arXiv subjects

Sheng Ran

Publications and source records attributed to Sheng Ran.

At least 55 records · Page 3Linked to original sources

arXiv2023

Identifying f-electron symmetries of UTe2 with O-edge resonant inelastic X-ray scattering

The recent discovery of spin-triplet superconductivity emerging from a non-magnetic parent state in UTe$_2$ has stimulated great interest in the underlying mechanism of Cooper pairing. Experimental characterization of short-range electronic and magnetic correlations is vital to understanding these phenomena. Here we use resonant inelastic X-ray scattering (RIXS), X-ray absorption spectroscopy (XAS), and atomic multiplet-based modeling to shed light on the active debate between 5$f^2$6$d^1$-based models with singlet crystal field states versus 5$f^3$-based models that predict atomic Kramers doublets and much greater 5$f$ itinerancy. The XAS and RIXS data are found to agree strongly with predictions for an 5$f^2$6$d^1$-like valence electron configuration with weak intra-dimer magnetic correlations, and provide new context for interpreting recent investigations of the electronic structure and superconducting pairing mechanism.

cond-mat.str-el↗

arXiv2022

Breakdown of the scaling relation of anomalous Hall effect in Kondo lattice ferromagnet USbTe

The interaction between strong correlation and Berry curvature is an open territory of in the field of quantum materials. Here we report large anomalous Hall conductivity in a Kondo lattice ferromagnet USbTe which is dominated by intrinsic Berry curvature at low temperatures. However, the Berry curvature induced anomalous Hall effect does not follow the scaling relation derived from Fermi liquid theory. The onset of the Berry curvature contribution coincides with the Kondo coherent temperature. Combined with ARPES measurement and DMFT calculations, this strongly indicates that Berry curvature is hosted by the flat bands induced by Kondo hybridization at the Fermi level. Our results demonstrate that the Kondo coherence of the flat bands has a dramatic influence on the low temperature physical properties associated with the Berry curvature, calling for new theories of scaling relations of anomalous Hall effect to account for the interaction between strong correlation and Berry curvature.

cond-mat.str-el↗

arXiv2022

Tuning the parity of superconductivity in CeRh2As2 via pressure

In the recently discovered heavy fermion superconductor CeRh2As2, a magnetic field induced phase transition has been observed inside the superconducting state, which is proposed to be a transition from an even to an odd-parity superconducting state. The odd-parity superconducting state and its large upper critical field has been explained by local inversion symmetry breaking and consequent Rashba spin-orbit coupling. Here we report the experimental tuning of the parity of superconductivity in CeRh2As2 via applied pressure. Superconductivity is enhanced by pressure after its initial suppression, forming a second dome. However, the odd-parity state only exists in the first dome and the second dome is dominated by the even-parity state. This parity switch of superconductivity under pressure is consistent with the competition between Rashba spin-orbit coupling and interlayer hopping in determining the parity of superconductivity in systems with local inversion symmetry breaking. Our results provide insightful guidance on how to look for new odd-parity superconductors.

cond-mat.supr-con↗

arXiv2022

Magnetic-field sensitive charge density wave orders in the superconducting phase of UTe2

The intense interest in triplet superconductivity partly stems from theoretical predictions of exotic excitations such as non-abelian Majorana modes, chiral supercurrents, and half-quantum vortices. However, fundamentally new, and unexpected states may emerge when triplet superconductivity appears in a strongly correlated system. In this work we use scanning tunneling microscopy to reveal an unusual charge density wave (CDW) order in the heavy fermion triplet superconductor, UTe2. Our high-resolution maps reveal a multi-component incommensurate CDW whose intensity get weaker with increasing field, eventually disappearing at the superconducting critical field, Hc2. To explain the origin and phenomenology of this unusual CDW, we construct a Ginzburg-Landau theory for a uniform triplet superconductor coexisting with three triplet pair density wave (PDW) states. This theory gives rise to daughter CDWs which would be sensitive to magnetic field due to their origin in a triplet PDW state, and naturally explains our data. Our discovery of a CDW sensitive to magnetic fields and strongly intertwined with superconductivity, provides important new information for understanding the order parameter of UTe2 and uncovers the possible existence of a new kind of triplet PDW order which has not been previously explored.

