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At least 415 records · Page 23
All sourcesNASA NTRSDOE OSTI
DOE OSTI2020

Superconducting pairing mechanism in CeCoIn 5 revisited

Spectroscopic-imaging scanning-tunneling microscopy (SI-STM) measurements have previously been applied to the study of the heavy-fermion system CeCoIn 5 to examine the superconducting gap structure and band dispersions via quasiparticle intereference. Here we directly measure the dispersing electron bands with angle-resolved-photoelectron spectroscopy (ARPES) and compare with first principles electronic structure calculations. By auto-correllating the ARPES-resolved bands with themselves we can measure the potential q-vectors and discern exactly which bands the STM is measuring. We find that the STM results are dominated by scattering associated with a cloverleaf shaped band centered at the zone corners. This same band is also a viable candidate to host the superconducting gap. Overall, the electronic structure calculations indicate that this region of the Fermi surface involves significant contributions from the Co d-electrons, an indication that the superconductivity in these materials is more 3-dimensional than that found in the related unconventional superconductors, the cuprates and the pnictides.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2017

Spectroscopic Study of the Charge Density Wave Order in 2H-TaS{sub 2}.

We conduct Angle Resolved Photoemission Spectroscopy (ARPES) investigation on 2H-TaS2, a prototypical incommensurate Charge Density Wave (CDW) material. A comparative study of the low-energy electronic structures of 2H-TaS2 and two other related compounds, 2H-TaSe2 and 2H-NbSe2, identifies several generic features of their CDW orders. Firstly, Fermi surface (FS) nesting alone doesn't seem to give rise to the CDW instability in these compounds. Secondly, partial gapping of the underlying FS surface in the CDW state is common to each of these materials. Finally, the CDW energy gap, unlike the energy gap in a superconductor, is not symmetric with respect to the chemical potential.

Angle Resolved Photoemission Spectroscopy↗
DOE OSTI2022

Observation of dimension-crossover of a tunable 1D Dirac fermion in topological semimetal NbSi x Te 2

Condensed matter systems in low dimensions exhibit emergent physics that does not exist in three dimensions. When electrons are confined to one dimension (1D), some significant electronic states appear, such as charge density wave, spin-charge separations, and Su-Schrieffer-Heeger (SSH) topological state. However, a clear understanding of how the 1D electronic properties connects with topology is currently lacking. Here we systematically investigated the characteristic 1D Dirac fermion electronic structure originated from the metallic NbTe 2 chains on the surface of the composition-tunable layered compound NbSi x Te 2 (x = 0.40 and 0.43) using angle-resolved photoemission spectroscopy. We found the Dirac fermion forms a Dirac nodal line structure protected by the combined $\widetilde {M_y}$ and time-reversal symmetry T and proves the NbSi x Te2 system as a topological semimetal, in consistent with the ab-initio calculations. As x decreases, the interaction between adjacent NbTe 2 chains increases and Dirac fermion goes through a dimension-crossover from 1D to 2D, as evidenced by the variation of its Fermi surface and Fermi velocity across the Brillouin zone in consistence with a Dirac SSH model. Our findings demonstrate a tunable 1D Dirac electron system, which offers a versatile platform for the exploration of intriguing 1D physics and device applications.

36 MATERIALS SCIENCE↗
DOE OSTI2026

Interplay of Kondo physics with incommensurate charge density waves in CeTe 3

CeTe 3 is a 2-dimensional (2D) Van der Waals (VdW) material with incommensurate charge density waves (CDW), extremely high transition temperature (T CDW ), and a large momentum-dependent CDW gap that leaves a significant portion of the Fermi surface intact. It is also considered to be a weak Kondo system, a property unexpected for a material with incommensurate CDW, where each atomic site is slightly different. Here, we study the properties of the CDW state in several RTe3 (R is rare earth) materials and examine the hybridization of itinerant states with the localized Ce 4f multiplet in CeTe 3 by using angle resolved photoemission spectroscopy. We find that the renormalization of the itinerant states originating from the hybridization with the deeper localized 4f states at −260 meV is k−dependent and extends to the Fermi level. As these localized states are far from the Fermi level, the observed hybridization affects the effective masses only marginally and does not lead to heavy fermions. However, since the same renormalizing mechanism normally leads to the heavy fermion physics when the localized 4f states are near the Fermi level, our observation of its strong k−dependence suggests that this could be the reason for discrepancy between the heavy masses in specific heat and light ones in Shubnikov de Haas oscillations, often observed in heavy fermions.

