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At least 127 records · Page 7
All sourcesNASA NTRSDOE OSTI
DOE OSTI2025

Renormalization-group approach to Kohn-Luttinger superconductivity: Amplification of the pairing gap from ℓ 4 to ℓ

Here, we revisit the renormalization group (RG) analysis of the Kohn-Luttinger (KL) mechanism for superconductivity. The KL mechanism leads to superconductivity in a system with a repulsive bare interaction. The key ingredient is the screening effect that renders the induced interaction attractive in channels with nonzero angular momentum ℓ ≠ 0, thereby triggering the Bardeen-Cooper-Schrieffer (BCS) instability. According to the original argument, the resulting gap is exponentially small, with its exponent scaling as −ℓ 4 . However, the KL mechanism was originally formulated within perturbation theory, where the series is known to converge poorly in certain cases—most notably, for the 𝑝-wave paring gap induced by a repulsive 𝑠-wave contact interaction. This poor convergence may be attributed to a divergent integrand in a specific class of diagrams containing both the BCS logarithm and the Kohn anomaly, suggesting that one must resum the Kohn anomaly contributions separately from the BCS logarithm. In this work, we incorporate the Kohn anomaly contribution into the 𝛽 function of the RG equation governing the BCS instability near the Fermi surface. Our solution shows that the KL gap exponent is then proportional to −ℓ, indicating a significant enhancement of the KL mechanism beyond the previously known result. To illustrate this, we study the spin-triplet 𝑝-wave pairing gap arising from a repulsive 𝑠-wave contact interaction and compare our RG-based results with those obtained from the Bethe-Salpeter equation in perturbation theory.

nuclear matter in neutron stars↗
DOE OSTI2025

QCD moat regime and its real-time properties

Dense quantum chromodynamics (QCD) matter may exhibit crystalline phases. Their existence is reflected in a moat regime, where mesonic correlations feature spatial modulations. We study the real-time properties of pions at finite temperature and density in QCD in order to elucidate the nature of this regime. We show that the moat regime arises from particle-hole-like fluctuations near the Fermi surface. This gives rise to a characteristic peak in the spectral function of the pion at nonzero spacelike momentum. This peak can be interpreted as a new quasi particle, the moaton. In addition, our framework also allows us to directly test the stability of the homogeneous chiral phase against the formation of an inhomogeneous condensate in QCD. We find that an inhomogeneous instability is highly unlikely for baryon chemical potentials μ B ≤ 630 MeV . Published by the American Physical Society 2025

73 NUCLEAR PHYSICS AND RADIATION PHYSICS↗
DOE OSTI2024

Quasi-Two-Dimensional Antiferromagnetic Spin Fluctuations in the Spin-Triplet Superconductor Candidate CeRh 2 As 2

The tetragonal heavy-fermion superconductor CeRh 2 ⁢As 2 (T c = 0.3 K) exhibits an exceptionally high critical field of 14 T for B ∥ c. It undergoes a field-driven first-order phase transition between superconducting states, potentially transitioning from spin-singlet to spin-triplet superconductivity. To further understand these superconducting states and the role of magnetism, we probe spin fluctuations in CeRh 2 ⁢As 2 using neutron scattering. Here, we find dynamic (π, π) antiferromagnetic (AFM) spin correlations with an anisotropic quasi-two-dimensional correlation volume. Our data place an upper limit of 0.31 μ B on the staggered magnetization of corresponding Néel orders at T = 0.08 K. Density functional theory calculations, treating Ce 4⁢ƒ electrons as core states, show that the AFM wave vector connects significant areas of the Fermi surface. Our findings indicate that the dominant excitations in CeRh 2 ⁢As 2 for ℏω⁢ < 1.2 meV are magnetic and suggest that superconductivity in CeRh 2⁢ As 2 is mediated by AFM spin fluctuations associated with a proximate quantum critical point.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2025