cond-mat.supr-con↗

arXiv2022

Multiple superconducting phases in heavy-fermion metals

Symmetry breaking beyond a global U(1) phase is the key signature of unconventional superconductors. As prototypical strongly correlated materials, heavy-fermion metals provide ideal platforms for realizing unconventional superconductivity. In this article, we review heavy-fermion superconductivity, with a focus on those materials with multiple superconducting phases. In this context, we highlight the role of orbital-selective (matrix) pairing functions, which are defined as matrices in the space of effective orbital degrees of freedom such as electronic orbitals and sublattices as well as equivalent descriptions in terms of intra- and inter-band pairing components in the band basis. The role of quantum criticality and the associated strange-metal physics in the development of unconventional superconductivity is emphasized throughout. We discuss in some detail the recent experimental observations and theoretical perspectives in the illustrative cases of UTe2, CeRh2As2, and CeCu2Si2, where applied magnetic fields or pressure induce a variety of superconducting phases. We close by providing a brief overview of overarching issues and implications for possible future directions.

cond-mat.supr-con↗

arXiv2022

The effect of the interlayer ordering on the Fermi surface of Kagome superconductor CsV$_3$Sb$_5$ revealed by quantum oscillations

The connection between unconventional superconductivity and charge density waves (CDW) has intrigued the condensed matter community and found much interest in the recently discovered superconducting Kagome family of AV$_3$Sb$_5$ (A = K, Cs, Rb). Xray diffraction and Raman spectroscopy measurements established that the CDW order in CsV$_3$Sb$_5$ comprises of a 2x2x4 structure with stacking of layers of star-of-David (SD) and inverse-star-of-David (ISD) pattern along the $c$-axis direction. Such interlayer ordering will induce a vast normalization of the electronic ground state; however, it has not been observed in Fermi surface measurements. Here we report quantum oscillations of CsV$_3$Sb$_5$ using tunnel diode oscillator frequency measurements. We observed a large number of frequencies, many of which were not reported. The number of frequencies can not be explained by DFT calculations when only SD or ISD distortion is considered. Instead, our results are consistent with calculations when interlayer ordering is taken into account, providing strong evidence that CDW phase of CsV$_3$Sb$_5$ has complicated structure distortion which in turn has dramatic effects on the Fermi surface properties.

cond-mat.str-el↗

arXiv2022

$c$-axis transport in UTe$_{2}$: Evidence of Three Dimensional Conductivity Component

We study the temperature dependence of electrical resistivity for currents directed along all crystallographic axes of the spin-triplet superconductor UTe$_{2}$. We focus particularly on an accurate determination of the resistivity along the $c$-axis ($ρ_c$) by using a generalized Montgomery technique that allows extraction of crystallographic resistivity components from a single sample. In contrast to expectations from the observed highly anisotropic band structure, our measurement of the absolute values of resistivities in all current directions reveals a surprisingly nearly isotropic transport behavior at temperatures above Kondo coherence, with $ρ_c \sim ρ_b \sim 2ρ_a$, that evolves to reveal qualitatively distinct behaviors on cooling. The temperature dependence of $ρ_c$ exhibits a peak at a temperature much lower than the onset of Kondo coherence observed in $ρ_a$ and $ρ_b$, consistent with features in magnetotransport and magnetization that point to a magnetic origin. A comparison to the temperature-dependent evolution of the scattering rate observed in angle-resolved photoemission spectroscopy experiments provides important insights into the underlying electronic structure necessary for building a microscopic model of superconductivity in UTe$_{2}$.

cond-mat.str-el↗

arXiv2022

Possible Coexistence of Antiferromagnetic and Ferromagnetic Spin Fluctuations in the Spin-triplet Superconductor UTe2 Revealed by 125Te NMR under Pressure

