmed with graphene/p-type silicon heterojunctions. The decrease of the junction current after the hydrazine treatment demonstrates an increase of the junction barrier between graphene and silicon, which is essentially due to the electron-doping of graphene and the resultant upshift of the Fermi level. Finally, partially doped graphene is realized and its electrical property is studied to demonstrate the potential of the hydrazine solutions to selectively electron-doping graphene for future electronic applications.

46 INSTRUMENTATION RELATED TO NUCLEAR SCIENCE AND ↗
DOE OSTI2018

Synthesis and Superconductivity of Electron-Doped β-ZrNCl with Partial Substitution of Ti on Zr Site

Highly crystallized α-TiNCl and ZrNCl with orthogonal network of [M{sub 2}N{sub 2}] (M: Ti and Zr) were used to synthesize the solid-solution phase α-Ti{sub y}Zr{sub 1−y}NCl, which was irreversibly transformed into its β-form polymorph with honeycomb network upon a heat treatment. The intercalation of lithium (Li) into the obtained powder samples was carried out to realize the electron doping in β-form compound. The measurements of magnetic susceptibility and electrical resistivity suggest that the intercalated compound β-Li{sub 0.43}Ti{sub 0.09}Zr{sub 0.91}NCl shows superconductivity with transition temperature (T{sub c}) of 15.0 K. Upon the cointercalation of solvent molecule of tetrahydrofuran (THF), two transitions were observed at 14.1 and ∼21 K, corresponding to the main phase of β-Li{sub x}(THF){sub y}ZrNCl and a THF-cointercalated phase of β-Ti{sub y}Zr{sub 1−y}NCl, respectively. We consider that such a tiny substitution of Ti on Zr site in β-form structure play an important role in the increase of T{sub c}.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2024

Prediction of s ± -Wave Superconductivity Enhanced by Electronic Doping in Trilayer Nickelates La 4 Ni 3 O 10 under Pressure

Motivated by the recently reported signatures of superconductivity in trilayer La 4 ⁢Ni 3 ⁢O 10 under pressure, here we comprehensively study this system using ab initio and random-phase approximation techniques. Without electronic interactions, the Ni d 3z 2 –r 2 orbitals show a bonding-antibonding and nonbonding splitting behavior via the O p z orbitals inducing a “trimer” lattice in La 4⁢ Ni 3 ⁢O 10 , analogous to the dimers of La 3 ⁢Ni 2 ⁢O 7 . The Fermi surface consists of three electron sheets with mixed e g orbitals, and a hole and an electron pocket made up of the d 3⁢z 2 –r 2 orbital, suggesting a Ni two-orbital minimum model. In addition, we find that superconducting pairing is induced in the s ± -wave channel due to partial nesting between the M = (π,π) centered pockets and portions of the Fermi surface centered at the Γ = (0,0) point. With changing electronic density n, the s ± instability remains leading and its pairing strength shows a domelike behavior with a maximum around n = 4.2 ( ~6.7% electron doping). The superconducting instability disappears at the same electronic density as that in the new 1313 stacking La 3 ⁢Ni 2 ⁢O 7 , correlated with the vanishing of the hole pocket that arises from the trilayer sublattice, suggesting that the high-T c superconductivity of La 3⁢ Ni 2 ⁢O 7 does not originate from a trilayer and monolayer structure. Furthermore, we confirm the experimentally proposed spin state in La 4 ⁢Ni 3 ⁢O 10 with an in-plane (π, π) order and antiferromagnetic coupling between the top and bottom Ni layers, and spin zero in the middle layer.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2018

Stabilization of s -wave superconductivity through arsenic p -orbital hybridization in electron-doped BaFe 2 As 2

