Superconducting Transition Metal Research Articles

Exploring topological phases in superconducting transition metal (Sc, Ti, V)-carbides

The combination of non trivial band topology and superconductivity, resulting in topological superconductivity, igniting a fervent pursuit in the realm of quantum materials. Through density functional theory and maximally localized Wannier functions, we delve into the electronic and topological properties of transition metal carbides ΛC (with Λ= Sc, Ti, V). Phonon dispersions guarantee the structural stability of these superconductors. Witness the presence of band inversion within the Brillouin Zone of TiC and VC, contrasting the absence of such band inversion in ScC. In addition, the nonzero Z2 topological invariant as well as the occurrence of Dirac cone in the surface spectrum of TiC and VC, unveil their topologically nontrivial character. These transition metal carbides emerge as promising candidates for probing the depths of topological superconductivity and unraveling the associated Majorana bound states.

A new Ni-based superconductor with tunable critical temperature

The search for new 3d transition metal superconductors, especially Ni-based superconductors, is of great significance for understanding the mechanism of high temperature superconductivity. Based on inorganic material database, we found a new superconductor Mo2NiB2 through structural design. The crystal structure of Mo2NiB2 was analyzed by synchrotron radiation, Mo2NiB2 is a superconductor with a quasi-one-dimensional structure and also a Ni-based superconductor with a new structure. We report that the Mo2NiB2 exhibits superconductivity of 4 K, which is a typical type Ⅱ superconductor, and the non-stoichiometric ratio of Ni sites can increase the superconducting critical temperature of Mo2NiB2.

Metallization and superconductivity with Tc > 12 K in transition metal dichalcogenide HfS2 under pressure

The application of high pressure not only drives the comprehension of the structure and triggers exotic electronic states in transition metal dichalcogenides, but also promotes the discovery of intriguing phenomena. Here, we show that HfS2 exhibits highly tunable electronic property under pressure with sequence of gradual narrowing of band-gap at <40 GPa, followed by pressure-induced metallization at above 40 GPa, and ultimately superconductivity starting at ∼115 GPa. Raman and x-ray diffraction experiments provide strong evidence for first-order structural phase transitions from a trigonal (P3‾m1) to an orthorhombic (Immm) at around 18 GPa and then to a tetragonal structure (I4/mmm) above 40 GPa. The disappearance of all the Raman modes supported the observed metallization at above 40 GPa. At the similar pressure, the carrier-type of the sample is found to transform from electron to hole according to the Hall coefficient. At a critical pressure of 115 GPa a superconducting state sets with a transition temperature (Tc) of 3 K. Further compression dramatically increases Tc up to a maximum value of 12.2 K at 173 GPa, setting a record of Tc for superconducting transition metal dichalcogenides with zero-resistance. The pressure-induced high Tc superconductivity is closely linked to structural reconstructions which trigger changes in electronic states near the Fermi surface. The efficient and remarkable manipulation on the transport properties of 1T-HfS2 not only shed lights on the broad perspective of layered materials but also provide crucial information towards their practical applications.

Two-dimensional multigap superconductivity in bulk 2H−TaSeS

Superconducting transition metal dichalcogenides emerged as a prime candidate for topological superconductivity. This work presents a detailed investigation of superconducting and transport properties on 2H-TaSeS single crystals using magnetization, transport, and specific heat measurements. These measurements suggest multigap anisotropic superconductivity with the upper critical field, breaking Pauli limiting field in both in-plane and out-of-plane directions. The angle dependence of the upper critical field suggests 2-dimensional superconducting nature in bulk 2H- TaSeS.