Physical properties of intergrowth compound Eu 2 CuZn 2 P 3

Here, the physical properties of single-crystalline Eu 2 ⁢CuZn 2 ⁢P 3 were inspected via thermodynamic, transport, and neutron diffraction measurements. Eu 2 ⁢ CuZn 2⁢ P 3 is composed of EuCuP and EuZn 2 ⁢ P 2 motifs stacked in a one-to-one ratio. The magnetic response of Eu 2 ⁢ CuZn 2 ⁢ P 3 is similar to that of EuZn 2 ⁢ P 2 in its A-type antiferromagnetic (semiconducting) state. A Néel temperature of T N = 40.3K is obtained from the specific heat capacity. Single-crystal neutron diffraction reveals a (001) magnetic propagation vector, with primarily in-plane moments coupled along [001] in an up-up-down-down pattern. This spin structure retains the ferromagnetic EuCuP motifs and antiferromagnetic coupling across the EuZn 2 ⁢P 2 motifs. The electrical resistivity is characterized by metallic behavior above 100 K, an increase in ρ upon cooling below ≈ 75K, and a large negative magnetoresistance below T N . First principles calculations evidence a narrow band gap for antiferromagnetic ordering and semimetallic behavior for the ferromagnetic state. This is consistent with the observed effect of magnetic field on the electrical resistivity. Hall effect measurements reveal a field-induced, negative anomalous Hall effect that increases on cooling down to 2 K. Also, an unconventional contribution to the Hall effect dominates the low-field Hall signal, particularly near T N , and is similar in appearance to a topological Hall effect. However, further work is necessary to understand if any of the transport properties have topological origins or if magnetism-induced modifications to the Fermi surface induce these unusual Hall effect signals in this and related materials.

36 MATERIALS SCIENCE↗
DOE OSTI2024

Intertwined charge and spin density waves in a topological kagome material

Using neutrons and x rays we show the topological kagome antiferromagnet Mn 3 Sn for T < 285 K forms a homogeneous spin and charge ordered state comprising a longitudinally polarized spin density wave with wave vector k β = k β c ̂ , a helical modulated version of the room temperature antichiral magnetic order with k χ = k χ c ̂ , and charge density waves with wave vectors 2 k β , 2 k χ , and k β + k χ . Though k χ and k β coincide for 200 K < T < 230 K , they exhibit distinct continuous T dependencies before locking to commensurate values of k β = 1 12 c * and k χ = 5 48 c * at low T . Density functional theory indicates this complex modulated state may be associated with the nesting of Fermi surfaces from correlated flat kagome bands, which host Weyl nodes that are annihilated as it forms. Published by the American Physical Society 2024

Chen, Y. (ORCID:0000000177169179)↗
DOE OSTI2025

Anisotropic field-induced changes in the superconducting order parameter of UTe 2

UTe 2 is a newly discovered unconventional superconductor, where electron Cooper pairs combine into a spin-triplet ground state. We study the specific heat C( H,T ) of a high-quality UTe 2 single crystal, with a single anomaly at a high superconducting transition temperature T c = 2.1 K and a low residual Sommerfeld coefficient γ 0 . Applying magnetic fields up to 12 T along the crystallographic axes a, b , and c , we observe highly anisotropic behavior in γ 0 ( H ). Below 4 T, γ 0 ( H ) ≈ γ 0 + λi √H (i = a, b, c) , which is expected for an unconventional superconductor with nodes on the Fermi surface away from the direction of the magnetic field. At 4 T ( a and b axes) and 5.5 T ( c axis), distinctive changes in the behavior of γ 0 (H) suggest that a stable low-field ground state begins to evolve above 4 T. The data analysis indicates that the nodes are located near the a–b plane, with possible order parameters d B2u + iεd B1u or d B2u + iεd Au . Our findings support B 2u as the primary superconducting order parameter in UTe 2 .

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2025

Thermodynamic Evidence of Fermionic Behavior in the Vicinity of One-Ninth Plateau in a Kagome Antiferromagnet

