Signature of quantum Griffiths singularity state in a layered quasi-one-dimensional superconductor

Quantum phase transitions in two-dimensional superconductors: a review on recent experimental progress

Superconductor-insulator/metal transition (SMT) as a paradigm of quantum phase transition has been a research highlight over the last three decades. Benefit from recent developments in the fabrication and measurements of two-dimensional (2D) superconducting films and nanodevices, unprecedented quantum phenomena have been revealed in the quantum phase transitions of 2D superconductors. In this review, we introduce the recent progress on quantum phase transitions in 2D superconductors, focusing on the quantum Griffiths singularity (QGS) and anomalous metal state. Characterized by a divergent critical exponent when approaching zero temperature, QGS of SMT is discovered in ultrathin crystalline Ga films and subsequently detected in various 2D superconductors. The universality of QGS indicates the profound influence of quenched disorder on quantum phase transitions. Besides, in a 2D superconducting system, whether a metallic ground state can exist is a long-sought mystery. Early experimental studies indicate an intermediate metallic state in the quantum phase transition of 2D superconductors. Recently, in high-temperature superconducting films with patterned nanopores, a robust anomalous metal state (i.e. quantum metal or Bose metal) has been detected, featured as the saturated resistance in the low temperature regime. Moreover, the charge-2equantum oscillations are observed in nanopatterned films, indicating the bosonic nature of the anomalous metal state and ending the debate on whether bosons can exist as a metal. The evidences of the anomalous metal states have also been reported in crystalline epitaxial thin films and exfoliated nanoflakes, as well as granular composite films. High quality filters are used in these works to exclude the influence of external high frequency noises in ultralow temperature measurements. The observations of QGS and metallic ground states in 2D superconductors not only reveal the prominent role of quantum fluctuations and dissipations but also provide new perspective to explore quantum phase transitions in superconducting systems.

Current-voltage characteristics of focused ion beam fabricated superconducting tungsten meanders

We report on the superconducting properties and intermediate resistive steps (IRS) observed in the current-voltage characteristics (IVC) of tungsten meander (MW) structures fabricated using focused ion beam (FIB) technique. Three number of MWs were studied with individual wire widths of 240 nm, 640 nm and 850 nm with superconducting transition temperatures (TC) of 4.5 K, 4.55 K and 4.60 K respectively. The measured normal state resistance values at 8 K for these wires are of ∼182 kΩ, ∼49 kΩ and ∼32 kΩ, respectively as a function of increasing wire widths; are higher than the quantum of resistance (h/4e2=6.45kΩ,his a Planck constant andeis electronic charge) indicating extreme disorder nature of the fabricated samples. The variation of resistance with respect to temperature (forT Collapse

Key Words

• focused ion beam deposition
• intermediate resistive steps (IRS)
• meander structures
• phase slip (PS)
• superconductivity

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Affiliation(s)

Abhishek Kumar Quantum Nanophotonics Metrology Division, CSIR-National Physical Laboratory, Dr K. S. Krishnan Marg, New Delhi 110012, India Academy of Scientific and Innovative Research (AcSIR), Ghaziabad 201002, India
Sudhir Husale Quantum Nanophotonics Metrology Division, CSIR-National Physical Laboratory, Dr K. S. Krishnan Marg, New Delhi 110012, India Academy of Scientific and Innovative Research (AcSIR), Ghaziabad 201002, India
M P Saravanan Low Temperature Laboratory, UGC-DAE Consortium for Scientific Research, University Campus, Khandwa Road, Indore 452001, India
Bikash Gajar Quantum Nanophotonics Metrology Division, CSIR-National Physical Laboratory, Dr K. S. Krishnan Marg, New Delhi 110012, India Academy of Scientific and Innovative Research (AcSIR), Ghaziabad 201002, India
Majid Yousuf Quantum Nanophotonics Metrology Division, CSIR-National Physical Laboratory, Dr K. S. Krishnan Marg, New Delhi 110012, India Academy of Scientific and Innovative Research (AcSIR), Ghaziabad 201002, India
Abhilasha Saini Quantum Nanophotonics Metrology Division, CSIR-National Physical Laboratory, Dr K. S. Krishnan Marg, New Delhi 110012, India Academy of Scientific and Innovative Research (AcSIR), Ghaziabad 201002, India
Mahesh Gaurav Yadav Quantum Nanophotonics Metrology Division, CSIR-National Physical Laboratory, Dr K. S. Krishnan Marg, New Delhi 110012, India Academy of Scientific and Innovative Research (AcSIR), Ghaziabad 201002, India
R P Aloysius Quantum Nanophotonics Metrology Division, CSIR-National Physical Laboratory, Dr K. S. Krishnan Marg, New Delhi 110012, India Academy of Scientific and Innovative Research (AcSIR), Ghaziabad 201002, India