Supercurrent diode effect and magnetochiral anisotropy in few-layer NbSe2

Nonreciprocal transport refers to charge transfer processes that are sensitive to the bias polarity. Until recently, nonreciprocal transport was studied only in dissipative systems, where the nonreciprocal quantity is the resistance. Recent experiments have, however, demonstrated nonreciprocal supercurrent leading to the observation of a supercurrent diode effect in Rashba superconductors. Here we report on a supercurrent diode effect in NbSe2 constrictions obtained by patterning NbSe2 flakes with both even and odd layer number. The observed rectification is a consequence of the valley-Zeeman spin-orbit interaction. We demonstrate a rectification efficiency as large as 60%, considerably larger than the efficiency of devices based on Rashba superconductors. In agreement with recent theory for superconducting transition metal dichalcogenides, we show that the effect is driven by the out-of-plane component of the magnetic field. Remarkably, we find that the effect becomes field-asymmetric in the presence of an additional in-plane field component transverse to the current direction. Supercurrent diodes offer a further degree of freedom in designing superconducting quantum electronics with the high degree of integrability offered by van der Waals materials.

Topological properties of bulk and bilayer 2M WS2: a first-principles study

Recently discovered 2M phase of bulk WS2 was observed to exhibit superconductivity with a critical temperature of 8.8 K, the highest reported among superconducting transition metal dichalcogenides. Also predicted to support protected surface states, it could be a potential topological superconductor. In the present study, we perform a detailed first-principles analysis of bulk and bilayer 2M WS2. We report a comprehensive investigation of the bulk phase, comparing structural and electronic properties obtained from different exchange correlation functionals to the experimentally reported values. By calculation of the Z 2 invariant and surface states, we give support for its non-trivial band nature. Based on the insights gained from the analysis of the bulk phase, we predict bilayer 2M WS2 as a new two-dimensional topological material. We demonstrate its dynamical stability from first-principles phonon computations and present its electronic properties, highlighting the band inversions between the W d and S p states. By means of Z 2 invariant computations and a calculation of the edge states, we show that bilayer 2M WS2 exhibits protected, robust edge states. The broken inversion symmetry in this newly proposed bilayer also leads to the presence of Berry curvature dipole and resulting non-linear responses. We compute the Berry curvature distribution and the dipole as a function of Fermi energy. We propose that Berry curvature dipole signals, which are absent in the centrosymmetric bulk 2M WS2, can be signatures of the bilayer. We hope our predictions lead to the experimental realization of this as-yet-undiscovered two-dimensional topological material.

Enhanced superconductivity in the Se-substituted 1T- PdTe2

Two-dimensional transition metal dichalcogenide PdTe$_2$ recently attracts much attention due to its phase coexistence of type-II Dirac semimetal and type-I superconductivity. Here we report a 67 % enhancement of superconducting transition temperature in the 1T-PdSeTe in comparison to that of PdTe2 through partial substitution of Te atoms by Se. The superconductivity has been unambiguously confirmed by the magnetization, resistivity and specific heat measurements. 1T-PdSeTe shows type-II superconductivity with large anisotropy and non-bulk superconductivity nature with volume fraction ~ 20 % estimated from magnetic and heat capacity measurements. 1T-PdSeTe expands the family of superconducting transition metal dichalcogenides and thus provides additional insights for understanding superconductivity and topological physics in the 1T-PdTe$_2$ system

Strain-stabilized superconductivity

Superconductivity is among the most fascinating and well-studied quantum states of matter. Despite over 100 years of research, a detailed understanding of how features of the normal-state electronic structure determine superconducting properties has remained elusive. For instance, the ability to deterministically enhance the superconducting transition temperature by design, rather than by serendipity, has been a long sought-after goal in condensed matter physics and materials science, but achieving this objective may require new tools, techniques and approaches. Here, we report the transmutation of a normal metal into a superconductor through the application of epitaxial strain. We demonstrate that synthesizing RuO2 thin films on (110)-oriented TiO2 substrates enhances the density of states near the Fermi level, which stabilizes superconductivity under strain, and suggests that a promising strategy to create new transition-metal superconductors is to apply judiciously chosen anisotropic strains that redistribute carriers within the low-energy manifold of d orbitals.