36 MATERIALS SCIENCE↗
DOE OSTI2019

Two dimensional electron gas in the $\delta$-doped iridates with strong spin–orbit coupling: La$_\delta$Sr 2 IrO 4

Iridates are of significant current interest because of the strong spin–orbit coupling that leads to a variety of new phenomena. Using density-functional studies, we suggest the formation of a spin-orbital entangled two dimensional electron gas (2DEG) in the -doped iridate La$_\delta$Sr 2 IrO 4 , where a single SrO layer is replaced by a LaO layer. The extra La electron resides close to the$\delta$-doped layer, partially occupying the J eff = 1/2 upper Hubbard band and thereby making the interface metallic. The magnetic structure of the bulk is destroyed near the interface, with the Ir 0 layer closest to the interface becoming non-magnetic, while the next layer (Ir 1 ) continues to maintain the AFM structure of the bulk, but with a reduced magnetic moment. The Fermi surface consists of a hole pocket and an electron pocket, located in two different Ir layers (Ir 0 and Ir 1 ), with both carriers derived from the J eff = 1/2 upper Hubbard band. The presence of both electrons and holes at the $\delta$-doped interface suggests unusual transport properties, leading to possible device applications.

2DEG↗
DOE OSTI2025

Electronic structure of the kagome metal YbTi 3 Bi 4 studied using torque magnetometry

This study investigates the electronic structure of the kagome metal YbTi 3 ⁢Bi 4 using high-field torque magnetometry. The torque signal measured at a maximum field of 41.5 T reveals clear de Haas–van Alphen (dHvA) oscillations with a major frequency peak at F δ ~ 130 T. By rotating the sample at various tilt angles Θ, we observed that F δ exhibits a nearly 1/cos⁡Θ dependence, indicating the presence of a quasi-two-dimensional (2D) Fermi surface (FS) in YbTi 3 ⁢Bi 4 . This argument is further supported by the detection of a forward-leaning, sawtoothlike waveform in the dHvA effect, a hallmark of 2D FS characteristics. Notably, we identified two high-frequency peaks near F χ ~ 1900 T and F λ ~ 5600 T; however, these peaks quickly disappear at Θ greater than 21°. To better understand experimental observations, we computed the electronic band structure and FS using ab initio density-functional theory (DFT). The electronic bands reveal the presence of several Dirac points, flat bands, and van Hove singularities near the Fermi level. Five bands cross the Fermi level and contribute to the FS of this material. The FS comprises cylindrical sheets, with theoretical frequencies from the FS pockets aligning well with the experimental dHvA frequencies. Several FS parameters characterizing F δ were determined by analyzing the temperature and field dependence of the dHvA oscillations using the Lifshitz-Kosevich theory. Further, the detailed electronic properties presented in this work provide critical insights into the electronic structure of YbTi 3 ⁢Bi 4 and other titanium-based kagome compounds.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2019

Instability of the f -electron state in URu 2 Si 2–x P x probed using high magnetic fields

