2D Superconductors Research Articles

Correlation between Nanoscale Domain Structures and Superconducting Phase Transitions in Highly Crystalline 2D Superconductors

AbstractThe domains and domain boundaries in 2D materials are known to play essential roles in investigating intriguing physical properties and have potential applications in nanoscale devices. Understanding the influence of individual domains on the superconducting properties of ultrathin 2D superconductors is of crucial importance for fundamental studies on mesoscopic superconductivity as well as applications in superconducting nanoelectronics. Here, low‐temperature electronic transport measurements of high quality ultrathin Mo2C crystals are presented that show clear evidence for the presence of multiple superconducting phase induced by the nanoscale domain structures. In particular, the observation of an anomalous resistance peak in the vicinity of the onset of the superconducting transition is reported. This resistive anomaly is interpreted as a consequence of nonequilibrium charge imbalance near the domain boundaries, which could induce effective normal‐superconducting interfaces in 2D Mo2C crystals. Moreover, the magnetic field‐tuned superconductor‐metal transition for ultrathin Mo2C crystals is examined. The observed scaling behavior is consistent with the appearance of quantum Griffiths singularity in 2D superconducting systems. This study sheds light on the understanding of the domain boundaries and their role on the transport properties of highly crystalline 2D superconductors, which may open potential application of domain structure in functional superconducting nanodevices.

Double Superconductivity in Nodal Line Material NaAlSi; Coexistence of Bulk and 2D Superconductivities

Unique diamagnetic torque signals are found in the nodal line material NaAlSi, which suggests the presence of double superconductivity; i.e., bulk superconductivity and 2D superconductivity on the crystal surface.

Hydrogenation induced high-temperature superconductivity in two-dimensional W2C3.

The discovery of highly crystalline two-dimensional (2D) superconductors provides a new alluring branch for exploring the fundamental significances. Based on first-principles calculations, we predict a new kind of 2D stable material W2C3, which is a semimetal but not a superconductor because of the weak electron-phonon coupling (EPC) strength. After hydrogenation, W2C3H2 possesses the intrinsic metallic properties with a large density of states (DOS) at the Fermi energy (EF). More interestingly, the EPC strength is greatly enhanced after hydrogenation and the calculated critical temperature (Tc) is 40.5 K. Furthermore, the compressive strain can obviously soften the low-frequency phonons and enhance the EPC strength. Then, the Tc of W2C3H2 can be increased from 40.5 K to 49.1 K with -4% compressive strain. This work paves the way for providing a new platform for 2D superconductivity.

A novel two-dimensional superconducting Ti layer: density functional theory and electron-beam irradiation.

Since the discovery of graphene in an atomic thin layer format, many investigations have been conducted to search for two-dimensional (2D) layered materials, in which 3d-transition metals offer much new physics and great freedom of tunability. In this work, through electron-beam irradiation, we enable the manufacture of a new 2D Ti nanosheet from a suspension of Ti0.91O2 nanosheets. In state-of-the-art density functional theory (DFT), both empirical and linear response theory predicted that Hubbard Ueff values would be imposed, resulting in unstable phonon dispersion curves. In the end, the newly found Ti monolayer is confirmed to be a non-magnetic superconductor, with a medium level of electron-phonon coupling. The newly established Ti layer is quite robust under strain, and the evolution of local Dirac points in electronic bands is also analyzed in terms of linearity and energetic shift near the Fermi energy. As suggested by the Fermi surface, this metal monolayer experiences an electronic topological transition under strain. Our findings will encourage many more explorations of pure d metal-based isotopic monolayers with diverse structures and open a new playground for 2D superconductors and ultra-thin sensoring components.