Magnetoelectric effects in gyrotropic superconductors

The magnetoelectric effect (or Edelstein effect) in noncentrosymmetric superconductors states that a supercurrent can induce spin magnetization. This is an intriguing phenomenon which has potential applications in superconducting spintronic devices. However, the original Edelstein effect only applies to superconductors with polar point group symmetry. In recent years, many new noncentrosymmetric superconductors have been discovered, such as superconductors with chiral lattice symmetry and superconducting transition metal dichalcogenides with various lattice structures. In this Rapid Communication, we provide a general framework to describe the supercurrent-induced magnetization in these recently discovered superconductors with gyrotropic point groups.

Molecular Beam Epitaxy of Transition Metal Nitrides for Superconducting Device Applications

Epitaxial integration of superconductors with semiconductors is expected to enable new device architectures and to increase electronic circuit and system functionality and performance in diverse fields, including sensing and quantum computing. Herein, radiofrequency plasma molecular‐beam epitaxy is used to epitaxially grow 3–200 nm‐thick metallic NbNx and TaNx thin films on hexagonal SiC substrates. Single‐phase cubic δ‐NbN and hexagonal TaNx films are obtained when the starting substrate temperature is ≈800 and ≈900 °C, respectively, and the active N to Nb or Ta ratio is ≈2.5–3. The films are characterized using in‐situ reflection high‐energy electron diffraction and ex‐situ atomic force microscopy, contactless sheet resistance, X‐ray diffraction, X‐ray photoelectron spectroscopy, secondary ion‐mass spectrometry, Rutherford backscattering spectrometry, cross‐sectional transmission electron microscopy, and low‐temperature electrical measurements. Smooth, epitaxial, low‐resistivity films of cubic δ‐NbN and hexagonal TaNx on SiC are demonstrated for films at least ≈50 nm‐thick, and their superconducting properties are reported. Epitaxy of AlN and GaN on δ‐NbN is also demonstrated, as well as integration of an epitaxial NbNx superconducting electrode layer under GaN high‐electron mobility transistors. These early demonstrations show the promise of direct epitaxial integration of superconducting transition metal nitrides with group III‐N semiconductors.

(Invited) 2D Materials: Crystal Growth for Future Device Structures

The isolation of graphene, now over decade ago, has given rise to the revitalization of an old full set of materials, two-dimensional materials (2DM), which have exceptional electrical, chemical and physical properties. Some of the materials under investigation in addition to graphene are hexagonal boron nitride (h-BN), semiconducting, metallic, and superconducting transition metal dichalcogenides (TMD) with a general chemical formula, MX2 where M is for example equal to Mo, W, Ta, Nb, Zr, Ti, and X = S, Se and Te, and others. While graphene is a material with many exceptional properties and h-BN is an excellent 2D insulator, TMD materials provide what neither graphene nor h-BN can, bandgap engineering that, in principle, can be used to create new devices that cannot be fabricated with h-BN and graphene alone. There is hope that 2DM can be integrated to fabricate numerous device types for many applications ranging from inkjet-printed circuits, photonic applications, flexible electronics, and high-performance electronics. However, to fully realize the benefits of these materials, the community will have to work together to define new device structures, device integration schemes, and materials growth processes to help the semiconductor industry make progress toward meeting the original goals of the International Technology Roadmap for Semiconductors (ITRS). A number of deposition/growth techniques have been used to prepare large area graphene, such as growth on SiC through the evaporation of Si at high temperatures, precipitation of carbon from metals, and catalytic chemical vapor deposition on Cu and Pt. Direct growth of good quality graphene on dielectrics/semiconductors other than SiC with reasonable properties has also been reported recently on Ge. Due to its insulating nature and compatibility with graphene and TMDs, the preparation of large area h-BN is also being developed on both metals and dielectrics. Transition metal dichalcogenides, however, present altogether different opportunities and difficulties in the preparation of low defect density large area single crystals. Vapor transport, chemical vapor deposition (CVD), and molecular beam epitaxy (MBE) are being developed to produce these materials for initial studies of materials physics and device fabrication. In addition, there is some effort in performing simulations to guide growth for both CVD and MBE growers. Therefore, there is an opportunity here to have the crystal growers and the modeling community collaborate to develop high quality materials and processes. A number of devices structures are currently under evaluation to take advantage of the basic properties of graphene, bi-layer graphene, h-BN and TMDs. Some of the devices are based on tunneling phenomena while others are based on excitonic phenomena. In this presentation I will present the state of the art results of graphene, h-BN, and a few TMD materials and their prospects for future electronic device applications.