A spin-triplet superconducting state mediated by ferromagnetic (FM) spin fluctuations has been suggested to occur in the newly discovered heavy-fermion superconductor UTe$_2$. However, the recent neutron scattering measurements revealed the presence of antiferromagnetic (AFM) spin fluctuations in UTe$_2$. Here, we report the $^{125}$Te nuclear magnetic resonance (NMR) studies of a single-crystal UTe$_2$, suggesting the coexistence of FM and AFM spin fluctuations in UTe$_2$. Owing to the two different Te sites in the compound, we conclude that the FM spin fluctuations are dominant within ladders and the AFM spin fluctuations originate from the inter-ladder magnetic coupling. Although AFM spin fluctuations exist in the system, the FM spin fluctuations in the ladders may play an important role in the appearance of the spin-triplet superconducting state of UTe$_2$.

cond-mat.supr-con↗

arXiv2022

Probing FeSi, a d-electron topological Kondo insulator candidate, with magnetic field, pressure, and microwaves

Recently, evidence for a conducting surface state below 19 K was reported for the correlated d-electron small gap semiconductor FeSi. In the work reported herein, the conducting surface state and the bulk phase of FeSi were probed via electrical resistivity measurements as a function of temperature T, magnetic field B to 60 T and pressure P to 7.6 GPa, and by means of a magnetic field modulated microwave spectroscopy (MFMMS) technique. The properties of FeSi were also compared to those of the Kondo insulator SmB6 to address the question of whether FeSi is a d-electron analogue of an f-electron Kondo insulator and, in addition, a topological Kondo insulator. The overall behavior of the magnetoresistance MR of FeSi at temperatures above and below the onset temperature (T_S) 19 K of the conducting surface state is similar to that of SmB6. The two energy gaps, inferred from the resistivity data in the semiconducting regime, increase with pressure up to about 7 GPa, followed by a drop which coincides with a sharp suppression of T_S. This behavior is similar to that reported for SmB6, except that the two energy gaps in SmB6 decrease with pressure before dropping abruptly at T_S. The MFMMS measurements showed a sharp feature at T_S (19 K) for FeSi, but no such feature was observed at T_S 4.5 K for SmB6. The absence of a feature at T_S for SmB6 may be due to experimental issues and will be the subject of a future investigation.

cond-mat.str-el↗

arXiv2022

Ubiquitous enhancement of nematic fluctuations across the phase diagram of iron based superconductors probed by the Nernst effect

The role of nematic fluctuations for unconventional superconductivity has been subject of intense discussions for many years. In iron-based superconductors, the most established probe for electronic-nematic fluctuations, i.e. the elastoresistivity seems to imply that superconductivity is reinforced by electronic-nematic fluctuations, since the elastoresistivity amplitude peaks at or close to optimal $T_C$. However, on the over-doped side of the superconducting dome, the diminishing elastoresistivity suggests a negligible importance in the mechanism of superconductivity. Here we introduce the Nernst coefficient as a genuine probe for electronic nematic fluctuations, and we show that the amplitude of the Nernst coefficient tracks the superconducting dome of two prototype families of iron-based superconductors, namely Rh-doped $BaFe_{2}As_{2}$ and Co-doped $LaFeAsO$. Our data thus provide fresh evidence that in these systems nematic fluctuations foster the superconductivity throughout the phase diagram.

cond-mat.supr-con↗

arXiv2022

An optical investigation of the heavy fermion normal state in superconducting UTe$_2$

The recently discovered superconductor, UTe$_2$, has attracted immense scientific interest due to the experimental observations that suggest odd-parity superconductivity. It is believed that the material becomes a heavy-fermion metal at low temperatures although details of the normal state are unclear. Using Fourier transform infrared spectroscopy (FTIR), the normal state electronic structure of UTe$_2$ was investigated at zero applied magnetic field. Combining the measured reflectivity with the dc resistivity, the complex optical conductivity was obtained over a large frequency range. The frequency dependence of the real part of the optical conductivity exhibits a MIR peak around 4000 cm$^{-1}$ and a narrow Drude peak that develops below 40 K. A combination of density functional and dynamic mean field theory (DFT + DMFT) gives spectra in close correspondence to the experiment. Via this comparison we attribute the prominent MIR peak to inter-band transitions involving a narrow U 5$f$ feature that develops near the Fermi level. In this regard, our data gives spectroscopic evidence for the existence of a low energy Kondo resonance at temperatures just above the onset of superconductivity and implicates heavy electrons in the formation of the superconducting state. We find that the coherent Kondo resonance is primarily associated with a collapse of scattering and less with a transfer of spectral weight.

