Noncentrosymmetric Superconductor Research Articles

Superconducting diode effect under time-reversal symmetry.

In noncentrosymmetric superconductors, superconducting and normal conductions can interchange on the basis of the current flow direction. This effect is termed a superconducting diode effect (SDE), which is a focal point of recent research. The broken inversion and time-reversal symmetry is believed to be the requirements of SDE, but their intrinsic role has remained elusive. Here, we report strain-controlled SDEs in a layered trigonal superconductor, PbTaSe2. The SDE was found exclusively in a strained device with its absence in an unstrained device despite that it is allowed in unstrained trigonal structure. Moreover, the zero-field or magnetic field-even (magnetic field-odd) SDE is observed when the strain and current are along armchair (zigzag) direction The results unambiguously demonstrate the intrinsic SDE under time-reversal symmetry and the critical role of strain-induced electric polarization.

Role of Higgs and Leggett modes for the third harmonic response in noncentrosymmetric superconductors

Role of Higgs and Leggett modes for the third harmonic response in noncentrosymmetric superconductors

Comment on “Fully gapped superconductivity and topological aspects of the noncentrosymmetric superconductor TaReSi”

Comment on “Fully gapped superconductivity and topological aspects of the noncentrosymmetric superconductor TaReSi”

Gate-controlled supercurrent effect in dry-etched Dayem bridges of non-centrosymmetric niobium rhenium

The application of a gate voltage to control the superconducting current flowing through a nanoscale superconducting constriction, named as gate-controlled supercurrent (GCS), has raised great interest for fundamental and technological reasons. To gain a deeper understanding of this effect and develop superconducting technologies based on it, the material and physical parameters crucial for the GCS effect must be identified. Top-down fabrication protocols should also be optimized to increase device scalability, although studies suggest that top-down fabricated devices are more resilient to show a GCS. Here, we investigate gated superconducting nanobridges made with a top-down fabrication process from thin films of the non-centrosymmetric superconductor niobium rhenium with varying ratios of the constituents (NbRe). Unlike other devices previously reported and made with a top-down approach, our NbRe devices systematically exhibit a GCS effect when they were fabricated from NbRe thin films with small grain size and etched in specific conditions. These observations pave the way for the realization of top-down-made GCS devices with high scalability. Our results also imply that physical parameters like structural disorder and surface physical properties of the nanobridges, which can be in turn modified by the fabrication process, are crucial for a GCS observation, providing therefore also important insights into the physics underlying the GCS effect.

A computational screening of Ta–Sb intermetallics at high pressure

The binary high-pressure phase diagram of the Ta–Sb system was constructed for the first time in this study, utilizing the evolutionary algorithm USPEX and density functional theory (DFT). Ten pressurized dynamically and mechanically stable or metastable novel phases of Ta–Sb were discovered, including I4/mmm-TaSb2, P4/nmm-TaSb, P-3-Ta2Sb7, I4/mmm-Ta2Sb3, P-4m2-Ta7Sb, Pm-3-Ta7Sb, Pmm2-Ta15Sb, P4/nmm-TaSb3, I4/mmm-Ta3Sb4, and I4/mmm-Ta2Sb5. The compounds P-4m2-Ta7Sb and Pmm2-Ta15Sb exhibit promising characteristics as non-centrosymmetric superconductors (NCSs), with their superconducting critical temperature (TC) being 3.831 and 3.221 K, respectively. The application of pressure tuning is predicted to transform the topological characteristics of P4/nmm-TaSb, causing it to transition from a topological insulator state to a Dirac semimetal state and ultimately reverting back to a topological insulator state. Therefore, the P4/nmm-TaSb compound is considered a promising candidate to investigate topological and superconducting excitations. Moreover, the mechanical and thermal properties of Ta–Sb binary phases were also investigated. The thermal conductivity of I4/mmm-TaSb2, P4/nmm-TaSb, and P4/nmm-TaSb3 all surpasses 20 W m−1 K−1 at 1000 K, showcasing their excellent thermal conductivity properties. The present study addresses the research gap concerning high-pressure structures in the Ta–Sb binary system, thereby offering valuable insights for the design and development of intermetallic compounds within this binary system.