The spin-1/2 kagome Heisenberg antiferromagnets are believed to host exotic quantum entangled states. Recently, the reports of 1/9 magnetization plateau and magnetic oscillations in a kagome antiferromagnet YCu 3 ⁢(OH) 6 ⁢Br 2 ⁢[Br 𝑥 ⁢(OH) 1−𝑥 ] (YCOB) have made this material a promising candidate for experimentally realizing quantum spin liquid states. Here, we present measurements of the specific heat 𝐶 𝑝 in YCOB in high magnetic fields (up to 41.5 T) down to 0.46 K, and the 1/9 plateau feature has been confirmed. Moreover, the temperature dependence of 𝐶 𝑝 /𝑇 in the vicinity of 1/9 plateau region can be fitted by a linear in 𝑇 term which indicates the presence of a Dirac spectrum, together with a constant term, which indicates a finite density of states contributed by other spinon Fermi surfaces. Surprisingly, the constant term is highly anisotropic in the direction of the magnetic field. Additionally, we observe a double-peak feature near 30 T above the 1/9 plateau which is another hallmark of fermionic excitations in the specific heat. This combination of gapless behavior and the double-peak structure strongly suggests that the 1/9 plateau in YCOB is nontrivial and hosts fermionic quasiparticles.

36 MATERIALS SCIENCE↗
DOE OSTI2025

Revealing the electronic structure of van der Waals antiferromagnetic NiPS 3 through synchrotron-based 𝜇-ARPES and alkali metal dosing

Antiferromagnetic NiPS 3 has recently emerged as a quantum material of considerable interest, thanks to the discovery of multiple new couplings involving electrons, spins, orbitals, phonons, and magnons. However, controversies and open questions persist concerning the fundamental origins of these couplings. A critical piece of information required to advance the understanding is the precise electronic band structure of NiPS 3 . Angle-resolved photoemission spectroscopy (ARPES), combined with alkali metal dosing (AMD), can enable us to directly observe the subtle electronic states that appear around the Fermi surface, offering valuable insights into the intriguing quantum properties and interplays of the examined material. Here, in this study, we present a comprehensive characterization and analysis of the band structure of van der Waals layered antiferromagnet NiPS 3 , leveraging state-of-the-art μ-ARPES measurements supported by density functional theory (DFT) calculations. Theoretical DFT results identify the orbital contributions to the observed bands, providing a precise understanding of the experimental ARPES data. Crucially, AMD enables the observation of conduction band and defect-related states above the valence band maximum in NiPS 3 . Furthermore, temperature dependent ARPES results across the Néel transition temperature of NiPS 3 reveal that the paramagnetic and antiferromagnetic phases have nearly identical band structures, underlining the highly localized character of Ni d states. These findings substantially deepen our understanding of the electronic properties of NiPS 3 and lay a vital foundation for exploring the intriguing quantum phenomena it exhibits.

Cao, Yifeng [Boston Univ., MA (United States); Law↗
DOE OSTI2026

Ultrafast Raman probe of the photoinduced superconducting to normal state transition in the cuprate Bi 2 ⁢Sr 2 ⁢CaCu 2 ⁢O 8+𝛿

Here, we report an ultrafast Time-Resolved Raman scattering study of the out-of-equilibrium photoinduced dynamics across the superconducting to normal state phase transition of the cuprate Bi 2 ⁢Sr 2⁢ CaCu 2 ⁢O 8+𝛿 . Using the polarization-resolved momentum space selectivity of Raman scattering, we track the superconducting condensate destruction and recovery dynamics with subpicoseconds time resolution in the antinodal region of the Fermi surface where the superconducting gap is maximum. Leveraging ultrafast Raman thermometry, we find a significant dichotomy between the superconducting condensate and the quasiparticle temperature dynamics near the antinodes, which cannot be framed in terms of a single effective electron temperature. The present work demonstrates the ability of Time-Resolved Raman scattering to selectively probe out-of-equilibrium pathways of different electronic subdegrees of freedom during a photoinduced phase transition.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2026

Beta-Delayed Neutron Emission of 𝑁=84 132Cd

Using the time-of-flight technique, we measured the beta-delayed neutron emission of 132 Cd . From our large-scale shell model (LSSM) calculation using the N3⁢LO interaction [, Phys. Rev. Lett. 131, 022501 (2023)], we suggest the decay is dominated by the transformation of a neutron in the 𝑔7/2 orbital, deep below the Fermi surface, into a proton in the 𝑔9/2 orbital. We compare the beta-decay half-lives and neutron branching ratios of nuclei with 𝑍<50 and 𝑁 ≥82 obtained with our LSSM with those of leading “global” models such as finite-range droplet model (FRDM). Our calculations match known half-lives and neutron branching ratios well and suggest that current leading models overestimate the yet-to-be-measured half-lives. Our model, backed by the 132 Cd decay data presented here, offers robust predictive power for nuclei of astrophysical interest such as 𝑟-process waiting points.