In this paper, using random-phase approximation spin-fluctuation theory, we study the influence of the hybridization between iron $d$ orbitals and pnictide $p$ orbitals on the superconducting pairing state in iron-based superconductors. The calculations are performed for a 16-orbital Hubbard-Hund tight-binding model of ${\mathrm{BaFe}}_{2}{\mathrm{As}}_{2}$ that includes the As-$p$ orbital degrees of freedom in addition to the Fe-$d$ orbitals and compared to calculations for a 10-orbital Fe-$d$ only model. In both models we find a leading ${s}^{±{}}$ pairing state and a subleading ${d}_{{x}^{2}{-}{y}^{2}}$-wave state in the parent compound. Upon doping, we find that the ${s}^{±{}}$ state remains the leading state in the 16-orbital model up to a doping level of 0.475 electrons per unit cell, at which the hole Fermi-surface pockets at the zone center start to disappear. This is in contrast to the 10-orbital model, where the $d$-wave state becomes the leading state at a doping of less than 0.2 electrons. This improved stability of ${s}^{±{}}$ pairing is found to arise from a decrease of ${d}_{xy}$ orbital weight on the electron pockets due to hybridization with the As-$p$ orbitals and the resulting reduction of near $({\pi},{\pi})$ spin-fluctuation scattering which favors the competing $d$-wave state. Finally, these results show that the orbital dependent hybridization of Fermi-surface Bloch states with the usually neglected $p$-orbital states is an important ingredient in an improved itinerant pairing theory.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2015

Enhanced carrier collection efficiency and reduced quantum state absorption by electron doping in self-assembled quantum dot solar cells

Reduced quantum dot (QD) absorption due to state filling effects and enhanced electron transport in doped QDs are demonstrated to play a key role in solar energy conversion. Reduced QD state absorption with increased n-doping is observed in the self-assembled In{sub 0.5}Ga{sub 0.5}As/GaAs QDs from high resolution below-bandgap external quantum efficiency (EQE) measurement, which is a direct consequence of the Pauli exclusion principle. We also show that besides partial filling of the quantum states, electron-doping produces negatively charged QDs that exert a repulsive Coulomb force on the mobile electrons, thus altering the electron trajectory and reducing the probability of electron capture, leading to an improved collection efficiency of photo-generated carriers, as indicated by an absolute above-bandgap EQE measurement. The resulting redistribution of the mobile electron in the planar direction is further validated by the observed photoluminescence intensity dependence on doping.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2023

Antiferromagnetic metal phase in an electron-doped rare-earth nickelate

Long viewed as passive elements, antiferromagnetic materials have emerged as promising candidates for spintronic devices due to their insensitivity to external fields and potential for high-speed switching. Recent work exploiting spin and orbital effects has identified ways to electrically control and probe the spins in metallic antiferromagnets, especially in non-collinear or non-centrosymmetric spin structures. The rare-earth nickelate NdNiO 3 is known to be a non-collinear antiferromagnet in which the onset of antiferromagnetic ordering is concomitant with a transition to an insulating state. In this work, we find that for low electron doping, the magnetic order on the nickel site is preserved, whereas electronically, a new metallic phase is induced. We show that this metallic phase has a Fermi surface that is mostly gapped by an electronic reconstruction driven by bond disproportionation. Furthermore, we demonstrate the ability to write to and read from the spin structure via a large zero-field planar Hall effect. Our results expand the already rich phase diagram of rare-earth nickelates and may enable spintronics applications in this family of correlated oxides.

36 MATERIALS SCIENCE↗
DOE OSTI2017

Pressure tuning of structure, superconductivity, and novel magnetic order in the Ce-underdoped electron-doped cuprate T ' - Pr 1.3 - x La 0.7 Ce x CuO 4 ( x = 0.1 )

