Session C14: 2D Materials (Metals, Superconductors, and Correlated Materials) -- Superconductivity

Probing the Superfluid Density and the Superconducting Gaps under Pressure in 2H-NbSe2

We report on high-pressure (p_max = 2.1 GPa) muon spin rotation experiments probing the temperature-dependent magnetic penetration depth λ(T) in the layered superconductor 2H-NbSe₂. Upon increasing the pressure, we observe a substantial increase of the superfluid density n_s/m* ∼ 1/λ², which we find to scale linearly with T_c. This linear scaling is considered a hallmark feature of unconventional superconductivity, especially in high-temperature cuprate superconductors. Our current results, along with our earlier findings on 1T'-MoTe₂ [1], demonstrate that this linear relation is also an intrinsic property of the superconductivity in transition metal dichalcogenides, whereas the ratio T_c/T_F is approximately a factor of 20 lower than the ratio observed in hole-doped cuprates. We, furthermore, find that the values of the superconducting gaps are insensitive to the suppression of the quasi-two-dimensional CDW state, indicating that the CDW ordering and the superconductivity in 2H-NbSe₂ are independent of each other.

[1] Z. Guguchia et. al., Nature Communications 8, 1082 (2017).   

Exponential decrease of the collective vortex state energy with layer number in 2H-NbSe2

In the last years, a new generation of superconductors truly in the two-dimensional limit has been discovered in van der Waals materials. Recently, it has been shown that these type-II SCs can be extremely sensitive to external perturbation which pushes the system into a non-equilibrium steady state exhibiting metallic-like behavior [1]. Here we study the equilibrium and non-equilibrium phase diagrams of few-layered 2H-NbSe2 with minimal external perturbations. At finite magnetic field we observe a current-induced crossover to a non-activated metallic-like regime with a critical current that reduces with layer number. We discuss our results both phenomenologically and in the vortex picture which we compare with time-dependent Ginzburg-Landau simulations. In the vortex-picture we attribute the crossover to a transition from a pinned to unpinned vortex liquid. Finally, we demonstrate that by spreading non-uniform currents, we stabilize different vortex steady states in different spatial locations, potentially leading to novel interfaces.

[1] - Tamir, I., Benyamini, A. et al. Sensitivity of the Superconducting State in Thin Films. (2018)   

Strongly enhanced Andreev reflection in ultra-transparent NbSe2/ bilayer graphene junctions

Van der Waals (vdW) layered materials with diverse properties provide an alternative route to construct future generations of miniaturized electronic devices. The prospect of constructing superconducting circuits from all vdW materials is attractive from many different viewpoints. Making transparent superconductor to normal conductor (S-N) junctions is a critical step in this endeavor. Here we present measurement and simulations of Andreev reflection (AR) in ultra-transparent NbSe₂/ bilayer grapehene (BLG) S-N junctions constructed via vdW stacking. The resistance across the junction is as low as 13 Ω, indicating a highly transparent NbSe₂/ BLG interface. In these devices, the proximity effect penetrates to the BLG side for at least 500 nm. We observe strongly enhanced AR in the differential conductance measurement across the junction. The zero-bias conductance peak reaches up to 1.7 times the normal value. We present a comprehensive study of the AR by systematically varying the carrier density and temperature in different devices. A model is constructed to capture the rich features of the experimental data. Important factors include the spatial decay of the superconducting gap, the effect of the carrier density profile near the interface and the interface transparency.   

Dynamically-created Josephson Junctions in thin layers of NbSe2

Niobium Diselenide (NbSe2) is a member of the transition metal dichalcogenides (TMDs) family which displays superconducting properties and is host to charge density waves (CDWs) down to its monolayer limit. These properties alongside its hexagonal structure make it an interesting material to study physics within its two dimensional limit. Here, we report on fabrication and low-temperature transport measurements of atomically-thin layers of NbSe2 in four terminal-configurations. We observe the formation of phase slip lines across our NbSe2 sheets which appear as non-zero resistances in the superconducting state. The formation of the phase slip lines dynamically creates Josephson junctions (JJs) . We interact with these dynamically-created JJs through the application of RF and magnetic fields. Interestingly, we observe a deviation from the conventional behavior of JJs in the presence of RF and a skewed Fraunhofer pattern in the presence of a magnetic field. We model our observations in the presence of RF by assuming a coupling of a JJ to a CDW. We found that the dynamics of the CDW influence the behavior of the JJ to produce results that qualitatively agree with our experiment.   

