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Fradkin, Eduardo

Publications and source records attributed to Fradkin, Eduardo.

17 records
All sourcesNASA NTRSDOE OSTI
DOE OSTI2024

Absence of a bulk signature of a charge density wave in hard x-ray measurements of UTe 2

The long-sought pair density wave (PDW) is an exotic phase of matter in which charge density wave (CDW) order is intertwined with the amplitude or phase of coexisting, superconducting order. Originally predicted to exist in copper-oxides, circumstantial evidence for PDW order now exists in a variety of materials. Recently, scanning tunneling microscopy (STM) studies have reported evidence for a three-component CDW at the surface of the heavy-fermion superconductor, UTe 2 , persisting below its superconducting transition temperature [Aishwarya et al., Nature 618, 928933 (2023); Gu et al., Nature 618, 921927 (2023); LaFleur et al., Nat. Commun. 15, 4456 (2024)]. Here, in this study, we use hard x-ray diffraction measurements on crystals of UTe 2 at 𝑇 = 1.9 and 12 K to search for a bulk signature of this CDW. Using STM measurements as a constraint, we calculate the expected locations of CDW superlattice peaks and sweep a large volume of reciprocal space in search of a signature. We fail to find any evidence for a CDW near any of the expected superlattice positions in many Brillouin zones. We estimate an upper bound on the CDW lattice distortion of 𝑢 max ≲ 4×10 −3 Å. Our results suggest that the CDW observed in STM is either purely electronic, somehow lacking a signature in the lattice, or is restricted to the material surface.

Kengle, Caitlin S.↗
DOE OSTI2022

Generic character of charge and spin density waves in superconducting cuprates

Charge density waves (CDWs) have been observed in nearly all families of copper-oxide superconductors. But the behavior of these phases across different families has been perplexing. In La-based cuprates, the CDW wavevector is an increasing function of doping, exhibiting the so-called Yamada behavior, while in Y- and Bi-based materials the behavior is the opposite. Here, we report a combined resonant soft X-ray scattering (RSXS) and neutron scattering study of charge and spin density waves in isotopically enriched La 1.8–x Eu 0.2 Sr x CuO 4 over a range of doping 0.07 ≤ x ≤ 0.20. In this work, we find that the CDW amplitude is temperature independent and develops well above experimentally accessible temperatures. Further, the CDW wavevector shows a nonmonotonic temperature dependence, exhibiting Yamada behavior at low temperature with a sudden change occurring near the spin ordering temperature. We describe these observations using a Landau–Ginzburg theory for an incommensurate CDW in a metallic system with a finite charge compressibility and spin-CDW coupling. Extrapolating to high temperature, where the CDW amplitude is small and spin order is absent, our analysis predicts a decreasing wavevector with doping, similar to Y and Bi cuprates. Our study suggests that CDW order in all families of cuprates forms by a common mechanism.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2021

Multiple Charge Density Waves and Superconductivity Nucleation at Antiphase Domain Walls in the Nematic Pnictide Ba 1– x Sr x Ni 2 As 2

How superconductivity interacts with charge or nematic order is one of the great unresolved issues at the center of research in quantum materials. Ba 1– x Sr x Ni 2 As 2 (BSNA) is a charge ordered pnictide superconductor recently shown to exhibit a sixfold enhancement of superconductivity due to nematic fluctuations near a quantum phase transition (at $x_c$ = 0.7). The superconductivity is, however, anomalous, with the resistive transition for $0.4 < x < x_c$ occurring at a higher temperature than the specific heat anomaly. Here, using x-ray scattering, we discovered a new charge density wave (CDW) in BSNA in this composition range. The CDWis commensurate with a period of two lattice parameters, and is distinct from the two CDWs previously reported in this material. We argue that the anomalous transport behavior arises from heterogeneous superconductivity nucleating at antiphase domain walls in this CDW. We also present new data on the incommensurate CDW, previously identified as being unidirectional, showing that it is a rotationally symmetric “4$\textit{Q}$” state with $C_4$ symmetry. Our study establishes BSNA as a rare material containing three distinct CDWs, and an exciting test bed for studying coupling between CDW, nematic, and SC orders.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2020

The Physics of Pair-Density Waves: Cuprate Superconductors and Beyond

Herein we review the physics of pair-density wave (PDW) superconductors. We begin with a macroscopic description that emphasizes order induced by PDW states, such as charge-density wave, and discuss related vestigial states that emerge as a consequence of partial melting of the PDW order. We review and critically discuss the mounting experimental evidence for such PDW order in the cuprate superconductors, the status of the theoretical microscopic description of such order, and the current debate on whether the PDW is a mother order or another competing order in the cuprates. In addition, we give an overview of the weak coupling version of PDW order, Fulde–Ferrell–Larkin–Ovchinnikov states, in the context of cold atom systems, unconventional superconductors, and noncentrosymmetric and Weyl materials.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2018