The widely studied hidden-order phase in URu 2 Si 2 exemplifies a broad class of emergent macroscopic states originating from d- and f-electron orbitals and their collective modes. This naturally has resulted in a focus on the associated elements (i.e., U and Ru), but recently it was discovered that electronic tuning on the more passive s and p orbitals fundamentally alters the electronic behavior and thus opens opportunities to disentangle the hidden-order conundrum. Here we present a detailed magnetoresistivity study of URu 2 Si 2–x P x , which focuses on two important aspects of this topic. First, we show that the anisotropy of the high field ordered states is preserved after hidden order is destroyed by Si → P chemical substitution, suggesting that this feature is mainly associated with the Kondo lattice, i.e., not hidden order itself. Second, Shubnikov–de Haas oscillations reveal that the observable parts of the Fermi surface topography are robust upon approaching the hidden-order phase boundary and the associated quasiparticle masses remain roughly constant. Furthermore, these observations place constraints on theories and prepare a path towards a resolution to the hidden-order problem.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2022

Disentangling the role of bond lengths and orbital symmetries in controlling $T_c$ of optimally doped YBa 2 Cu 3 O 7

Optimally doped YBa 2 Cu 3 O 7 (YBCO) has a high critical temperature, at 92 K. It is largely believed that Cooper pairs form in YBCO and other cuprates because of spin fluctuations, but the issue and the detailed mechanism are far from settled. In the present work, we employ a state-of-the-art first-principles ability to compute both the low- and high-energy spin fluctuations in optimally doped YBCO. We benchmark our results against recent inelastic neutron scattering and resonant inelastic x-ray scattering measurements. Further, we use strain as an external parameter to modulate the spin fluctuations and superconductivity. We disentangle the roles of barium-apical oxygen hybridization, interlayer coupling, and orbital symmetries by applying an idealized strain, and also a strain with a fully relaxed structure. We show that shortening the distance between Cu layers is conducive to enhanced Fermi surface nesting, which increases spin fluctuations and drives up $T_c$. However, when the structure is fully relaxed, electrons flow to the $d_{z^2}$ orbital as a consequence of a shortened Ba-O bond, which is detrimental for superconductivity.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2015

Gutzwiller charge phase diagram of cuprates, including electron–phonon coupling effects

Besides significant electronic correlations, high-temperature superconductors also show a strong coupling of electrons to a number of lattice modes. Combined with the experimental detection of electronic inhomogeneities and ordering phenomena in many high-T c compounds, these features raise the question as to what extent phonons are involved in the associated instabilities. Here we address this problem based on the Hubbard model including a coupling to phonons in order to capture several salient features of the phase diagram of hole-doped cuprates. Charge degrees of freedom, which are suppressed by the large Hubbard U near half-filling, are found to become active at a fairly low doping level. We find that possible charge order is mainly driven by Fermi surface nesting, with competition between a near-(π, π) order at low doping and antinodal nesting at higher doping, very similar to the momentum structure of magnetic fluctuations. The resulting nesting vectors are generally consistent with photoemission and tunneling observations, evidence for charge density wave order in YBa₂Cu₃O 7-δ including Kohn anomalies, and suggestions of competition between one- and two-q-vector nesting.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2019

Suppression of hybridization by Cd doping in CeCoIn 5

The interaction between superconductivity and magnetism in heavy fermion compounds has remained a challenge ever since the discovery of unconventional superconductivity in such systems. The heavy fermion Ce T In 5 ( T = Co,Rh,Ir) family offers a good platform to study the relationship between superconductivity and magnetism, whose ground state can be easily tuned by chemical doping, making the ambient-pressure experimental techniques like angle-resolved photoemission spectroscopy possible to address this old but engaging question. By Cd doping, the superconducting state of CeCoIn 5 can be smoothly tuned to an antiferromagnetic ground state, which can be reversed by applying pressure. In this study, we present an electronic structure study of CeCo ( In 0.85 Cd 0.15 ) 5 with an antiferromagnetic ground state and the results are compared with CeCoIn 5 . We found that the hybridization strength between the f electrons and conduction electrons in CeCoIn 5 has been suppressed by Cd doping, but still the Fermi surface includes a significant contribution from the Ce f electrons. Our results find that the electronic tuning effect by Cd doping arises from the change in the hybridization strength between the Ce f electrons and conduction electrons caused by the Cd dopant, and the subtle interplay between itinerancy and localization of the f electrons determines the ground state.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2017

Hall number across a van Hove singularity

In this paper, in the context of the relaxation time approximation to Boltzmann transport theory, we examine the behavior of the Hall number n H of a metal in the neighborhood of a Lifshitz transition from a closed Fermi surface to open sheets. We find a universal nonanalytic dependence of n H on the electron density in the high-field limit, but a nonsingular dependence at low fields. Finally, the existence of an assumed nematic transition produces a doping dependent n H similar to that observed in recent experiments in the high-temperature superconductor YBa 2 Cu 3 O 7-x .

