Heavy-fermion Superconductor Research Articles

Local characterization of a heavy-fermion superconductor via sub-Kelvin magnetic force microscopy

Using magnetic force microscopy operating at sub-Kelvin temperatures, we characterize the heavy fermion superconductor CeCoIn5. We pinpoint the absolute London penetration depth of λ(0)=435 ± 20 nm and report its temperature dependence, which is closely linked to the symmetry of the superconducting gap. In addition, we directly measure the pinning force of individual Abrikosov vortices and estimate the critical current density of jc=9×104 A/cm2. In contrast to the related, well-established tunnel diode oscillator technique, our method is capable of resolving inhomogeneities locally on the micrometer scale at ultra-low temperature.

Incommensurate Spin Fluctuations in the Spin-Triplet Superconductor Candidate UTe_{2}.

Spin-triplet superconductors are of extensive current interest because they can host topological state and Majorana fermions important for quantum computation. The uranium-based heavy-fermion superconductor UTe_{2} has been argued as a spin-triplet superconductor similar to UGe_{2}, URhGe, and UCoGe, where the superconducting phase is near (or coexists with) a ferromagnetic (FM) instability and spin-triplet electron pairing is driven by FM spin fluctuations. Here we use neutron scattering to show that, although UTe_{2} exhibits no static magnetic order down to 0.3K, its magnetism in the [0,K,L] plane is dominated by incommensurate spin fluctuations near an antiferromagnetic ordering wave vector and extends to at least 2.6meV. We are able to understand the dominant incommensurate spin fluctuations of UTe_{2} in terms of its electronic structure calculated using a combined density-functional and dynamic mean-field theory.

Phenomenological theory in reentrant uranium-based superconductors

We develop a phenomenological theory for the family of uranium-based heavy fermion superconductors ($URhGe$, $UCoGe$, and $UTe_2$ ). The theory unifies the understanding of both superconductivity(SC) with a weak magnetic field and reentrant superconductivity(RSC) that appears at the first-order transition line with a high magnetic field. It is shown that the magnetizations along the easy and hard axis have opposite effects on superconductivity. The RSC is induced by the fluctuation parallel to the direction of the magnetic field. The theory makes specific predictions about the variation of triplet superconductivity order parameters $\vec{d}$ with applied external magnetic fields and the existence of a metastable state for the appearance of the RSC.

Successive magnetic transitions in the heavy-fermion superconductor Ce3PtIn11 studied by In115 nuclear quadrupole resonance

Nuclear quadrupole resonance (NQR) measurements were performed on the heavy fermion superconductor Ce3PtIn11 with Tc = 0.32 K. The temperature dependence of both spin-lattice relaxation rate 1/T1 and NQR spectra evidences the occurrence of two successive magnetic transitions with TN1 = 2.2 K and TN2 = 2.0 K. In successive magnetic transitions, even though the magnetic moment at the Ce(2) site plays a major role, the magnetic moment at the Ce(1) site also contributes to some extent. While a commensurate antiferromagnetic ordered state appears for TN2 < T < TN1, a partially incommensurate antiferromagnetic ordered state is suggested for T < TN2.

Intrinsic Bulk Quantum Oscillations in a Bulk Unconventional Insulator SmB6.

SummaryThe finding of bulk quantum oscillations in the Kondo insulator SmB6 proved a considerable surprise. Subsequent measurements of bulk quantum oscillations in other correlated insulators including YbB12 lent support to our discovery of a class of bulk unconventional insulators that host bulk quantum oscillations. Here we perform a series of experiments to examine evidence for the intrinsic character of bulk quantum oscillations in floating zone-grown single crystals of SmB6 that have been the subject of our quantum oscillation studies. We present results of thermodynamic, transport, and composition analysis experiments on pristine floating zone-grown single crystals of SmB6 and compare quantum oscillations with metallic LaB6 and elemental aluminum. These results establish the intrinsic origin of quantum oscillations from the insulating bulk of floating zone-grown SmB6. The similarity of the Fermi surface in insulating SmB6 with the conduction-electron Fermi surface in metallic hexaborides is at the heart of a theoretical mystery.