Theoretical Prediction of Superconductivity in Boron Kagome Monolayer: MB3 (M = Be, Ca, Sr) and the Hydrogenated CaB3

Using first-principles calculations, we predict a new type of two-dimensional (2D) boride MB3 (M = Be, Ca, Sr), constituted by boron kagome monolayer and the metal atoms adsorbed above the center of the boron hexagons. The band structures show that the three MB3 compounds are metallic, thus the possible phonon-mediated superconductivity is explored. Based on the Eliashberg equation, for BeB3, CaB3, and SrB3, the calculated electron–phonon coupling constants λ are 0.46, 1.09, and 1.33, and the corresponding superconducting transition temperatures T c are 3.2, 22.4, and 20.9 K, respectively. To explore superconductivity with higher transition temperature, hydrogenation and charge doping are further considered. The hydrogenated CaB3, i.e., HCaB3, is stable, with the enhanced λ of 1.39 and a higher T c of 39.3 K. Moreover, with further hole doping at the concentration of 5.8 × 1011 hole/cm2, the T c of HCaB3 can be further increased to 44.2 K, exceeding the McMillan limit. The predicted MB3 and HCaB3 provide new platforms for investigating 2D superconductivity in boron kagome lattice since superconductivity based on monolayer boron kagome lattice has not been studied before.

Nonstoichiometric Salt Intercalation as a Means to Stabilize Alkali Doping of 2D Materials.

Although doping with alkali atoms is a powerful technique for introducing charge carriers into physical systems, the resulting charge-transfer systems are generally not air stable. Here we describe computationally a strategy towards increasing the stability of alkali-doped materials that employs stoichiometrically unbalanced salt crystals with excess cations (which could be deposited during, e.g., insitu gating) to achieve doping levels similar to those attained by pure alkali metal doping. The crystalline interior of the salt crystal acts as a template to stabilize the excess dopant atoms against oxidation and deintercalation, which otherwise would be highly favorable. We characterize this doping method for graphene, NbSe_{2}, and Bi_{2}Se_{3} and its effect on direct-to-indirect band gap transitions, 2D superconductivity, and thermoelectric performance. Salt intercalation should be generally applicable to systems which can accommodate this "ionic crystal" doping (and particularly favorable when geometrical packing constraints favor nonstoichiometry).

High-temperature superconductivity in two-dimensional hydrogenated titanium diboride: Ti2B2H4

In recent years, superconductivity in two-dimensional (2D) layered hexagonal metal borides has aroused much interests. Here, based on first-principles calculations, we theoretically report a new type of 2D hydrogenated transition-metal diboride: Ti2B2H4. Results reveal that the introduction of hydrogen atoms expands the frequency range of phonon spectrum of monolayer Ti2B2, and significantly increases the electron-phonon coupling (EPC). The calculated EPC constant λ of Ti2B2H4 is 1.18, and the corresponding superconducting critical temperature (Tc) is 48.6 K, exceeding the McMillan limit (39 K). The EPC mainly originates from the coupling between electrons in Ti-3d orbitals and the low-frequency vibration modes of Ti atoms as well as high-frequency vibration modes of H atoms. Additionally, the coupling strength can be further increased to 1.61 by applying appropriate biaxial compressive strain, and the corresponding Tc can be boosted to 69.4 K, approaching to liquid nitrogen (N2) temperature. The predicted Ti2B2H4 provides a new platform for 2D superconductivity and may have potential applications in 2D nanodevices.

The encapsulation of atomically-thin MgB2-based superconductors in two-dimensional material bilayers

Two-dimensional (2D) superconductors are a class of materials with unique properties that can potentially exhibit novel superconductivity at a reduced dimensionality, as well as pave the way for miniaturizing superconducting devices at the atomic limit. However, such materials are highly sensitive to interaction with their substrates and the environment, and therefore it is necessary to find potential methods for chemically isolating them. We address this problem by examining the possibility of isolating the medium-temperature 2D MgB2-based superconductors. We examine two possible structures: the monolayer stoichiometric MgB2 and the B-caged Mg3B8 structure. By solving the anisotropic Eliashberg equations, we predict that the latter possesses K upon application of a small lateral (3% tensile) strain, with an electron–phonon coupling of 0.88. To investigate protecting the superconductivity of these two layers, we apply density functional theory calculations to examine their encapsulation in a graphene bilayer or a hexagonal boron nitride bilayer. We find that both have high potential as encapsulation systems to protect the superconductivity of the 2D MgB2-based superconductors, in particular B-caged (B–Mg) n systems.