Complete zero-energy flat bands of surface states in fully gapped chiral noncentrosymmetric superconductors

Complete zero-energy flat bands of surface states in fully gapped chiral noncentrosymmetric superconductors

Nonreciprocal superconducting transport and the spin Hall effect in gyrotropic structures

The search for superconducting systems exhibiting nonreciprocal transport and, specifically, the diode effect, has proliferated in recent years. This trend has encompassed a wide variety of systems, including planar hybrid structures, asymmetric SQUIDs, and certain noncentrosymmetric superconductors. A common feature of such systems is a gyrotropic symmetry, realized on different scales and characterized by a polar vector. Alongside time-reversal symmetry breaking, the presence of a polar axis allows for magnetoelectric effects, which, when combined with proximity-induced superconductivity, results in spontaneous non-dissipative currents that underpin the superconducting diode effect. With this symmetry established, we present a comprehensive theoretical study of transport in a lateral Josephson junction composed of a normal metal supporting the spin Hall effect, and attached to a ferromagnetic insulator. Due to the presence of the latter, magnetoelectric effects arise without requiring external magnetic fields. We determine the dependence of the anomalous currents on the spin relaxation length and the transport parameters commonly used in spintronics to characterize the interface between the metal and the ferromagnetic insulator. Therefore, our theory naturally unifies nonreciprocal transport in superconducting systems with classical spintronic effects, such as the spin Hall effect, spin galvanic effect, and spin Hall magnetoresistance. We propose an experiment involving measurements of magnetoresistance in the normal state and nonreciprocal transport in the superconducting state. Such experiment would, on the one hand, allow for determining the parameters of the model and thus verifying with a greater precision the theories of magnetoelectric effects in normal systems. On the other hand, it would contribute to a deeper understanding of the underlying microscopic origins that determine these parameters.

Superconductivity of amorphous and crystalline Re–Lu films

We report on superconducting properties of a novel material: rhenium-lutetium films. Different compositions of Re x Lu binary are explored from x ≈ 3.8 to close to pure Re stoichiometry. The highest critical temperature, up to Tc≈ 7 K, is obtained for x ≈ 10.5 in accordance with electron dispersive spectroscopy results. Depending on the deposition conditions, polycrystalline or amorphous films are obtainable, both of which are interesting for practical use. Crystalline structure of polycrystalline phase is identified as a non-centrosymmetric superconductor using grazing incidence x-ray diffractometry. Superconducting properties were characterized both resistively and magnetically. Magnetoresistivity and AC/DC susceptibility measurements allowed us to determine Hc1 and Hc2 of these films, as well as estimate coherence length ξ(0) and magnetic penetration depth λL(0) . We also provide information on surface morphology of these films. Demonstration of superconductivity in this material justifies the point of view that Lu plays a role of group 3 transition metal in period 6 of the Periodic table of elements. Then, in analogy with Re–Nb, Re–Ti, Re–Hf and Re–Zr, one can expect that crystalline Re–Lu also breaks time-reversal symmetry. If that is proven by future experiments, in combination with noncentrosymmetric feature, these films could be used for forming nonreciprocal current devices, such as superconducting diodes, without involvement of external magnetic fields.

Crystal growth and Hall effect of the non-centrosymmetric superconductor α-BiPd and the topological superconductor β-Bi2Pd

Due to the rarity of triplet pairing, topological superconductors (SCs) have primarily been realized experimentally in manufactured topological phases, such as heterostructures, where the proximity effect causes triplet pairing with traditional s-wave SCs. Recently, there has been a lot of interest in investigating novel quantum materials with spin-triplet pairs in order to develop Majorana physics and creating quantum computers. Bi-based superconductors are speculated to hold promise for realizing spin-triplet pairing due to its large spin orbit coupling. In this work, we present the growth of α-BiPd and β-Bi2Pd single crystals using the optical floating zone technique and their characterization. The single crystals of α-BiPd and β-Bi2Pd are found to crystallize in monoclinic and tetragonal crystalline system with space group P21 and I4/mmm, respectively. The composition and microstructure of the grown crystals were analyzed with a scanning electron microscope, through energy dispersive spectroscopy (EDS) and electron backscattered diffraction (EBSD) analysis. The superconducting behavior and Hall effect of both α-BiPd and β-Bi2Pd single crystals have been investigated through resistivity measurements.

Physical properties of newly synthesized noncentrosymmetric TaIr2B2 and NbIr2B2 superconductors: an extensive comparison of GGA and LDA functional investigations.