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2019

Quantum oscillations and a nontrivial Berry phase in the noncentrosymmetric topological superconductor candidate BiPd

Here, we report the measurements of de Haas-van Alphen (dHvA) oscillations in the noncentrosymmetric superconductor BiPd. Several pieces of a complex multisheet Fermi surface are identified, including a small pocket (α ~ 40 T) which is three dimensional and anisotropic. From the temperature dependence of the amplitude of the oscillations, the cyclotron effective mass is (0.18 ± 0.02) m e . Further analysis revealed a nontrivial π-Berry phase is associated with the 40 T pocket, which strongly supports the presence of topological states in bulk BiPd.

Quantum oscillation techniques↗
DOE OSTI2019

Dirac fermions and possible weak antilocalization in LaCuSb 2

Layered heavy-metal square-lattice compounds have recently emerged as potential Dirac fermion materials due to bonding within those sublattices. We report quantum transport and spectroscopic data on the layered Sb square-lattice material LaCuSb 2 . Linearly dispersing band crossings, necessary to generate Dirac fermions, are experimentally observed in the electronic band structure observed using angle-resolved photoemission spectroscopy, along with a quasi-two-dimensional Fermi surface. Weak antilocalization that arises from two-dimensional transport is observed in the magnetoresistance, as well as regions of linear dependence, both of which are indicative of topologically nontrivial effects. Measurements of the Shubnikov–de Haas quantum oscillations show low effective mass electrons on the order of 0.065m e , further confirming the presence of Dirac fermions in this material.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2021

Interorbital nematicity and the origin of a single electron Fermi pocket in FeSe

The electronic structure of the enigmatic iron-based superconductor FeSe has puzzled researchers since spectroscopic probes failed to observe the expected electron pocket at the Y point in the 1-Fe Brillouin zone. It has been speculated that this pocket, essential for an understanding of the superconducting state, is either absent or incoherent. In this study, we perform a theoretical study of the preferred nematic order originating from nearest-neighbor Coulomb interactions in an electronic model relevant for FeSe. We find that at low temperatures the dominating nematic components are of interorbital d x z – d x y and d y z – d x y character, with spontaneously broken amplitudes for these two components. This interorbital nematic order naturally leads to distinct hybridization gaps at the X and Y points of the 1-Fe Brillouin zone, and may thereby produce highly anisotropic Fermi surfaces with only a single electron pocket at one of these momentum-space locations. The associated superconducting gap structure obtained with the generated low-energy electronic band structure from spin-fluctuation mediated pairing agrees well with that measured experimentally. Finally, from a comparison of the computed spin susceptibility to available neutron scattering data, we discuss the necessity of additional self-energy effects, and explore the role of orbital-dependent quasiparticle weights as a minimal means to include them.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2022

Complex structure due to As bonding and interplay with electronic structure in superconducting BaNi 2 As 2

BaNi 2 As 2 is a superconductor chemically related to the Fe-based superconductors, with a complex and poorly understood structural phase transition. We show, based on first-principles calculations, that, in fact, there are two distinct competing structures. These structures are different from the electronic, transport, and bonding points of view but are close in energy. These arise due to complex As bonding patterns and drive distortions of the Ni layers. This is supported by photoemission experiments. This is very distinct from views of the distortion as being primarily driven by electrons at the Fermi surface as in a classical charge density wave, or by correlated electron physics associated with the Ni d electrons and the Ni d orbitals. The structural distortion, although connected with As does lead to an interplay of electronic and structural behavior including induced anisotropic electronic transport. Here, the local bonding nature of the instabilities and the competition between distortions is discussed in the context of the complex behavior observed in BaNi 2 As 2 samples.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2018