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Observation of thickness-tuned universality class in superconducting β-W thin films

The interplay between quenched disorder and critical behavior in quantum phase transitions is conceptually fascinating and of fundamental importance for understanding phase transitions. However, it is still unclear whether or not the quenched disorder influences the universality class of quantum phase transitions. More crucially, the absence of superconducting-metal transitions under in-plane magnetic fields in 2D superconductors imposes constraints on the universality of quantum criticality. Here, we observe the thickness-tuned universality class of superconductor-metal transition by changing the disorder strength in β-W films with varying thickness. The finite-size scaling uncovers the switch of universality class: quantum Griffiths singularity to multiple quantum criticality at a critical thickness of t_c⊥1~8nm and then from multiple quantum criticality to single criticality at t_c⊥2~16nm. Moreover, the superconducting-metal transition is observed for the first time under in-plane magnetic fields and the universality class is changed at t_c‖~8nm. The observation of thickness-tuned universality class under both out-of-plane and in-plane magnetic fields provides broad information for the disorder effect on superconducting-metal transitions and quantum criticality.

Observation of In-Plane Quantum Griffiths Singularity in Two-Dimensional Crystalline Superconductors

Quantum Griffiths singularity (QGS) reveals the profound influence of quenched disorder on the quantum phase transitions, characterized by the divergence of the dynamical critical exponent at the boundary of the vortex glasslike phase, named as quantum Griffiths phase. However, in the absence of vortices, whether the QGS can exist under a parallel magnetic field remains a puzzle. Here, we study the magnetic field induced superconductor-metal transition in ultrathin crystalline PdTe_{2} films grown by molecular beam epitaxy. Remarkably, the QGS emerges under both perpendicular and parallel magnetic field in four-monolayer PdTe_{2} films. The direct activated scaling analysis with a new irrelevant correction has been proposed, providing important evidence of QGS. With increasing film thickness to six monolayers, the QGS disappears under perpendicular field but persists under parallel field, and this discordance may originate from the differences in microscopic processes. Our work demonstrates the universality of parallel field induced QGS and can stimulate further investigations on novel quantum phase transitions under parallel magnetic field.

High-Throughput Screening of Element-Doped Carbon Nanotubes Toward an Optimal One-Dimensional Superconductor

In order to search for optimal one-dimensional (1D) superconductors with a high transition temperature (T_c), we performed high-throughput computation on the phonon dispersion, electron-phonon coupling (EPC), and superconducting properties of (5,0), (3,3), and element-doped (3,3) carbon nanotubes (CNTs) based on first-principles calculations. We find that the CNT (5,0) is superconductive with T_c of 7.9 K, while the (3,3) CNT has no superconductivity. However, by high-throughput screening of about 50 elements in the periodic table, we identified that 14 elemental dopants can make the (3,3) CNT dynamically stable and superconducting. The high T_c ≈ 28 K suggests that the Si-doped (3,3) CNT is an excellent one-dimensional (1D) superconductor. In addition, the Al-, In-, and La-doped (3,3) CNTs are also great 1D superconductor candidates with a T_c of about 18, 17, and 29 K, respectively. These results may inspire the synthesis and discovery of optimal high-T_c 1D superconductors experimentally.

Chemical Vapor Deposition of Superconducting FeTe1-xSex Nanosheets

FeTe_1-xSe_x, a promising layered material used to realize Majorana zero modes, has attracted enormous attention in recent years. Pulsed laser deposition (PLD) and molecular-beam epitaxy (MBE) are the routine growth methods used to prepare FeTe_1-xSe_x thin films. However, both methods require high-vacuum conditions and polished crystalline substrates, which hinder the exploration of the topological superconductivity and related nanodevices of this material. Here we demonstrate the growth of the ultrathin FeTe_1-xSe_x superconductor by a facile, atmospheric pressure chemical vapor deposition (CVD) method. The composition and thickness of the two-dimensional (2D) FeTe_1-xSe_x nanosheets are well controlled by tuning the experimental conditions. The as-prepared FeTe_0.8Se_0.2 nanosheet exhibits an onset superconducting transition temperature of 12.4 K, proving its high quality. Our work offers an effective strategy for preparing the ultrathin FeTe_1-xSe_x superconductor, which could become a promising platform for further study of the unconventional superconductivity in the FeTe_1-xSe_x system.