In recent years, noncentrosymmetric (NCS) structural compounds have received much attention from the scientific community in the exploration for the unconventional nature of superconductivity with exciting physical properties. This study uses the comprehensive generalized gradient approximation (GGA) and local density approximation (LDA) to gain insights into the physical properties of two recently synthesized Ir-based NCS superconductors, TaIr2B2 and NbIr2B2. The structural parameters, mechanical performance, electronic structure, Debye temperature, melting temperature, electronic specific heat, and electron-phonon coupling constant of TaIr2B2 and NbIr2B2 are explored and discussed in detail. Density functional theory (DFT) optimized structural parameters of both NCS phases agree well with experimental observation. Both GGA and LDA calculations show that the compounds are ductile, machinable, mechanically stable, and anisotropic in nature. The elastic moduli and hardness calculations reveal that TaIr2B2 is harder than NbIr2B2. The calculation of the melting temperature reveals that TaIr2B2 is more suitable for high temperature technology applications compared to NbIr2B2. Both GGA and LDA functionals reveal that the optical functions are very similar. Both compounds display a significant amount of reflectivity spectra over a wide range of photon energies. The GGA functional reveals a somewhat higher density of states value compared to that of LDA. The present calculated values of the electron-phonon coupling constant of both compounds are consistent with values previously reported from experimental studies.

Unequivocal Identification of Spin-Triplet and Spin-Singlet Superconductors with Upper Critical Field and Flux Quantization.

Spin-triplet superconductors play central roles in Majorana physics and quantum computing but are difficult to identify. We show the methods of kink-point upper critical field and flux quantization in superconducting rings can unequivocally identify spin-singlet, spin-triplet in centrosymmetric superconductors, and singlet-triplet admixture in noncentrosymmetric superconductors, as realized in γ-BiPd, β-Bi_{2}Pd, and α-BiPd, respectively. Our findings are essential for identifying triplet superconductors and exploring their quantum properties.

Linear optical response from the odd-parity Bardasis-Schrieffer mode in locally non-centrosymmetric superconductors

On the recent report of a field-induced first order transition in the superconducting state of CeRh2As2, which is a possible indication of a parity-switching transition of the superconductor, the microscopic physics is still under investigation. However, if two competing paring channels of opposite parities do exist, a particle-particle collective mode referred to as the Bardasis-Schrieffer (BS) mode should generically exist below the pair-breaking continuum. The BS mode of the CeRh2As2 superconductor can couple to the light, as it arises from a pairing channel with the parity opposite to that of the superconducting condensate. Here, by using a generic model Hamiltonian we carry out a qualitative investigation on the excitation energy of the BS mode with respect to the out-of-plane magnetic fields and its contribution to the optical conductivity. Our findings indicate that the distinct coupling between the BS mode and the light can serve as evidence for the competing odd-parity channels of CeRh2As2 and other locally non-centrosymmetric superconductors.

Spontaneous superconducting diode effect in non-magnetic Nb/Ru/Sr2RuO4 topological junctions

Non-reciprocal electronic transport in a material occurs if both time reversal and inversion symmetries are broken. The superconducting diode effect (SDE) is an exotic manifestation of this type of behavior where the critical current for positive and negative currents are mismatched, as recently observed in some non-centrosymmetric superconductors with a magnetic field. Here, we demonstrate a SDE in non-magnetic Nb/Ru/Sr2RuO4 Josephson junctions without applying an external magnetic field. The cooling history dependence of the SDE suggests that time-reversal symmetry is intrinsically broken by the superconducting phase of Sr2RuO4. Applied magnetic fields modify the SDE dynamically by randomly changing the sign of the non-reciprocity. We propose a model for such a topological junction with a conventional superconductor surrounded by a chiral superconductor with broken time reversal symmetry.

Ultralow-temperature heat transport study of noncentrosymmetric superconductor CaPtAs

The noncentrosymmetric superconductor CaPtAs with time-reversal symmetry breaking in its superconducting state was previously proposed to host nodal superconductivity. Here, by employing ultralow-temperature thermal conductivity measurement on CaPtAs single crystal, we study its superconducting gap structure. A negligible residual linear term of thermal conductivity (κ 0/T) in zero magnetic field and the field dependence of κ 0/T indicate that CaPtAs has multiple superconducting gaps with a dominant s-wave component. This is consistent with recent nuclear quadrupole resonance measurements on CaPtAs. Our work puts a strong constraint on the theories to describe the superconducting pairing symmetry of CaPtAs.