Thinning-Induced Metastable Superconductivity in a Correlated 2D Material: 1T-IrTe2

Recently, reduced thickness was found to dramatically impact not only the static electronic structure (e.g. graphene, MoS2), but also the dynamic ordering kinetics (e.g. 1T-TaS2) [1]. The ordering kinetics of first-order phase transitions becomes significantly slowed with decreasing thickness, and metastable supercooled states can be realized by thinning alone. We therefore focus on layered iridium ditelluride (IrTe2), a charge-ordering system that is transformed into a superconductor by suppressing its first-order transition. Here, we discovered a persistent superconducting zero-resistance state in mechanically-exfoliated IrTe2 thin flakes [2]. The maximum superconducting critical temperature was identical to that of the bulk crystal which is chemically optimized, and the emergent superconductivity was revealed to have a metastable nature. The discovered robust metastable superconductivity suggests that 2D material is a new platform to induce, control, and functionalize metastable electronic states that are inaccessible in bulk crystals. [1. M. Yoshida et al. Sci. Adv. 1, e1500606 (2015); 2. M. Yoshida et al., Nano Lett. 18, 3113 (2018)]   

Dynamic phase diagram at zero magnetic field in 2D superconducting MoS2

Recent discoveries of two-dimensional (2D) crystalline superconductors [1] have led to clarifications of various intrinsic nature in magnetic fields. For example, 2D crystalline superconductors with weak pinning are very fragile against out-of-plane magnetic fields, and thus show a quantum metallic state due to quantum fluctuations [2] (though this phenomenon is still under debate [3]). However, there has been no report for how the BKT state at zero magnetic field evolves into the normal state through the dynamic states with increasing current in such systems. In this talk, we report on transport properties at zero magnetic field in ion-gated MoS₂ as a function of current and temperature. We show a clear BKT transition and anomalous kinks observed in resistance-temperature and current-voltage curves, and discuss the dynamic phase diagram based on these data, which contains a variety of current-induced nonequilibrium states originating from not only thermal but also quantum dissociation of vortices and antivortices.
[1] Y Saito et al. Nature Rev. Mat. 2 16094 (2016), [2] Y. Saito et al. Science 350, 409 (2015). A. W. Tsen, Nature Phys. 12, 208 (2016), [3] I. Tamir et al. arXiv: 1804.04648   

Superconductivity in WTe2

The layered semimetal WTe_2 was recently found to be a two-dimensional topological insulator (2D TI) when thinned down to a single monolayer, with conducting helical edge channels. We report that intrinsic superconductivity can be induced in this monolayer 2D TI by mild electrostatic doping, at temperatures below 1 K. The 2D TI-superconductor transition can be easily driven by applying a just a small gate voltage, offering an effective experimental platform to investigate the 2D superconductor-insulator transition in detail. This discovery offers new possibilities for gate-controlled devices combining superconductivity and topology, and could provide a basis for quantum information schemes based on topological protection.   

Quantum and classical vortex ratchets in a trigonal 2D superconductor

One of the unique features of recently emerging 2D superconductors is the quantum metallic state, which is a temperature-independent finite resistive state that appears once finite magnetic field is switched on [1, 2]. This quantum metallic state exhibits a sharp contrast with the conventional superconductor-insulator transition in conventional 2D systems, where the metallic state appears only at a single critical point.
To investigate the vortex dynamics which governs the resistance in the quantum metallic state, we have investigated the nonreciprocal transport in gated MoS₂, an archetypal noncentrosymmetric 2D superconductor with trigonal symmetry. We found that the second harmonic resistance R_xx^2ω appears when the vortex motion is controlled by the classical vortex flow, while R_xx^2ω is substantially suppressed when the vortex motion is in quantum creep region. The present result indicates that the trigonal 2D superconductor is a new model system for investigation of quantum and classical ratchets.
[1] Y. Saito et al., Science. 350, 409–413 (2015).
[2] A. W. Tsen et al., Nat. Phys. 12, 208–212 (2016).   