Madurga, M [Department of Physics and Astronomy, U↗
DOE OSTI2025

Good plasmons in a bad metal

Correlated metals may exhibit unusually high resistivity that increases linearly in temperature, breaking through the Mott-Ioffe-Regel bound, above which coherent quasiparticles are destroyed. The fate of collective charge excitations, or plasmons, in these systems is a subject of debate. Several studies have suggested that plasmons are overdamped, whereas other studies have detected propagating plasmons. Here, in this work, we present direct nano-optical images of low-loss hyperbolic plasmon polaritons (HPPs) in the correlated van der Waals metal MoOCl 2 . HPPs are plasmon-photon modes that waveguide through extremely anisotropic media and are remarkably long-lived in MoOCl 2 . Photoemission data presented here reveal a highly anisotropic Fermi surface, reconstructed and made partly incoherent, likely through electronic interactions as explained by many-body theory. HPPs remain long-lived despite this, revealing previously unseen imprints of many-body effects on plasmonic collective modes.

36 MATERIALS SCIENCE↗
DOE OSTI2024

Emergence of two distinct phase transitions in monolayer CoSe2 on graphene

Abstract Dimensional modifications play a crucial role in various applications, especially in the context of device miniaturization, giving rise to novel quantum phenomena. The many-body dynamics induced by dimensional modifications, including electron-electron, electron-phonon, electron-magnon and electron-plasmon coupling, are known to significantly affect the atomic and electronic properties of the materials. By reducing the dimensionality of orthorhombic CoSe 2 and forming heterostructure with bilayer graphene using molecular beam epitaxy, we unveil the emergence of two types of phase transitions through angle-resolved photoemission spectroscopy and scanning tunneling microscopy measurements. We disclose that the 2 × 1 superstructure is associated with charge density wave induced by Fermi surface nesting, characterized by a transition temperature of 340 K. Additionally, another phase transition at temperature of 160 K based on temperature dependent gap evolution are observed with renormalized electronic structure induced by electron-boson coupling. These discoveries of the electronic and atomic modifications, influenced by electron-electron and electron-boson interactions, underscore that many-body physics play significant roles in understanding low-dimensional properties of non-van der Waals Co-chalcogenides and related heterostructures. Graphical Abstract

36 MATERIALS SCIENCE↗
DOE OSTI2024

Hole doping and electronic correlations in Cr substituted BaFe$_{2}$As$_{2}$

For a significant composition range, the suppression of the spin density wave transition temperature (T SDW ) in Cr- and Mn-substituted BaFe 2 As 2 (CrBFA and MnBFA, respectively) coincides as a function of Cr/Mn content, despite the distinct electronic effects of these substitutions. Additionally, for any Cr/Mn content superconductivity (SC) is absent and this topic is particularly less explored in the case of CrBFA. Here, in this work, we employ angle-resolved photoemission spectroscopy (ARPES) and combined density functional theory plus dynamical mean field theory (DFT+DMFT) to address the evolution of the Fermi surface (FS) and electronic correlations in CrBFA. Our findings reveal that incorporating Cr leads to an effective hole doping of the states near the FS, which is well described within the virtual crystal approximation (VCA). Moreover, analysis of the ARPES spectra of the bands with main d yz -orbital character reveals a fractional scaling of the imaginary part of self-energy as a function of the binding energy, a signature property of Hund's correlations. Our DFT+DMFT calculations support these experimental findings. We conclude that CrBFA is a correlated electron system for which the changes in the FS as a function of Cr are unrelated to the suppression of T SDW . In addition, we suggest that the absence of SC is primarily due to the competition between Cr local moments and the Fe-derived itinerant spin fluctuations.