Boson–fermion duality in a gravitational background

We study the 2+1 dimensional boson–fermion duality in the presence of background curvature and electromagnetic fields. The main players are, on the one hand, a massive complex |&straightphi;|{sup 4} scalar field coupled to a U(1) Maxwell–Chern–Simons gauge field at level 1, representing a relativistic composite boson with one unit of attached flux, and on the other hand, a massive Dirac fermion. We show that, in a curved background and at the level of the partition function, the relativistic composite boson, in the infinite coupling limit, is dual to a short-range interacting Dirac fermion. The coupling to the gravitational spin connection arises naturally from the spin factors of the Wilson loop in the Chern–Simons theory. A non-minimal coupling to the scalar curvature is included on the bosonic side in order to obtain agreement between partition functions. Although an explicit Lagrangian expression for the fermionic interactions is not obtained, their short-range nature constrains them to be irrelevant, which protects the duality in its strong interpretation as an exact mapping at the IR fixed point between a Wilson–Fisher–Chern–Simons complex scalar and a free Dirac fermion. We also show that, even away from the IR, keeping the |&straightphi;|{sup 4} term is of key importance as it provides the short-range bosonic interactions necessary to prevent intersections of worldlines in the path integral, thus forbidding unknotting of knots and ensuring preservation of the worldline topologies.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2018

Establishing the consequences of intertwined order parameters in spatially modulated superconductors. Final Report

This the Final Report for the grant DE-SC0012368. The purpose of this project was to establish the generality and full implications of phasesensitive coupling between charge and superconducting order parameters in materials exhibiting charge inhomogeneity. Specifically, our targeted goal was to first confirm experimentally a number of unique phenomena predicted by Berg et al for the cuprate material La 2-x Ba x CuO 4 (LBCO), commonly believed to contain such coupling and then extend the work to include several other superconducting families. More generally, our expanded research program sought to identify material families with co-existing charge and superconducting orders, and to understand the nature of the coupling between the order parameters. In the long term, we hope these are the first steps in identifying symmetry-based unifying principles, which might provide the foundation for a universal phenomenological theory, appropriate to describe to multiple families containing unconventional superconductivity.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2018

Pair density waves in superconducting vortex halos

We analyze the interplay between a d -wave uniform superconducting and a pair-density-wave (PDW) order parameter in the neighborhood of a vortex. We develop a phenomenological nonlinear sigma model, solve the saddle-point equation for the order-parameter configuration, and compute the resulting local density of states in the vortex halo. The intertwining of the two superconducting orders leads to a charge density modulation with the same periodicity as the PDW, which is twice the period of the charge density wave that arises as a second harmonic of the PDW itself. Lastly, we discuss key features of the charge density modulation that can be directly compared with recent results from scanning tunneling microscopy and speculate on the role PDW order may play in the global phase diagram of the hole-doped cuprates.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2017

Measurement of the dynamic charge response of materials using low-energy, momentum-resolved electron energy-loss spectroscopy (M-EELS)

One of the most fundamental properties of an interacting electron system is its frequency- and wave-vector-dependent density response function, \chi({\bf q},\omega) . The imaginary part, \chi''({\bf q},\omega) , defines the fundamental bosonic charge excitations of the system, exhibiting peaks wherever collective modes are present. \chi χ quantifies the electronic compressibility of a material, its response to external fields, its ability to screen charge, and its tendency to form charge density waves. Unfortunately, there has never been a fully momentum-resolved means to measure \chi({\bf q},\omega) at the meV energy scale relevant to modern electronic materials. Here, we demonstrate a way to measure \chi χ with quantitative momentum resolution by applying alignment techniques from x-ray and neutron scattering to surface high-resolution electron energy-loss spectroscopy (HR-EELS). This approach, which we refer to here as “M-EELS”, allows direct measurement of \chi''({\bf q},\omega) with meV resolution while controlling the momentum with an accuracy better than a percent of a typical Brillouin zone. We apply this technique to finite-q excitations in the optimally-doped high temperature superconductor, Bi _2 2 Sr _2 2 CaCu _2 2 O _{8+x} 8 + x (Bi2212), which exhibits several phonons potentially relevant to dispersion anomalies observed in ARPES and STM experiments. Our study defines a path to studying the long-sought collective charge modes in quantum materials at the meV scale and with full momentum control.