Magnetic and transport properties of new ternary uranium-based germanide U2Rh3Ge5

A new ternary uranium germanide U2Rh3Ge5 has been successfully synthesized and investigated by means of magnetic susceptibility χ(T, H), isothermal magnetization M(T, H), electrical resistivity ρ(T), and specific heat C(T, H) measurements. This compound is found to crystallize in the U2Co3Si5-type orthorhombic structure. The low-field χ(T) shows a clear peak at T N = 41.5 K corresponding to an antiferromagnetic transition. The M(H) curve measured up to 70 kOe exhibits an H-linear behavior at 2 K with very small induced magnetic moments, while it shows upward curvature with increasing temperature, implying the possible presence of a metamagnetic transition in high-field region above 70 kOe. As the temperature decreases, ρ(T) increases slowly at T > T N and decreases rapidly at T < T N, which can be understood based on a semiconductor-like narrow band gap model (or the c-f hybridization effect) and an antiferromagnetic spin-wave model, respectively. No evidence of heavy-fermion behavior or superconductivity transition is observed at temperatures as low as 0.4 K. The obtained experimental results are discussed by comparing with those reported for the isomorphic compound U2Ir3Si5 and the quasi-isomorphic compound U2Rh3Si5.

Non-Fermi liquid transport in the vicinity of the nematic quantum critical point of superconducting FeSe1−xSx

Non-Fermi liquids are strange metals whose physical properties deviate qualitatively from those of conventional metals due to strong quantum fluctuations. In this paper, we report transport measurements on the FeSe$_{1-x}$S$_x$ superconductor, which has a quantum critical point of a nematic order without accompanying antiferromagnetism. We find that in addition to a linear-in-temperature resistivity $\rho_{xx}\propto T$, which is close to the Planckian limit, the Hall angle varies as $\cot \theta_{\rm H} \propto T^2$ and the low-field magnetoresistance is well scaled as $\Delta\rho_{xx}/\rho_{xx}\propto \tan^2 \theta_{\rm H}$ in the vicinity of the nematic quantum critical point. This set of anomalous charge transport properties shows striking resemblance with those reported in cuprate, iron-pnictide and heavy fermion superconductors, demonstrating that the critical fluctuations of a nematic order with ${\bf q} \approx 0$ can also lead to a breakdown of the Fermi liquid description.

Fermi Surface of the Heavy-fermion Superconductor PrTi2Al20

We have investigated the Fermi surface properties in the Pr-based heavy-fermion superconductor PrTi2Al20 and its reference compound LaTi2Al20 by means of the de Haas–van Alphen effect experiments a...

Electronic structure of LaIrIn5 and f-electron character in its related Ce-115 compounds

LaIrIn$_5$ is a reference compound of the heavy-fermion superconductor CeIrIn$_5$.The lack of f electrons in LaIrIn$_5$ indicates that there should not be any f electron participatingin the construction of its Fermi surface. Thus the electronic structure comparison between LaIrIn$_5$and CeIrIn$_5$ provides a good platform to study the properties of f electrons. Here angle-resolvedphotoemission spectroscopy (ARPES) measurements and density functional theory (DFT)calculations are performed to study the electronic structures of LaIrIn$_5$ and CeIrIn$_5$.We find the valence band structures of the two materials are similar to each other, exceptfor the absence of f bands in LaIrIn$_5$. By analyzing the Fermi crossings of the threeconduction bands of the two materials quantitatively, we find the volumes of the electronpockets $\\upalpha$ and $\\upbeta$ around the M$'$ point become larger from LaIrIn$_5$ to CeIrIn$_5$,while the hole pocket $\\upgamma$ around the $\\Gamma'$ point becomes smaller. Together with thecalculation results, we confirm that this is mainly originated from the f-electron contribution,while the lattice-constant difference between LaIrIn$_5$ and CeIrIn$_5$only has a finite influence.We also give a summary of the f-electron character in its related Ce-115 heavy fermion compounds. Our results may be essential for the complete microscopic understandingof the 115 compounds and the related heavy-fermion systems.

Strong in-plane anisotropy in the electronic structure of fixed-valence β−LuAlB4

The origin of intrinsic quantum criticality in the heavy-fermion superconductor $\beta$-YbAlB$_4$ has been attributed to strong Yb valence fluctuations and its peculiar crystal structure. Here, we assess these contributions individually by studying the isostructural but fixed-valence compound $\beta$-LuAlB$_4$. Quantum oscillation measurements and DFT calculations reveal a Fermi surface markedly different from that of $\beta$-YbAlB$_4$, consistent with a `large' Fermi surface there. We also find an unexpected in-plane anisotropy of the electronic structure, in contrast to the isotropic Kondo hybridization in $\beta$-YbAlB$_4$.