Magnetic-Field-Induced Re-entrance of Superconductivity in Ta2PdS5 Nanostrips

The motion of Abrikosov vortices is the dominant origin of dissipation in type II superconductors subjected to a magnetic field, which leads to a finite electrical resistance. It is generally believed that the increase in the magnetic field results in the aggravation of energy dissipation through the increase in vortex density. Here, we show a distinctive re-entrance of the dissipationless state in quasi-one-dimensional superconducting Ta₂PdS₅ nanostrips. Utilizing magnetotransport measurements, we unveil a prominent magnetoresistance drop with the increase in the magnetic field below the superconducting transition temperature, manifesting itself as a giant re-entrance to the superconducting phase. Time-dependent Ginzburg-Landau calculations show that this is originated from the suppression of the vortex motion by the increased energy barrier on the edges. Interestingly, both our experiments and simulations demonstrate that this giant re-entrance of superconductivity occurs only in certain geometrical regimes because of the finite size of the vortex.

Disorder-Induced Quantum Griffiths Singularity Revealed in an Artificial 2D Superconducting System

Disorder-induced Griffiths singularity of quantum phase transition (QPT) is a crucial issue in 2D superconductors (2DSC). In a superconducting system, the strength of disorder is found to be associated with the vortex pinning energy, which is closely related to the quantum Griffiths singularity; however, a direct study to elucidate the role of vortex pinning energy on the quantum Griffiths singularity in 2DSC remains to be undertaken. Here, an artificial 2DSC system is designed by randomly depositing superconducting nanoislands on 2Delectron gas (2DEG). Quantum Griffiths singularity is present in a graphene/Pb-islands-array hybrid, where the superconducting behavior transits to weakly localized metallic behavior induced by the vertical magnetic field and exhibits critical behavior with a diverging dynamical critical exponent approaching zero temperature. Compared to the study of graphene/Sn-islands-array hybrid where the sharp QPT is observed, the vortex pinning energy acquired from the Arrhenius plot analysis is greater in graphene/Pb-islands-array hybrid, which may contribute to the presence of the quantum Griffiths singularity. This work may provide a comprehensive interpretation of the QPT in 2DSC.

Two-dimensional ferromagnetic superlattices

Mechanically exfoliated two-dimensional ferromagnetic materials (2D FMs) possess long-range ferromagnetic order and topologically nontrivial skyrmions in few layers. However, because of the dimensionality effect, such few-layer systems usually exhibit much lower Curie temperature (T C) compared to their bulk counterparts. It is therefore of great interest to explore effective approaches to enhance their T C, particularly in wafer-scale for practical applications. Here, we report an interfacial proximity-induced high-T C 2D FM Fe3GeTe2 (FGT) via A-type antiferromagnetic material CrSb (CS) which strongly couples to FGT. A superlattice structure of (FGT/CS)n, where n stands for the period of FGT/CS heterostructure, has been successfully produced with sharp interfaces by molecular-beam epitaxy on 2-inch wafers. By performing elemental specific X-ray magnetic circular dichroism (XMCD) measurements, we have unequivocally discovered that T C of 4-layer Fe3GeTe2 can be significantly enhanced from 140 K to 230 K because of the interfacial ferromagnetic coupling. Meanwhile, an inverse proximity effect occurs in the FGT/CS interface, driving the interfacial antiferromagnetic CrSb into a ferrimagnetic state as evidenced by double-switching behavior in hysteresis loops and the XMCD spectra. Density functional theory calculations show that the Fe-Te/Cr-Sb interface is strongly FM coupled and doping of the spin-polarized electrons by the interfacial Cr layer gives rise to the T C enhancement of the Fe3GeTe2 films, in accordance with our XMCD measurements. Strikingly, by introducing rich Fe in a 4-layer FGT/CS superlattice, T C can be further enhanced to near room temperature. Our results provide a feasible approach for enhancing the magnetic order of few-layer 2D FMs in wafer-scale and render opportunities for realizing realistic ultra-thin spintronic devices.

General Nanomolding of Ordered Phases

Large-scale, controlled fabrication of ordered phases is challenging at the nanoscale, yet highly demanded as their well-ordered structure and chemistry is the key for advanced functionality. Here, we demonstrate a general nanomolding process of ordered phases based on atomic diffusion. Resulting nanowires are single crystals and maintain their composition and structure throughout their length, which we explain by a self-ordering process originating from their narrow Gibbs free energy. The versatility, control, and precision of this thermomechanical nanomolding method of ordered phases provides new insights into single crystal growth and suggest itself as a technology to enable wide spread usage for nanoscale and quantum devices.

Signature of Superconductivity in Orthorhombic CoSb Monolayer Films on SrTiO3(001)

We prepare orthorhombic CoSb monolayer films on SrTiO₃(001) substrate by molecular beam epitaxy and observe symmetric gap around the Fermi level with coherence peaks at ± (6-7) meV by in situ scanning tunneling spectroscopy. Ex situ magnetization measurements of the films protected by Te and Si films consistently reveal a diamagnetic transition at 14 K. These results suggest the occurrence of superconductivity in orthorhombic CoSb monolayers on SrTiO₃(001).