36 MATERIALS SCIENCE↗
DOE OSTI2017

Vestigial nematicity from spin and/or charge order in the cuprates

Nematic order has manifested itself in a variety of materials in the cuprate family. We propose an effective field theory of a layered system with incommensurate, intertwined spin- and charge-density wave (SDW and CDW) orders, each of which consists of two components related by C4 rotations. Using a variational method (which is exact in a large N limit), we study the development of nematicity from partially melting those density waves by either increasing temperature or adding quenched disorder. As temperature decreases we first find a transition to a nematic phase, but depending on the range of parameters (e.g. doping concentration) the strongest fluctuations associated with this phase reflect either proximate SDW or CDW order. We also discuss the changes in parameters that can account for the differences in the SDW-CDW interplay between the (214) family and the other hole-doped cuprates.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2017

Influence of Domain Walls in the Incommensurate Charge Density Wave State of Cu Intercalated 1 T –TiSe 2

We report a low-temperature scanning tunneling microscopy study of the charge density wave (CDW) order in 1T–TiSe 2 and Cu 0.08 TiSe 2 . In pristine 1T–TiSe 2 we observe a long-range coherent commensurate CDW (CCDW) order. In contrast, Cu 0.08 TiSe 2 displays an incommensurate CDW (ICDW) phase with localized CCDW domains separated by domain walls. Furthermore, density of states measurements indicate that the domain walls host an extra population of fermions near the Fermi level which may play a role in the emergence of superconductivity in this system. Fourier transform scanning tunneling spectroscopy studies suggest that the dominant mechanism for CDW formation in the ICDW phase may be electron-phonon coupling.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2016

Numerical evidence for a chiral spin liquid in the XXZ antiferromagnetic Heisenberg model on the kagome lattice at m = 2 3 magnetization

In this work, we perform an exact-diagonalization study of the spin- 1 2 XXZ Heisenberg antiferromagnet on the kagome lattice at finite magnetization m = 2 3 with an emphasis on the X Y point ( J z = 0 ) and in the presence of a small chiral term. Recent analytic work by Kumar et al. [K. Kumar, K. Sun, and E. Fradkin, Phys. Rev. B 90 , 174409 (2014) ] on the same model, using a newly developed flux attachment transformation, predicts a plateau at this value of the magnetization described by a chiral spin liquid (CSL) with a spin Hall conductance of σ x y = 1 2 . Such a state is topological in nature, has a ground-state degeneracy, and exhibits fractional excitations. We analyze the degeneracy structure in the low-energy manifold, identify the candidate topological states, and use them to compute the modular matrices and Chern numbers, all of which strongly agree with expected theoretical behavior for the σ x y = 1 2 CSL. In the limit of zero chirality, we find on most (not all) clusters that the topological invariants are still those of a CSL.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2016

Reexamination of the effective fine structure constant of graphene as measured in graphite

Here we present a refined and improved study of the influence of screening on the effective fine structure constant of graphene, α*, as measured in graphite using inelastic x-ray scattering. This followup to our previous study [J. P. Reed et al., Science 330, 805 (2010)] was carried out with two times better energy resolution, five times better momentum resolution, and an improved experimental setup with lower background. We compare our results to random-phase approximation (RPA) calculations and evaluate the relative importance of interlayer hopping, excitonic corrections, and screening from high energy excitations involving the sigma bands. We find that the static, limiting value of α* falls in the range 0.25-0.35, which is higher than our previous result of 0.14, but still below the value expected from RPA. We show the reduced value is not a consequence of interlayer hopping effects, which were ignored in our previous analysis, but of a combination of excitonic effects in the π→ π* particle-hole continuum, and background screening from the σ-bonded electrons. We find that σ-band screening is extremely strong at distances of less than a few nanometers, and should be highly effective at screening out short-distance, Hubbard-like interactions in graphene as well as other carbon allotropes.

36 MATERIALS SCIENCE↗
DOE OSTI2016

Basic Research Needs Workshop on Quantum Materials for Energy Relevant Technology

Imagine future computers that can perform calculations a million times faster than today’s most powerful supercomputers at only a tiny fraction of the energy cost. Imagine power being generated, stored, and then transported across the national grid with nearly no loss. Imagine ultrasensitive sensors that keep us in the loop on what is happening at home or work, warn us when something is going wrong around us, keep us safe from pathogens, and provide unprecedented control of manufacturing and chemical processes. And imagine smart windows, smart clothes, smart buildings, supersmart personal electronics, and many other items — all made from materials that can change their properties “on demand” to carry out the functions we want. The key to attaining these technological possibilities in the 21st century is a new class of materials largely unknown to the general public at this time but destined to become as familiar as silicon. Welcome to the world of quantum materials — materials in which the extraordinary effects of quantum mechanics give rise to exotic and often incredible properties. To realize the tantalizing potential of quantum materials, there is much basic scientific research to be done. Recognizing the high potential impact of quantum materials, nations around the world are already investing in this effort. We must learn how the astonishing properties of quantum materials can be tailored to address our most pressing technological needs, and we must dramatically improve our ability to synthesize, characterize, and control quantum materials. To accelerate the progress of quantum materials research, the U.S. Department of Energy’s Office of Science, Office of Basic Energy Sciences (BES), sponsored a “Basic Research Needs Workshop on Quantum Materials for Energy-relevant Technology,” which was held near Washington, D.C. on February 8–10, 2016. Attended by more than 100 leading national and international scientific experts in the synthesis, characterization, and theory of quantum materials, the workshop identified four priority research directions (PRDs) that will lay the foundation to better understand quantum materials and harness their rich technological potential.

36 MATERIALS SCIENCE↗