Proximity-induced mixed odd- and even-frequency pairing in monolayer NbSe2

Monolayer superconducting transition metal dichalcogenide NbSe$_2$ is a candidate for a nodal topological superconductor by magnetic field. Because of the so-called Ising spin-orbit coupling that strongly pins the electron spins to the out-of-plane direction, Cooper pairs in monolayer superconductor NbSe$_2$ are protected against an applied in-plane magnetic field much larger than the Pauli limit. In monolayer NbSe$_2$, in addition to the Fermi pockets at the corners of Brillouin zone with opposite crystal momentum similar to other semiconducting transition metal dichalcogenides, there is an extra Fermi pocket around the $\Gamma$ point with much smaller spin splitting, which could lead to an alternative strategy for pairing possibilities that are manipulable by a smaller magnetic field. By considering a monolayer NbSe$_2$-ferromagnet substrate junction, we explore the modified pairing correlations on the pocket at $\Gamma$ point in hole-doped monolayer NbSe$_2$. The underlying physics is fascinating as there is a delicate interplay of the induced exchange field and the Ising spin-orbit coupling. We realize a mixed singlet-triplet superconductivity, s+f, due to the Ising spin-orbit coupling. Moreover, our results reveal the admixture state including both odd- and even-frequency components, associated with the ferromagnetic proximity effect. Different frequency symmetries of the induced pairing correlations can be realized by manipulating the magnitude and direction of the induced magnetization.

Local corrugation and persistent charge density wave in ZrTe3 with Ni intercalation

The mechanism of emergent bulk superconductivity in transition metal intercalated ZrTe3 is investigated by studying the effect of Ni doping on the band structure and charge density wave (CDW). The study reports theoretical and experimental results in the range of Ni0.01ZrTe3 to Ni0.05ZrTe3. In the highest doped samples bulk superconductivity with Tc < TCDW is observed, while TCDW is strongly reduced. Relativistic ab-initio calculations reveal Ni incorporation occurs preferentially through intercalation in the van-der-Waals gap. Analysis of the structural and elec- tronic effects of intercalation, indicate buckling of the Te-sheets adjacent to the Ni site akin to a locally stabilised CDW-like lattice distortion. Experiments by low temperature x-ray diffraction, angle-resolved-photoemission spectroscopy (ARPES) as well as temperature dependent resistivity reveal the nearly unchanged persistence of the CDW into the regime of bulk superconductivity. The CDW gap is found to be unchanged in its extent in momentum space, with the gap size also unchanged or possibly slightly reduced on Ni intercalation. Both experimental observations suggest that superconductivity coexists with the CDW in NixZrTe3.

Sc–Zr–Nb–Rh–Pd and Sc–Zr–Nb–Ta–Rh–Pd High-Entropy Alloy Superconductors on a CsCl-Type Lattice

We have synthesized previously unreported high-entropy alloys (HEAs) in the pentanary (ScZrNb)1–x[RhPd]x and hexanary (ScZrNbTa)1–x[RhPd]x systems. The materials have CsCl-type structures and mixed site occupancies. Both HEAs are type-II superconductors with strongly varying critical temperatures (Tc’s) depending on the valence electron count (VEC); the Tc’s increase monotonically with decreasing VEC within each series, and do not follow the trends seen for either crystalline or amorphous transition metal superconductors. The (ScZrNb)0.65[RhPd]0.35 HEA with the highest Tc, ∼9.3 K, also exhibits the largest μ0Hc2(0) = 10.7 T. The pentanary and hexanary HEAs have higher superconducting transition temperatures than their simple binary intermetallic relatives with the CsCl-type structure and a surprisingly ductile mechanical behavior. The presence of niobium, even at the 20% level, has a positive impact on the Tc. Nevertheless, niobium-free (ScZr)0.50[RhPd]0.50, as mother-compound of both superconducting HEAs...