Anisotropic Vortex Squeezing in Synthetic Rashba Superconductors: A Manifestation of Lifshitz Invariants

Most of 2D superconductors are of type II, i.e., they are penetrated by quantized vortices when exposed to out-of-plane magnetic fields. In presence of a supercurrent, a Lorentz-like force acts on the vortices, leading to drift and dissipation. The current-induced vortex motion is impeded by pinning at defects, enabling the use of superconductors to generate high magnetic fields without dissipation. Usually, the pinning strength decreases upon any type of pair-breaking. Here we show that in Rashba superconductors the application of an in-plane field leads, instead, to an unexpected enhancement of pinning. When rotating the in-plane component of the field with respect to the current direction, the vortex inductance turns out to be highly anisotropic. We explain this phenomenon as a manifestation of Lifshitz invariant terms in the Ginzburg-Landau free energy, which are enabled by inversion and time-reversal symmetry breaking and lead to an elliptic squeezing of vortex cores. Our experiment provides access to a fundamental property of Rashba superconductors and offers an entirely new approach to vortex manipulation.

Ionic Modulation at the LaAlO3/KTaO3 Interface for Extreme High‐Mobility Two‐Dimensional Electron Gas

AbstractDue to the coexistence of many emergent phenomena, including 2D superconductivity and a large Rashba spin‐orbit coupling, 5d transition metal oxides‐based two‐dimensional electron systems (2DESs) have been prospected as one of the potential intrants for modern electronics. However, despite the lighter electron mass, the mobility of carriers, a key requisite for high‐performance devices, in 5d‐oxides devices remains far behind their 3d‐oxides analogs. The carriers’ mobility in these oxides is significantly hampered by the inevitable presence of defects. Here, very high mobility (≈22 650 cm2 V−1 s−1) of 5d‐2DES confined at the LaAlO3/KTaO3 interface is reported. The high mobility, which is beyond the values observed in SrTiO3 2DESs in the same carrier‐density range, is achieved using the ionic‐liquid gating at room temperature. The authors postulate that the ionic‐liquid gating affects the oxygen vacancies and efficiently reduces any disorder at the interface. Investigating density and mobility in a broad range of back‐gate voltage, the authors reveal that the mobility follows the power‐law µ ∝ n1.2, indicating the very high quality of ionic‐liquid‐gated LaAlO3/KTaO3 devices, consistent with the postulate. Furthermore, the analysis of the quantum oscillations confirms that the high‐mobility electrons occupy the electronic sub‐bands emerging from the Ta:5d orbitals of KTaO3.

Tunable two-dimensional superconductivity and spin-orbit coupling at the EuO/KTaO3(110) interface

Unconventional quantum states, most notably the two-dimensional (2D) superconductivity, have been realized at the interfaces of oxide heterostructures where they can be effectively tuned by the gate voltage (VG). Here we report that the interface between high-quality EuO (111) thin film and KTaO3 (KTO) (110) substrate shows superconductivity with onset transition temperature T_{{{mathrm{c}}}}^{{{{mathrm{onset}}}}} = 1.35 K. The 2D nature of superconductivity is verified by the large anisotropy of the upper critical field and the characteristics of a Berezinskii–Kosterlitz-Thouless transition. By applying VG, T_{{{mathrm{c}}}}^{{{{mathrm{onset}}}}} can be tuned from ~1 to 1.7 K; such an enhancement can be possibly associated with a boosted spin-orbit energy varepsilon _{{{{mathrm{so}}}}} = hbar /tau _{{{{mathrm{so}}}}}, where τso is the spin-orbit relaxation time. Further analysis of τso based on the upper critical field (Hc2) and magnetoconductance reveals complex nature of spin-orbit coupling (SOC) at the EuO/KTO(110) interface with different mechanisms dominating the influence of SOC effects on the superconductivity and the magnetotransport in the normal state. Our results demonstrate that the SOC should be considered an important factor in determining the 2D superconductivity at oxide interfaces.