High-pressure neutron powder diffraction, muon-spin rotation, and magnetization studies of the structural, magnetic, and the superconducting properties of the Ce-underdoped superconducting (SC) electron-doped cuprate system with the Nd 2 CuO 4 (the so-called T ' ) structure T ' - Pr 1.3 - x La 0.7 Ce x CuO 4 with x = 0.1 are reported. A strong reduction of the in-plane and out-of-plane lattice constants is observed under pressure. However, no indication of any pressure-induced phase transition from T' to the K 2 NiF 4 (the so-called T) structure is observed up to the maximum applied pressure of p = 11 GPa. Large and nonlinear increase of the short-range magnetic order temperature T so in T ' - Pr 1.3 - x La 0.7 Ce x CuO 4 ( x = 0.1) was observed under pressure. Simultaneous pressure causes a nonlinear decrease of the SC transition temperature T c . All these experiments establish the short-range magnetic order as an intrinsic and competing phase in SC T' - Pr 1.3 - x La 0.7 Ce x CuO 4 ( x = 0.1 ). The observed pressure effects may be interpreted in terms of the improved nesting conditions through the reduction of the in-plane and out-of-plane lattice constants upon hydrostatic pressure.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2018

Electron Doping of Proposed Kagome Quantum Spin Liquid Produces Localized States in the Band Gap

Carrier doping of quantum spin liquids is a long-proposed route to the emergence of high-temperature superconductivity. Electrochemical intercalation in kagome hydroxyl halide materials shows that samples remain insulating across a wide range of electron counts. In this work, we demonstrate through first-principles density-functional calculations, corrected for self-interaction, the mechanism by which electrons remain localized in various Zn-Cu hydroxyl halides, independent of the chemical identity of the dopant - the formation of polaronic states with attendant lattice displacements and a dramatic narrowing of bandwidth upon electron addition. The same theoretical method applied to electron doping in cuprate Nd 2 CuO 4 correctly produces a metallic state when the initially formed polaron dissolves into an extended state. In conclusion, our general findings explain the insulating behavior in a wide range of 'doped' quantum magnets and demonstrate that new quantum spin liquid host materials are needed to realize metallicity borne of a spin liquid.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2026

Emergence of low-energy spin waves in superconducting electron-doped cuprates

In order to fully utilize the technological potential of unconventional superconductors, an enhanced understanding of the superconducting mechanism is necessary. In the best performing superconductors, the cuprates, superconductivity is intimately linked with magnetism, although the details of this coupling remain elusive. Here, we address this gap by studying the electron-doped cuprate Nd 1.85 Ce 0.15 CuO 4−δ that has an antiferromagnetic ground state when synthesized and only becomes superconducting after a reductive annealing process. Using neutron spectroscopy, we show that the as-grown crystal exhibits a large spin pseudogap in the magnetic fluctuation spectrum. Annealing removes defects introduced by the commonly employed synthesis method and significantly reduces the spin pseudogap. While the spin pseudogap in the annealed sample likely arises from superconductivity, in the as-grown sample it results from the absence of long-wavelength spin waves. These results reveal a direct connection between defects, magnetism, and superconductivity, offering new insight into the mechanisms underlying high-temperature superconductivity and guiding the design of improved superconducting materials.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2016

Superconductivity below 20 K in heavily electron-doped surface layer of FeSe bulk crystal

A superconducting transition temperature (T c ) as high as 100 K was recently discovered in one monolayer FeSe grown on SrTiO 3 . The discovery ignited efforts to identify the mechanism for the markedly enhanced T c from its bulk value of 8 K. There are two main views about the origin of the T c enhancement: interfacial effects and/or excess electrons with strong electron correlation. Here, we report the observation of superconductivity below 20 K in surface electron-doped bulk FeSe. The doped surface layer possesses all the key spectroscopic aspects of the monolayer FeSe on SrTiO 3 . Without interfacial effects, the surface layer state has a moderate T c of 20 K with a smaller gap opening of 4.2 meV. Our results show that excess electrons with strong correlation cannot induce the maximum T c , which in turn reveals the need for interfacial effects to achieve the highest T c in one monolayer FeSe on SrTiO 3 .