cond-mat.supr-con↗

arXiv2021

Symmetry of magnetic correlations in spin-triplet superconductor UTe2

The temperature dependence of the low-energy magnetic excitations in the spin-triplet superconductor UTe$_2$ was measured via inelastic neutron scattering in the normal and superconducting states. The imaginary part of the dynamic susceptibility follows the behavior of interband correlations in a hybridized Kondo lattice with an appropriate characteristic energy. These excitations are a lower-dimensional analogue of phenomena observed in other Kondo lattice materials, such that their presence is not necessarily due to dominance of ferromagnetic or antiferromagnetic correlations. The onset of superconductivity alters the magnetic excitations noticeably on the same energy scales, suggesting that these changes originate from additional electronic structure modification.

cond-mat.str-el↗

arXiv2021

Interplay between magnetism and superconductivity in UTe2

Time-reversal symmetry breaking (TRSB) in UTe2 was inferred from observations of a spontaneous Kerr response in the superconducting state after cooling in zero magnetic field, while a finite c-axis magnetic field training was further used to determine the nature of the non-unitary composite order-parameter of this material. Here we present an extensive study of the magnetic-field-trained Kerr effect, which unveils a unique critical state of pinned ferromagnetic vortices. We show that a remanent Kerr signal that appears following the removal of a training magnetic field, which reflects the response of the TRSB order parameter and the external magnetic field through the paramagnetic susceptibility. This unambiguously demonstrate the importance of the ferromagnetic fluctuations and their intimate relation to the composite order parameter. Focusing the measurement to the center of the sample, we are able to accurately determine the maximum field that is screened by the critical state and the respective critical current. Measurements in the presence of magnetic field show the tendency of the superconductor to produce shielding currents that oppose the increase in vortex-induced magnetization due to the diverging paramagnetic susceptibility.

cond-mat.supr-con↗

arXiv2021

From hidden-order to antiferromagnetism: electronic structure changes in Fe-doped URu$_{2}$Si$_{2}$

In matter, any spontaneous symmetry breaking induces a phase transition characterized by an order parameter, such as the magnetization vector in ferromagnets, or a macroscopic many-electron wave-function in superconductors. Phase transitions with unknown order parameter are rare but extremely appealing, as they may lead to novel physics. An emblematic, and still unsolved, example is the transition of the heavy fermion compound URu$_2$Si$_2$ (URS) into the so-called hidden-order (HO) phase when the temperature drops below $T_0 = 17.5$K. Here we show that the interaction between the heavy fermion and the conduction band states near the Fermi level has a key role in the emergence of the HO phase. Using angle resolved photoemission spectroscopy, we find that while the Fermi surfaces of the HO and of a neighboring antiferromagnetic (AFM) phase of well-defined order parameter have the same topography, they differ in the size of some, but not all, of their electron pockets. Such a non-rigid change of the electronic structure indicates that a change in the interaction strength between states near the Fermi level is a crucial ingredient for the HO-to-AFM phase transition.

cond-mat.str-el↗

arXiv2021

Expansion of the high field-boosted superconductivity in UTe2 under pressure

Magnetic field induced superconductivity is a fascinating quantum phenomenon, whose origin is yet to be fully understood. The recently discovered spin triplet superconductor, UTe2, exhibits two such superconducting phases, with the second one reentering in the magnetic field of 45 T and persisting up to 65 T. More surprisingly, in order to induce this superconducting phase, the magnetic field has to be applied in a special angle range, not along any high symmetry crystalline direction. Here we investigated the evolution of this high-field induced superconducting phase under pressure. Two superconducting phases merges together under pressure, and the zero resistance persists up to 45 T, the field limit of the current study. We also reveal that the high field-induced superconducting phase is completely decoupled from the first order field polarized phase transition, different from previously known example of field induced superconductivity in URhGe, indicating a superconductivity boosted by a different paring mechanism.