Observation of Superconducting Collective Modes from Competing Pairing Instabilities in Single-Layer NbSe2.

In certain unconventional superconductors with sizable electronic correlations, the availability of closely competing pairing channels leads to characteristic soft collective fluctuations of the order parameters, which leave fingerprints in many observables and allow the phase competition to be scrutinized. Superconducting layered materials, where electron-electron interactions are enhanced with decreasing thickness, are promising candidates to display these correlation effects. In this work, the existence of a soft collective mode in single-layer NbSe2 , observed as a characteristic resonance excitation in high-resolution tunneling spectra is reported. This resonance is observed along with higher harmonics, its frequency Ω/2Δis anticorrelated with the local superconducting gap Δ, and its amplitude gradually vanishes by increasing the temperature and upon applying a magnetic field up to the critical values (TC and HC2 ), which sets an unambiguous link to the superconducting state. Aided by a microscopic model that captures the main experimental observations, this resonance is interpreted as a collective Leggett mode that represents the fluctuation toward a proximate f-wave triplet state, due to subleading attraction in the triplet channel. These findings demonstrate the fundamental role of correlations in superconducting 2D transition metal dichalcogenides, opening a path toward unconventional superconductivity in simple, scalable, and transferable 2D superconductors.

Effect of Impurity Scattering on Percolation of Bosonic Islands and Superconductivity in Fe Implanted NbN Thin Films.

A reentrant temperature dependence of the thermoresistivity between an onset local superconducting ordering temperature and a global superconducting transition at has been reported in disordered conventional 3-dimensional (3D) superconductors. The disorder of these superconductors is a result of either an extrinsic granularity due to grain boundaries, or of an intrinsic granularity ascribable to the electronic disorder originating from impurity dopants. Here, the effects of Fe doping on the electronic properties of sputtered NbN layers with a nominal thickness of 100 nm are studied by means of low-T/high- magnetotransport measurements. The doping of NbN is achieved via implantation of 35 keV Fe ions. In the as-grown NbN films, a local onset of superconductivity at is found, while the global superconducting ordering is achieved at , with a normal state resistivity . Moreover, upon Fe doping of NbN, is estimated, while and are measured to be 15.1 K and 13.5 K, respectively. In Fe:NbN, the intrinsic granularity leads to the emergence of a bosonic insulator state and the normal-metal-to-superconductor transition is accompanied by six different electronic phases characterized by a N-shaped T dependence of . The bosonic insulator state in a s-wave conventional superconductor doped with dilute magnetic impurities is predicted to represent a workbench for emergent phenomena, such as gapless superconductivity, triplet Cooper pairings and topological odd frequency superconductivity.

Local imaging of diamagnetism in proximity-coupled niobium nanoisland arrays on gold thin films

In this work we study the effect of engineered disorder on the local magnetic response of proximity coupled superconducting island arrays by comparing scanning Superconducting Quantum Interference Device (SQUID) susceptibility measurements to a model in which we treat the system as a network of one-dimensional (1D) superconductor-normal metal-superconductor (SNS) Josephson junctions, each with a Josephson coupling energy $E_J$ determined by the junction length or distance between islands. We find that the disordered arrays exhibit a spatially inhomogeneous diamagnetic response which, for low local applied magnetic fields, is well described by this junction network model, and we discuss these results as they relate to inhomogeneous two-dimensional (2D) superconductors. Our model of the static magnetic response of the arrays does not fully capture the onset of nonlinearity and dissipation with increasing applied field, as these effects are associated with vortex motion due to the dynamic nature of the scanning SQUID susceptometry measurement. This work demonstrates a model 2D superconducting system with engineered disorder and highlights the impact of dissipation on the local magnetic properties of 2D superconductors and Josephson junction arrays.