Nearly isotropic superconductivity in the layered Weyl semimetal WTe2 at 98.5 kbar

Layered transition metal dichalcogenide WTe$_2$ has recently attracted significant attention due to the discovery of an extremely large magnetoresistance, a predicted type-II Weyl semimetallic state, and the pressure-induced superconducting state. By a careful measurement of the superconducting upper critical fields as a function of the magnetic field angle at a pressure as high as 98.5 kbar, we provide the first detailed examination of the dimensionality of the superconducting condensate in WTe$_2$. Despite the layered crystal structure, the upper critical field exhibits a negligible field anisotropy. The angular dependence of the upper critical field can be satisfactorily described by the anisotropic mass model from 2.2 K ($T/T_c\sim0.67$) to 0.03 K ($T/T_c\sim0.01$), with a practically identical anisotropy factor $\gamma\sim1.7$. The temperature dependence of the upper critical field, determined for both $H\perp ab$ and $H\parallel ab$, can be understood by a conventional orbital depairing mechanism. Comparison of the upper critical fields along the two orthogonal field directions results in the same value of $\gamma\sim1.7$, leading to a temperature independent anisotropy factor from near $T_c$ to $<0.01T_c$. Our findings thus identify WTe$_2$ as a nearly isotropic superconductor, with an anisotropy factor among one of the lowest known in superconducting transition metal dichalcogenides.

Elastic, thermodynamic, electronic, and optical properties of recently discovered superconducting transition metal boride NbRuB: An ab-initio investigation

The elastic, thermodynamic, electronic, and optical properties of recently discovered and potentially technologically important transition metal boride NbRuB, are investigated using the density functional formalism. Both generalized gradient approximation (GGA) and local density approximation (LDA) are used for optimizing the geometry and for estimating various elastic moduli and constants. The optical properties of NbRuB are studied for the first time with different photon polarizations. The frequency (energy) dependence of various optical constants complement quite well the essential features of the electronic band structure calculations. Debye temperature of NbRuB is estimated from the thermodynamical study. All these theoretical estimates are compared with published results, where available, and discussed in detail. Both electronic band structure and optical conductivity reveal robust metallic characteristics. The NbRuB possesses significant elastic anisotropy. Electronic features, on the other hand, are almost isotropic in nature. The effects of electronic band structure and Debye temperature on the emergence of superconductivity are also analyzed.

Superconductivity in transition metals.

A qualitative account of the occurrence and magnitude of superconductivity in the transition metals is presented, with a primary emphasis on elements of the first row. Correlations of the important parameters of the Bardeen-Cooper-Schrieffer theory of superconductivity are highlighted with respect to the number of d-shell electrons per atom of the transition elements. The relation between the systematics of superconductivity in the transition metals and the periodic table high-lights the importance of short-range or chemical bonding on the remarkable natural phenomenon of superconductivity in the chemical elements. A relationship between superconductivity and lattice instability appears naturally as a balance and competition between localized covalent bonding and so-called broken covalency, which favours d-electron delocalization and superconductivity. In this manner, the systematics of superconductivity and various other physical properties of the transition elements are related and unified.

Anharmonic suppression of charge density waves in 2H-NbS2

The temperature dependence of the phonon spectrum in the superconducting transition metal dichalcogenide 2H-NbS$_2$ is measured by diffuse and inelastic x-ray scattering. A deep, wide and strongly temperature dependent softening, of the two lowest energy longitudinal phonons bands, appears along the $\mathrm{\Gamma M}$ symmetry line in reciprocal space. In sharp contrast to the iso-electronic compounds 2H-NbSe$_2$, the soft phonons energies are finite, even at very low temperature, and no charge density wave instability occurs, in disagreement with harmonic ab-initio calculations. We show that 2H-NbS$_2$ is at the verge of the charge density wave transition and its occurrence is only suppressed by the large anharmonic effects. Moreover, the anharmonicity and the electron phonon coupling both show a strong in-plane anisotropy.