cond-mat.supr-con↗

arXiv2021

Anomalous normal fluid response in a chiral superconductor UTe2

A chiral superconductor has been proposed as one pathway to realize topological quantum computation utilizing the predicted Majorana normal fluid at its boundary (i.e., a point, edge, or surface). The search for experimental realizations has led to the discovery of 1D chiral superconducting systems. However, the long-sought 2D and 3D chiral superconductors with edge and surface Majorana normal fluid are yet to be conclusively found. Here we report evidence for a chiral spin-triplet pairing state of UTe$_2$ with significant surface normal fluid response. The microwave surface impedance of the UTe$_2$ crystal was measured and converted to complex conductivity, which is sensitive to both normal and superfluid responses. The anomalous residual normal fluid conductivity in the zero temperature limit supports the presence of a significant normal fluid response. The superfluid conductivity follows the low temperature behavior predicted for the axial spin-triplet state, which is further narrowed down to the chiral spin-triplet state with evidence of broken time-reversal symmetry. The temperature dependence of the superfluid conductivity also reveals a low bulk impurity scattering rate and low frequency-to-energy-gap ratio, implying that the observed normal fluid response does not have a trivial origin. Our findings suggest that UTe$_2$ can be a new platform to study exotic topological excitations in higher dimension, and may play the role of a versatile 3D building block in the future era of topological quantum computation.

cond-mat.supr-con↗

arXiv2020

Dual nature of 5$f$ electrons in the isostructural UM$_2$Si$_2$ family: from antiferro- to Pauli paramagnetism via hidden order

Using inelastic x-ray scattering beyond the dipole limit and hard x-ray photoelectron spectroscopy we establish the dual nature of the U $5f$ electrons in UM$_2$Si$_2$ (M = Pd, Ni, Ru, Fe), regardless of their degree of delocalization. We have observed that the compounds have in common a local atomic-like state that is well described by the U $5f^2$ configuration with the $Γ_1^{(1)}$ and $Γ_2$ quasi-doublet symmetry. The amount of the U 5$f^3$ configuration, however, varies considerably across the UM$_2$Si$_2$ series, indicating an increase of U5$f$ itineracy in going from M=Pd to Ni to Ru, and to the Fe compound. The identified electronic states explain the formation of the very large ordered magnetic moments in UPd$_2$Si$_2$ and UNi$_2$Si$_2$, the availability of orbital degrees of freedom needed for the hidden order in URu$_2$Si$_2$ to occur, as well as the appearance of Pauli paramagnetism in UFe$_2$Si$_2$. A unified and systematic picture of the U$M_2$Si$_2$ compounds may now be drawn, thereby providing suggestions for new experiments to induce hidden order and/or superconductivity in U compounds with the tetragonal body-centered ThCr$_2$Si$_2$ structure.

cond-mat.str-el↗

arXiv2020

Nature of the spin resonance mode in CeCoIn$_5$

Spin-fluctuation-mediated unconventional superconductivity can emerge at the border of magnetism, featuring a superconducting order parameter that changes sign in momentum space. Detection of such a sign-change is experimentally challenging, since most probes are not phase-sensitive. The observation of a spin resonance mode (SRM) from inelastic neutron scattering is often seen as strong phase-sensitive evidence for a sign-changing superconducting order parameter, by assuming the SRM is a spin-excitonic bound state. Here, we show that for the heavy fermion superconductor CeCoIn$_5$, its SRM defies expectations for a spin-excitonic bound state, and is not a manifestation of sign-changing superconductivity. Instead, the SRM in CeCoIn$_5$ likely arises from a reduction of damping to a magnon-like mode in the superconducting state, due to its proximity to magnetic quantum criticality. Our findings emphasize the need for more stringent tests of whether SRMs are spin-excitonic, when using their presence to evidence sign-changing superconductivity.

cond-mat.supr-con↗