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2019

From the Ternary Phase Ca 14 Zn 1+δ Sb 11 (δ ≈ 0.4) to the Quaternary Solid Solutions Ca 14– x RE x ZnSb 11 (RE = La–Nd, Sm, Gd, x ≈ 0.9). A Tale of Electron Doping via Rare-Earth Metal Substitutions and the Concomitant Structural Transformations

In this work, the ternary compound Ca 14 Zn 1.37(1) Sb 11 and its six rare-earth metal substituted derivatives Ca 14–x RE x ZnSb 11 (RE = La–Nd, Sm, Gd; x ≈ 0.90±0.06) have been synthesized and structurally characterized by single-crystal X-ray diffraction methods. All com-pounds formally crystallize in the tetragonal Ca 14 AlSb 11 structure type (space group $I$4 1 $/ acd$, No. 142, $Z$ = 8). The crystal structure of Ca 14 Zn 1.37(1) Sb 11 subtly differs from the structure of the remaining six, as well as from the structure of the archetype, due to the presence of a partially occupied interstitial Zn position. The extra zinc atom is needed in this structure to alleviate the unfavorable number of valence electrons in the imaginary Ca 14 ZnSb 11 . Electron doping, via substitution of RE 3+ ions on Ca 2+ sites is shown as an alternative route to achieve electron balance in these Zn-based analogs of the Ca 14 AlSb 11 structure, which does not require the in-corporation of interstitial atoms. Electrical resistivity measurements done on single-crystalline samples are in agreement with the notion that Ca 14–x RE x ZnSb 11 behave as either bad metals or heavily-doped semi-conductors. Magnetization measurements show Curie-Weiss paramagnetic behavior related to the local-moment magnetism of the RE 3+ ions.

36 MATERIALS SCIENCE↗
DOE OSTI2019

Correlation between scale-invariant normal-state resistivity and superconductivity in an electron-doped cuprate

An understanding of the normal state in the high-temperature superconducting cuprates is crucial to the ultimate understanding of the long-standing problem of the origin of the superconductivity itself. This so-called “strange metal” state is thought to be associated with a quantum critical point (QCP) hidden beneath the superconductivity. In electron-doped cuprates—in contrast to hole-doped cuprates—it is possible to access the normal state at very low temperatures and low magnetic fields to study this putative QCP and to probe the T → 0 K state of these materials. We report measurements of the low-temperature normal-state magnetoresistance (MR) of the n-type cuprate system La 2– x Ce x CuO 4 and find that it is characterized by a linear-in-field behavior, which follows a scaling relation with applied field and temperature, for doping ( x ) above the putative QCP ( x = 0.14). The magnitude of the unconventional linear MR decreases as T c decreases and goes to zero at the end of the superconducting dome ( x ~ 0.175) above which a conventional quadratic MR is found. These results show that there is a strong correlation between the quantum critical excitations of the strange metal state and the high- T c superconductivity.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2021

Hidden strange metallic state in underdoped electron-doped cuprates

The low-temperature linear-in-T resistivity of “strange metals,” such as the metallic state of the cuprate high-temperature superconductors, has long been thought to be associated with a quantum critical point. However, recent transport studies of the cuprates have found this behavior persists over a finite range of overdoping. Here, in this work, we report magnetoresistance and Hall effect results for electron-doped films of the cuprate superconductor La 2-x ⁢ Ce x CuO 4 (LCCO) for temperatures from 0.7 to 45 K and magnetic fields up to 65 T. For x = 0.12 and 0.13, just below the Fermi surface reconstruction (FSR) at x = 0.14, the normal state in-plane resistivity exhibits a well-known upturn at low temperature. Our new results show that this resistivity upturn is eliminated at high magnetic field and the resistivity becomes linear-in-temperature from ~40 K down to 0.7 K. The magnitude of the linear coefficient scales with Tc and doping, as found previously for dopings above the FSR. This striking observation suggests that the strange metal is not confined to a single “critical point” in the phase diagram, but rather is a robust universal feature of the metallic ground state of the cuprates.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