Angle-dependent magnetoresistance as a sensitive probe of the charge density wave in quasi-one-dimensional semimetal Ta 2 NiSe 7

The behavior of charge density waves (CDWs) in an external magnetic field is dictated by both orbital and Pauli (Zeeman) effects. A quasi-one-dimensional (Q1D) system features Q1D Fermi surfaces that allow these effects to be distinguished, which in turn can provide a sensitive probe to the underlying electronic states. Furthermore, we studied the field dependence of an incommensurate CDW in a transition-metal chalcogenide Ta 2 NiSe 7 with a Q1D chain structure. The angle-dependent magnetoresistance (MR) is found to be very sensitive to the relative orientation between the magnetic field and the chain direction. With an applied current fixed along the b axis (the chain direction), the angle-dependent MR shows a striking change of the symmetry below T CDW only for a rotating magnetic field in the ac plane. In contrast, the symmetry axis remains unchanged for other configurations (H in ab and bc planes). The orbital effect conforms to the lattice symmetry, while the Pauli effect in the form of μ B B/ℏv F can be responsible for such symmetry change, provided that the Fermi velocity v F is significantly anisotropic and the nesting vector changes in a magnetic field, which is corroborated by our first-principles calculations. Our results show that the angle-dependent MR is a sensitive transport probe of CDW and can be useful for the study of low-dimensional systems in general.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2017

Tunability of the topological nodal-line semimetal phase in ZrSi X -type materials ( X = S , Se , Te )

The discovery of a topological nodal-line (TNL) semimetal phase in ZrSiS has invigorated the study of other members of this family. In this paper, we present a comparative electronic structure study of $\mathrm{ZrSi}X$ (where $X=\text{S}$, Se, Te) using angle-resolved photoemission spectroscopy (ARPES) and first-principles calculations. Our ARPES studies show that the overall electronic structure of $\mathrm{ZrSi}X$ materials comprises the diamond-shaped Fermi pocket, the nearly elliptical-shaped Fermi pocket, and a small electron pocket encircling the zone center ($\mathrm{{\Gamma}}$) point, the $M$ point, and the $X$ point of the Brillouin zone, respectively. We also observe a small Fermi surface pocket along the $M{-}\mathrm{{\Gamma}}{-}M$ direction in ZrSiTe, which is absent in both ZrSiS and ZrSiSe. Furthermore, our theoretical studies show a transition from nodal-line to nodeless gapped phase by tuning the chalcogenide from S to Te in these material systems. Finally, our findings provide direct evidence for the tunability of the TNL phase in $\mathrm{ZrSi}X$ material systems by adjusting the spin-orbit coupling strength via the $X$ anion.

36 MATERIALS SCIENCE↗
DOE OSTI2023

Band structure, superconductivity, and polytypism in AuSn 4

The orthorhombic compound AuSn 4 is compositionally similar to the Dirac node arc semimetal PtSn 4 . AuSn 4 is, contrary to PtSn 4 , superconducting with a critical temperature of T c = 2.35 K. Recent measurements present indications for quasi-two-dimensional superconducting behavior in AuSn 4 . Here we present measurements of the superconducting density of states and the band structure of AuSn 4 through scanning tunneling microscopy and angular resolved photoemission spectroscopy (ARPES). The superconducting gap values in different portions of the Fermi surface are spread around Δ 0 = 0.4 meV, which is close to but somewhat larger than Δ = 1.76k B T c expected from BCS theory. We observe superconducting features in the tunneling conductance at the surface up to temperatures about 20% larger than bulk T c . The band structure calculated with density functional theory follows well the results of ARPES. The crystal structure presents two possible stackings of Sn layers, giving two nearly degenerate polytypes. This makes AuSn 4 a rather unique case with a three-dimensional electronic band structure but properties ressembling those of low-dimensional layered compounds.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