Unique properties of the optical activity in noncentrosymmetric superconductors: Sum rule, missing area, and relation with the superconducting Edelstein effect

Unique properties of the optical activity in noncentrosymmetric superconductors: Sum rule, missing area, and relation with the superconducting Edelstein effect

A high-throughput screening and discovery of lanthanum based ternary noncentrosymmetric superconductors with ZrNiAl-structure from ab initio calculations

Noncentrosymmetric superconductors act as promising candidates for exploring unconventional properties like spin-triplet pairing states and intrinsic topological superconductivity. In this work, we provide an illustration of a comprehensive materials discovery process for the development of lanthanum-based superconductors without inversion center similar to ZrNiAl. Our approach begins with a comprehensive screening based on the first principles calculations by substituting different element species and materials database. Throughout a series of strict criteria, we expand the range of materials in this family to 35 compounds with superior stabilities which indicate that they are highly likely to be synthesized. Among these compounds we have identified 15 promising candidates with large energy splitting at the Fermi level. Moreover, the conventional superconducting transition temperatures have also been explored, the results show that 30 compounds are superconducting phases. This work may help with the design and the expansion of the viable phase space of La-based ZrNiAl-like materials for investigating novel quantum phenomena in noncentrosymmetric superconductors.

Pressure-dependent structural, electronic, optical, and mechanical properties of superconductor [formula omitted]: A first-principles study

The structural, electronic, mechanical, thermodynamic, and superconducting properties of a noncentrosymmetric heavy-fermion CeRh2As2 superconductor under pressure were studied using first-principles calculations based on density functional theory. The results indicate that the CeRh2As2 superconductor is mechanically and thermodynamically stable in the pressure ranges of 0–40 GPa. For the first time, this analysis provides invaluable, detailed insights into various physical features of this recently reported superconducting material CeRh2As2. It belongs to the well-known CaBe2Ge2-type family. The rising trend of elastic moduli with applied pressure indicates that CeRh2As2 stiffens up under applied pressure. Ductility of the studied superconductor rises significantly with pressure. The study of various anisotropy indices show that the compound is significantly anisotropic under ambient or applied pressure. Various mechanical features of CeRh2As2 are analyzed according to the results of elastic constants and adequately explained. The melting temperature rises with the applied pressure, making CeRh2As2 more suitable for high temperature applications. The electronic properties investigation greatly supports the calculated optical functions. As the applied pressure increases, the absorption and reflectivity spectra shift to higher energy regions. The computed TDOS at 0 GPa pressure at EF is ∼5.07 states/eV/f.u. and applied pressure has a negligible effect on the value of DOS. A pressure-induced increasing trend in the Debye temperature has also been observed.

Adherence of the rotating vortex lattice in the noncentrosymmetric superconductor Ru7B3 to the London model

The noncentrosymmetric superconductor Ru7B3 has in previous studies demonstrated remarkably unusual behaviour in its vortex lattice (VL), where the nearest neighbour directions of the vortices dissociate from the crystal lattice and instead show a complex field-history dependence, and the VL rotates as the field is changed. In this study, we look at the VL form factor of Ru7B3 during this field-history dependence, to check for deviations from established models, such as the London model. We find that the data is well described by the anisotropic London model, which is in accordance with theoretical predictions that the alterations to the structure of the vortices due to broken inversion symmetry should be small. From this, we also extract values for the penetration depth and coherence length.

Second-harmonic generation in fluctuating Ising superconductors

In a two-dimensional noncentrosymmetric transition metal dichalcogenide Ising superconductor in the fluctuating regime under the action of a uniform external electromagnetic field, a second-harmonic generation (SHG) effect takes place. There emerge two contributions to this effect, one conventional, which is due to the electron gas in its normal state, and the other one is of the Aslamazov–Larkin nature. Namely, it originates from the presence of fluctuating Cooper pairs in the system when the temperature approaches the temperature of the superconducting transition in the sample from above. Employing a usual approach to Ising superconductors, we lift the valley degeneracy by application of a weak out-of-plane external magnetic field, which produces a Zeeman effect. In calculations, we use the Boltzmann equations approach for the electron gas in the normal state, and the time-dependent Ginzburg–Landau equations for the fluctuating Cooper pairs, and show the emergence of SHG electric current characterized by a temperature-dependent broadening and a redshift.

Counterpart of the Chandrasekhar-Kendall state in noncentrosymmetric superconductors

Counterpart of the Chandrasekhar-Kendall state in noncentrosymmetric superconductors