Temperature versus doping phase diagrams forBa(Fe1−xTMx)2As2(TM=Ni,Cu,Cu/Co)single crystals

Microscopic, structural, transport, and thermodynamic measurements of single crystalline $\text{Ba}{({\text{Fe}}_{1\ensuremath{-}x}{\text{TM}}_{x})}_{2}{\text{As}}_{2}$ ($\text{TM}=\text{Ni}$ and Cu) series, as well as two mixed $\text{TM}=\text{Cu}/\text{Co}$ series, are reported. In addition, high-magnetic field, anisotropic ${H}_{c2}(T)$ data were measured up to 33 T for the optimally Ni-doped ${\text{BaFe}}_{2}{\text{As}}_{2}$ sample. All the transport and thermodynamic measurements indicate that the structural and magnetic phase transitions at 134 K in pure ${\text{BaFe}}_{2}{\text{As}}_{2}$ are monotonically suppressed and increasingly separated in a similar manner by these dopants. In the $\text{Ba}{({\text{Fe}}_{1\ensuremath{-}x}{\text{Ni}}_{x})}_{2}{\text{As}}_{2}$ $(x\ensuremath{\le}0.072)$, superconductivity, with ${T}_{c}$ up to 19 K, is stabilized for $0.024\ensuremath{\le}x\ensuremath{\le}0.072$. In the $\text{Ba}{({\text{Fe}}_{1\ensuremath{-}x}{\text{Cu}}_{x})}_{2}{\text{As}}_{2}$ $(x\ensuremath{\le}0.356)$ series, although the structural and magnetic transitions are suppressed, there is only a very limited region of superconductivity: a sharp drop of the resistivity to zero near 2.1 K is found only for the $x=0.044$ samples. In the $\text{Ba}{({\text{Fe}}_{1\ensuremath{-}x\ensuremath{-}y}{\text{Co}}_{x}{\text{Cu}}_{y})}_{2}{\text{As}}_{2}$ series, superconductivity, with ${T}_{c}$ values up to 12 K ($x\ensuremath{\sim}0.022$ series) and 20 K ($x\ensuremath{\sim}0.047$ series), is stabilized. Quantitative analysis of the detailed temperature-dopant concentration $(T\ensuremath{-}x)$ and temperature-extra electrons $(T\ensuremath{-}e)$ phase diagrams of these series shows that there exists a limited range of the number of extra electrons added, inside which the superconductivity can be stabilized if the structural and magnetic phase transitions are suppressed enough. Moreover, comparison with pressure-temperature phase diagram data, for samples spanning the whole doping range, further re-enforces the conclusion that suppression of the structural/magnetic phase transition temperature enhances ${T}_{c}$ on the underdoped side, but for the overdoped side ${T}_{C}^{\text{max}}$ is determined by $e$. Therefore, by choosing the combination of dopants that are used, we can adjust the relative positions of the upper phase lines (structural and magnetic phase transitions) and the superconducting dome to control the occurrence and disappearance of the superconductivity in transition metal, electron-doped ${\text{BaFe}}_{2}{\text{As}}_{2}$.

Superconductivity in Transition Metal Doped MoB4

Polycrystalline ingots of MoB4 have been found to be superconducting near 6 to 8 K when doped with Ti or Nb. Magnetic susceptibility measurements performed on these alloys indicate that a substantial fraction of the sam- ple volume is superconducting below the transition temper- ature, while X-ray diffraction scans provide insight into the crystallographic effects of the dopants on the layered MoB4 structure, revealing both a systematic increase of the lattice constant in the direction of the boron planes and a system- atic decrease in the inter-plane spacing as the dopant amount is increased. This structural expansion and contraction cor- responds to an increase in Tc until the dopant solubility limit is reached. These findings present a stark contrast to the ef- fects of lattice parameter modification on superconductivity in the structurally related AlB2-type alloys.