Superconductivity and topological aspects of two-dimensional transition-metal monohalides

Two-dimensional (2D) superconducting states have attracted much recent interest, especially when they coexist with nontrivial band topology which affords a promising approach towards Majorana fermions. Using first-principles calculations, we predict van der Waals monolayered transition-metal monohalides MX (M = Zr, Mo; X = F, Cl) as a class of 2D superconductors with remarkable transition temperature (5.9–12.4 K). Anisotropic Migdal-Eliashberg theory reveals that ZrCl have a single superconducting gap ∆ ~ 2.14 meV, while MoCl is a two-gap superconductor with ∆ ~ 1.96 and 1.37 meV. The Z2 band topology of 2D MX is further demonstrated that MoF and MoCl are candidates for realizing topological superconductivity. Moreover, the Dirac phonons of ZrCl and MoCl contribute w-shape phononic edge states, which are potential for an edge-enhanced electron-phonon coupling. These findings demonstrate that 2D MX offers an attractive platform for exploring the interplay between superconductivity, nontrivial electronic and phononic topology.

Dephasing-enhanced Majorana zero modes in two-dimensional and three-dimensional higher-order topological superconductors

The 1D Kitaev model in the topological phase, with open boundary conditions, hosts strong Majorana zero modes. These are fermion parity-odd operators that almost commute with the Hamiltonian and manifest in long coherence times for edge degrees of freedom. We obtain higher-dimensional counterparts of such Majorana operators by explicitly computing their closed form expressions in models describing 2D and 3D higher-order superconductors. Due to the existence of such strong Majorana zero modes, the coherence time of the infinite temperature autocorrelation function of the corner Majorana operators in these models diverges with the linear system size. In the presence of a certain class of orbital-selective dissipative dynamics, the coherence times of half of the corner Majorana operators is enhanced, while the time correlations corresponding to the remaining corner Majoranas decay much faster as compared with the unitary case. We numerically demonstrate robustness of the coherence times to the presence of disorder.

Zero-field superconducting diode effect in small-twist-angle trilayer graphene

The critical current of a superconductor can be different for opposite directions of current flow when both time-reversal and inversion symmetry are broken. %When time-reversal and inversion symmetry are simultaneously broken, the critical current of a 2D superconductor is expected to depend on the directions of current flow. Such nonreciprocal behavior in superconducting transport, which creates a superconducting diode, has recently been demonstrated experimentally by breaking these symmetries with an applied magnetic field \cite{Ando2020diodes} or by construction of a magnetic tunnel junction \cite{Diez2021magnetic}. Here we report an intrinsic superconducting diode effect which is present at zero external magnetic field in mirror symmetric twisted trilayer graphene (tTLG). Such nonreciprocal behavior, with sign that can be reversed through training with an out-of-plane magnetic field, provides direct evidence of the microscopic coexistence between superconductivity and time-reversal symmetry breaking. In addition to the magnetic-field trainability, we show that the zero-field diode effect can be controlled by varying carrier density or twist angle. In accordance with these experimental controls, a natural interpretation for the origin of the intrinsic diode effect is an imbalance in valley occupation of the underlying Fermi surface, which likely leads to finite-momentum Cooper pairing and nematicity in the superconducting phase.