Superconductivity in TaSe2 epitaxial thin films

Transition metal dichalcogenides (TMDs) draw much attention because of their intriguing properties emerging at two-dimensional (2D) limit. Nowadays, some groups have succeeded in exfoliation of metallic TMDs such as NbSe₂ and TaS₂ down to a monolayer limit, unveiling new aspects of 2D physical properties. One remarkable example is unconventional superconductivity achieved by combination of large spin-orbit coupling and broken inversion symmetry, where spin-momentum-locked Cooper pairs play an important role in their superconducting states. Another unique feature is the relationship between superconductivity and charge-density wave (CDW), which has been discussed both in NbSe₂ and TaS₂. In this work, we focus on TaSe₂, which has been also known to have similar electronic structure and exhibit both superconductivity and CDW in its bulk form. There is one paper reporting electronic structure of monolayer TaSe₂ [1], but electrical transport properties of TaSe₂ ultrathin films have not been investigated so far. In this presentation, we report on fabrication of high-quality TaSe₂ thin films by molecular beam epitaxy, and show their transport properties with reduced dimensions.

[1] H. Ryu et al., Nano Lett. 18, 689-694 (2018)   

Sign reversing Hall effect in atomically thin high temperature superconductors

We fabricate van der Waals heterostructure devices using few unit cell thick Bi₂Sr₂CaCu₂O_8+δ for magnetotransport measurements. The superconducting transition temperature and carrier density in atomically thin samples can be maintained to close to that of the bulk samples. As in the bulk sample, the sign of the Hall conductivity is found to be opposite to the normal state near the transition temperature but with a drastic enlargement of the region of Hall sign reversal in the temperature-magnetic field phase diagram as the thickness of samples decreases. Quantitative analysis of the Hall sign reversal based on the excess charge density in the vortex core and superconducting fluctuations suggests a renormalized superconducting gap in atomically thin samples at the two-dimensional limit. Preprint available at arXiv:1809.06944 .   

Spatially Resolved Electronic Structure of Bi2Sr2-xLaxCuO6+δ in the Two-dimensional Limit

Dimensionality plays a fundamental role in high-temperature superconductivity; all high-temperature superconductors adopt a layered crystal structure, and much of high-temperature superconductor theory is based on purely two-dimensional (2D) models. Here we exfoliate Bi₂Sr_2-xLa_xCuO_6+δ (referred to as La-Bi2201) single crystals down to monolayers (half unit cell) at liquid nitrogen temperature under ultra-high vacuum. A monolayer La-Bi2201 contains only a single layer of CuO₂ plane, and therefore represents a cuprate superconductor in the ultimate 2D limit. We study the electronic structure of monolayer La-Bi2201 at atomic scale using scanning tunnelling microscopy and spectroscopy.   

Visualizing the electronic structure of monolayer Bi2Sr2CaCu2O8+δ

Although high-temperature superconductors are a complex and diverse family of materials, they all adopt a layered crystal structure, in which two-dimensional lattices stack together to form the three-dimensional bulk. This curious fact begs the question: does high-temperature superconductivity (HTS) exist in an isolated monolayer, and if so, is the two-dimensional HTS—and various other correlated phenomena related to HTS—different from its three-dimensional counterpart? The answer to these questions may provide important insights on the role of dimensionality in HTS. Here, we fabricate atomically thin cuprate Bi₂Sr₂CaCu₂O_8+δ that retains HTS down to monolayer (i.e. half unit cell) limit. The electronic structure of monolayer Bi₂Sr₂CaCu₂O_8+δ is probed with scanning tunneling microscopy and spectroscopy. Survey of the electronic phases – superconductivity, pseudogap, charge order and Mott insulating state – reveals that they are indistinguishable from those in the bulk. Out results, therefore, indicates that essential physics of HTS is contained in a monolayer Bi₂Sr₂CaCu₂O_8+δ.   

Tunable high-temperature superconductivity in monolayer Bi2Sr2CaCu2O8+δ

Dimensionality plays a central role in high-temperature superconductivity (HTS). Layered cuprate superconductors hold the record of T_c at ambient pressure. Cuprates in the two-dimensional (2D) limit would provide valuable insights on the nature of HTS and offer unprecedented tunability in their material properties. Half-unit-cell-thick single crystal of Bi₂Sr₂CaCu₂O_8+δ (referred to as monolayer Bi-2212) is such an ideal 2D system. Here we fabricate transport devices from monolayer Bi-2212 samples obtained by exfoliation and study the evolution of superconductivity as the dimensionality and doping level is varied. We find that superconductivity remains intact in monolayer Bi-2212. Finite size scaling analysis at superconductor-to-insulator transition (SIT) reveals distinct critical behaviors at different disorder levels. Our study leads to a unified picture of SITs previously reported in cuprate systems.