46 INSTRUMENTATION RELATED TO NUCLEAR SCIENCE AND ↗
DOE OSTI2025

Decay spectroscopy of an isomeric state in 251 Md

Superheavy elements ( Z ≥ 104) owe their existence to quantum shell effects which stabilize them against fission. Experimental studies of their structure are challenging, and only recently has direct information begun to emerge. Meanwhile, the comparative ease of producing the transfermium nuclei offers an alternative approach. These nuclei are well-deformed ( β 2 ≈ 0.28) owing to the existence of deformed gaps in the single particle spectrum near Z = 100 and N = 152. Because of this, the proton orbitals near the Fermi surface in this region are expected to originate from the spherical sub shells which define the shell gaps near Z = 114. In this sense, the transfermium nuclei act as a testing ground for theoretical models used to describe the superheavy region. Deformed odd-mass nuclei are of particular interest since they can provide direct insight into the single particle structure.

73 NUCLEAR PHYSICS AND RADIATION PHYSICS↗
DOE OSTI2025

Diagrammatic Monte Carlo Approach to Real‐frequency Response Functions and the Spin‐Fermion Model of Hot Spots (Final Report)

This three‐year research project was focused on two outstanding condensed matter problems (i) Real‐frequency response functions in Coulomb systems at finite temperature, and Fermi surface (FS) reconstruction in the Spin-Fermion (SF) model. The uniform electron gas (jellium) model, describing electrons interacting via long‐range Coulomb forces on positive neutralizing background, is fundamentally important both for understanding the physics of correlated electrons and for formulation of the time‐dependent density functional theory (TDDFT). The spin-fermion model has found a wide range of applications in the physics of cuprates and iron‐based superconductors to explain the “strange metal" behavior and to suggest a possible pairing mechanism for high‐temperature superconductivity. The project’s goals were development and application of the Diagrammatic Monte Carlo (DiagMC) techniques for solving the above problems.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2026

Zentropy Theory for Transformative Functionalities of Magnetic and Superconducting Materials

The proposed research developed the zentropy theory through applications to complex magnetic materials and superconductors under the hypothesis that the emergent properties of complex magnetic materials and superconductors can be predicted by statistical mechanics of ergodic microstates with their partition functions computed from DFT-predicted free energies. The key objective is to develop approaches to systematically determine the types and number of microstates and the supercell size in DFT-based calculations through convergency of macroscopic functionalities, with the incorporation of our mixed-space approach accounting for the interactions between periodic supercells. In addition to use scientific intuitions to guide the design of important microstates, the key innovation of the proposed research is to integrate the domain knowledge and the material-property-descriptor database (MPDD) with 4 million microstates, which is supported by our deep neural network machine learning models (SIPFENN: structure-informed prediction of formation energy using neural networks) and integrated with our high throughput DFT Tool Kit (DFTTK). For complex magnetic materials, one of the objectives is to develop approaches to calculate short-range ordering from the statistical distribution of each microstate. For superconductors, the divergency of quasiparticle effective mass at a quantum critical point will be investigated, and the superconducting and non-superconducting microstates will be delineated through analysis of electronic band structure, density of states, charge density, and Fermi surface.

36 MATERIALS SCIENCE↗
DOE OSTI2025

Hund's coupling assisted orbital-selective superconductivity in Ba1-xKxFe2As2

While the superconducting transition temperature of hole-doped Ba_{1-x}K_{x}Fe_{2}As_{2} decreases past optimal doping, superconductivity does not completely disappear even for the fully doped KFe_{2}As_{2} compound. In fact, superconductivity is robust through a Lifshitz transition where electron bands become hole-like around the zone corner at around x=0.7, thus challenging the conventional understanding of superconductivity in iron-based systems. High-resolution angle-resolved photoemission spectroscopy is used to investigate the superconducting gap structure, as well as the normal state electronic structure, around optimal doping and across the Lifshitz transition. Our findings reveal a largely orbital-dependent superconducting gap structure, where the more strongly correlated d_{xy} band has a vanishing superconducting gap at higher doping, aligning with the Hund's metal behavior observed in the normal state. Notably, the superconducting gap on the d_{xy} band disappears before the Lifshitz transition, suggesting that the Fermi surface topology may play a secondary role. We discuss how these results point to orbital-selective superconducting pairing and how strong correlations via Hund's coupling may shape superconducting gap structures in iron-based and other multiorbital superconductors.

FOS: Physical sciences↗