3D Graphical Optimization for Molecular Effect Model in High Temperature Superconductors Thallium Class [ TC < 0° , TC > 0° ] without Lattice Tetragonal Compounds

In subsequent study series for 3D High Temperature Superconductors (HTCSs), Inverse Least Squares (ILS) and 3D Numerical/Graphical Optimization is the objective for this Molecular Effect Model (MEM) contribution. HTSCs class selected is Thallium group of [ Tl-Sn-Pb-Ba-Si-Mn-Mg-Cu-O ], constrained/extended to [T<sub>C</sub> ? 0° , T<sub>C</sub> > 0° ]. Tetragonal Lattice and Amorphous compounds of this Type II Thallium HTSCs class are not included in this study. Results obtained for Matlab 3D Graphical Optimization techniques are proven/presented. Numerical calculations include 3D Tikhonov Regularization algorithms with mathematical developments for this Thallium class. Thus, results show two strands, 3D surfaces for Graphical Optimization MEM Thallium HTSCs modelling, and MEM analysis of numerical equations. 3D MEM and Numerical Optimization get low error and residuals. Electronics Physics applications emerge from allteh research.

Density-induced BCS-Bose evolution in gated two-dimensional superconductors: The role of the interaction range in the Berezinskii-Kosterlitz-Thouless transition

We discuss the evolution from Bardeen-Cooper-Schrieffer (BCS) to Bose superconductivity vs. carrier density n in two-dimensional (2D) gated superconductors and address the fundamental role that the interaction range plays in the Berezinskii-Kosterlitz-Thouless transition. We investigate the density dependence of the critical temperature (T c ), superfluid density, order parameter modulus, chemical potential and pair size. Our most important finding is that it is absolutely essential to include classical and quantum phase fluctuations, as well as finite-ranged interactions to explain the non-monotonic behavior of T c vs. n and to guarantee that the upper bound on T c is not exceeded in 2D superconductors, as experimentally observed in (Nakagawa Y. et al., Science, 372 (2021) 190), a lithium-intercalated layered nitride, and in magic-angle twisted trilayer graphene (Park J. M. et al., Nature, 590 (2021) 249). Furthermore, we show that we can extract, from measurements of T c and the order parameter modulus, the effective mass of charge carriers and their interaction strength and range.

Electron–phonon coupling constant and BCS ratios in LaH10−y doped with magnetic rare-earth element

Stoichiometric near-room-temperature superconductors (NRTS) (for instance, H3S and LaH10) exhibit a high ground-state upper critical field, B c2(0) ⩾ 100 T, so that the magnetic phase diagram in these materials cannot be measured in non-destructive experiments. However, (Semenok et al 2022 Adv. Mater.) proposed the idea of exploring the full magnetic phase diagram in NRTS samples, in which the superconducting order parameter is suppressed by magnetic element doping. If the elements areuniformly distributed in the material, then the theory of electron–phonon mediated superconductivity predicts the suppression of the order parameter in a 3D s-wave superconductor. (Semenok et al 2022 Adv. Mater.) experimentally proved this idea by substituting lanthanum with the magnetic rare-earth neodymium in (La1−x Nd x )H10−y . As a result, the transition temperature in (La1−x Nd x )H10−y (x = 0.09) was suppressed to T c ∼ 120 K, and the upper critical field decreased to B c2(T= 41 K) = 55 T. While the exact hydrogen content should be further established in the (La1−x Nd x )H10−y (x = 0.09) (because similar T c suppression was observed in hydrogen-deficient LaH10−y samples reported by Drozdov et al (2019 Nature 569 528)), a significant part of the full magnetic phase diagram for the (La1−x Nd x )H10−y (x = 0.09) sample was measured. Here, we analyzed the reported (Semenok et al 2022 Adv. Mater.) magnetoresistance data for (La1−x Nd x )H10−y (x = 0.09) compressed at P = 180 GPa and deduced: (a) Debye temperature, ; (b) the electron–phonon coupling constant, ; (c) the ground-state superconducting energy gap, ; (d) the gap-to-transition temperature ratio, ; and (e) the relative jump in specific heat at transition temperature, . The deduced values indicate that (La1−x Nd x )H10−y (x = 0.09; P = 180 GPa) is a moderately strongly coupled s-wave superconductor.