Electronic structure evolution accompanying heavy fermion formation in CeCu2Si2

The cooper pairs in the heavy-fermion superconductor CeCu2Si2 are formed of heavy fermions. Therefore, the heavy fermions are fundamental to the emergence of unconventional superconductivity and associated non-Fermi-liquid behavior in the normal state. The interplay between localization and itinerancy manifested on the electronic structure is key for understanding the heavy-fermion behavior. Here, via the first-principle density functional theory (DFT) combined with single-site dynamical mean-field theory (DMFT), we investigate the temperature ( T ) evolution of the electronic structure of CeCu2Si2 in the normal state, focusing on the role of the 4 f states in the low energy regime. Two characteristic temperature scales of this evolution, which accompanied the heavy-fermion formation, are established. The coherence onset temperature is around 130 K, whereas the heavy-fermion band formation temperature is between 40 and 80 K; both characteristic temperature scales are higher than the transport coherence temperature. Furthermore, the heavy-fermion formation is confirmed by calculating its effective mass variation with the temperature. Based on the calculated T- dependent evolution of the 4 f orbital occupancy and electronic structure, an explanation on the behavior of the temperature evolution of the correlation strength of CeCu2Si2 is provided. Our results offer a comprehensive microscopic picture of the heavy-fermion formation in CeCu2Si2, which is essential for further understanding the emergent superconducting pairing mechanism.

Nature of the spin resonance mode in CeCoIn5

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 CeCoIn5, its SRM defies expectations for a spin-excitonic bound state, and is not a manifestation of sign-changing superconductivity. Instead, the SRM in CeCoIn5 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.

Anisotropy of the Upper Critical Field in the Heavy-Fermion Superconductor UTe2 under Pressure

We studied the anisotropy of the superconducting upper critical field $H_{\rm c2}$ in the heavy-fermion superconductor UTe$_2$ under hydrostatic pressure by magnetoresistivity measurements. In agreement with previous experiments we confirm that superconductivity disappears near a critical pressure $p_{\rm c} \approx 1.5$~GPa, and a magnetically ordered state appears. The unusual $H_{\rm c2}(T)$ at low temperatures for $H \parallel a$ suggests that the multiple superconducting phases which appear under pressure have quite different $H_{\rm c2}$. For a field applied along the hard magnetization $b$ axis $H_{\rm c2} (0)$ is glued to the metamagnetic transition $H_{\rm m}$ which is suppressed near $p_{\rm c}$. The suppression of $H_{\rm m}$ with pressure follows the decrease of temperature $T_{\chi}^{\rm max}$, at the maximum in the susceptibility along $b$. The strong reinforcement of $H_{\rm c2}$ at ambient pressure for $H \parallel b$ above 16~T is rapidly suppressed under pressure due to the increase of $T_{\rm sc}$ and the decrease of $H_{\rm m}$. The change in the hierarchy of the anisotropy of $H_{\rm c2}(0)$ on approaching $p_{\rm c}$ points out that the $c$ axis becomes the hard magnetization axis.

Multiple Superconducting Phases and Unusual Enhancement of the Upper Critical Field in UTe2

We performed AC calorimetry and magnetoresistance measurements under pressure for H || a-axis (easy-magnetization axis) in the novel heavy-fermion superconductor UTe2. Thanks to the thermodynamic information, multiple superconducting phases have been revealed under pressure and magnetic field. The (H,T) phase diagram of superconductivity under pressure displays an abrupt increase of the upper critical field (Hc2) at low temperature and in the high field region, and a strong convex curvature of Hc2 at high temperature. This behavior of Hc2 and the multiple superconducting phases require a state for the superconducting order parameter more complex than a spin-triplet equal spin pairing. Above the superconducting critical pressure, Pc, we find strong indications that the possible magnetic order is closer to antiferromagnetism than to ferromagnetism.

Type-II t−J model in superconducting nickelate Nd1−xSrxNiO2

The recent observation of superconductivity at relatively high temperatures in hole doped NdNiO$_2$ has generated considerable interest, particularly due to its similarity with the infinite layer cuprates. Building on the observation that the Ni$^{2+}$ ions resulting from hole doping are commonly found in the spin-triplet state, we introduce and study a variant of the $t-J$ model in which the holes carry S=1. We name this new model the Type II $t-J$ model. We find two distinct mechanisms for $d$ wave superconductivity. In both scenarios the pairing is driven by the spin coupling $J$. However, coherence is gained in distinct ways in these two scenarios. In the first case, the spin-one holes condense leading to a $d$ wave superconductor along with spin-symmetry breaking. Different orders including spin-nematic orders are possible. This scenario is captured by a spin one slave boson theory. In the second scenario, a coherent and symmetric $d$ wave superconductor is achieved from "Kondo resonance": spin one holes contribute two electrons to form large Fermi surface together with the spin 1/2 singly occupied sites. The large Fermi surface then undergoes $d$ wave pairing because of spin coupling $J$, similar to heavy fermion superconductor. We propose a three-fermion parton theory to treat these two different scenarios in one unified framework and calculate its doping phase diagram within a self consistent mean field approximation. Our study shows that a combination of "cuprate physics" and "heavy fermion physics" may emerge in the type II $t-J$ model.

Chiral superconductivity in heavy-fermion metal UTe2.

Spin-triplet superconductors are condensates of electron pairs with spin 1 and an odd-parity wavefunction1. An interesting manifestation of triplet pairing is the chiral p-wave state, which is topologically non-trivial and provides a natural platform for realizing Majorana edge modes2,3. However, triplet pairing is rare in solid-state systems and has not been unambiguously identified in any bulk compound so far. Given that pairing is usually mediated by ferromagnetic spin fluctuations, uranium-based heavy-fermion systems containing f-electron elements, which can harbour both strong correlations and magnetism, are considered ideal candidates for realizing spin-triplet superconductivity4. Here we present scanning tunnelling microscopy studies of the recently discovered heavy-fermion superconductor UTe2, which has a superconducting transition temperature of 1.6 kelvin5. We find signatures of coexisting Kondo effect and superconductivity that show competing spatial modulations within one unit cell. Scanning tunnelling spectroscopy at step edges reveals signatures of chiral in-gap states, which have been predicted to exist at the boundaries of topological superconductors. Combined with existing data that indicate triplet pairing in UTe2, the presence of chiral states suggests that UTe2 is a strong candidate for chiral-triplet topological superconductivity.

Three-dimensional and temperature-dependent electronic structure of the heavy-fermion compound CePt2In7 studied by angle-resolved photoemission spectroscopy

The three-dimensional electronic structure and Ce 4f electrons of the heavy fermion superconductor CePt2In7 is investigated. Angle-resolved photoemission spectroscopy using variable photon energy establishes the existence of quasi-two and three dimensional Fermi surface topologies. Temperature-dependent 4d-4f on-resonance photoemission spectroscopies reveal that heavy quasiparticle bands begin to form at a temperature well above the characteristic (coherence) temperature T*. T* emergence may be closely related to crystal electric field splitting, particularly the low-lying heavy band formed by crystal electric field splitting.

Fermi-Surface Instability in the Heavy-Fermion Superconductor UTe_{2}.

Transport measurements are presented up to fields of 29T in the recently discovered heavy-fermion superconductor UTe_{2} with magnetic field H applied along the easy magnetization a axis of the body-centered orthorhombic structure. The thermoelectric power varies linearly with temperature above the superconducting transition, T_{SC}=1.5 K, indicating that superconductivity develops in a Fermi liquid regime. As a function of field the thermoelectric power shows successive anomalies which appear at critical values of the magnetic polarization. Remarkably, the lowest magnetic field instability for H∥a occurs for the same critical value of the magnetization (0.4 μ_{B}) than the first order metamagnetic transition at 35T for field applied along the b axis. It can be clearly identified as a Lifshitz transition. The estimated number of charge carriers at low temperature reveals a metallic ground state distinct from LDA calculations indicating that strong electronic correlations are a major issue.

Pseudo-Triplet 5fElectron State in the Heavy Fermion Superconductor NpPd5Al2

Pseudo-triplet ground state in the heavy-fermion superconductor NpPd5Al2 was proposed from the macroscopic data analyzed by the crystalline electric field (CEF) level scheme deduced from the system...

F-Electron States of Heavy-Fermion Superconductor NpPd5Al2 and Rare-Earth- and Actinide-Based Isostructural Compounds

Good correspondence of the LS and j–j coupling schemes can be realized in the f-electron states of the heavy-fermion superconductor NpPd5Al2 and the isostructural family. The rare-earth and actinid...