The layered transition metal dichalcogenides host a rich collection of charge density wave phases in which both the conduction electrons and the atomic structure display translational symmetry breaking. Manipulating these complex states by purely electronic methods has been a long-sought scientific and technological goal. Here, we show how this can be achieved in 1T-TaS 2 in the 2D limit. We first demonstrate that the intrinsic properties of atomically thin flakes are preserved by encapsulation with hexagonal boron nitride in inert atmosphere. We use this facile assembly method together with transmission electron microscopy and transport measurements to probe the nature of the 2D state and show that its conductance is dominated by discommensurations. The discommensuration structure can be precisely tuned in few-layer samples by an in-plane electric current, allowing continuous electrical control over the discommensuration-melting transition in 2D.two-dimensional materials | strongly correlated systems | charge density waves L ayered 1T-TaS 2 exhibits a number of unique structural and electronic phases. At low temperature and ambient pressure, the ground state is a commensurate (C) charge density wave (CDW). On heating, it undergoes a sequence of first-order phase transitions to a nearly commensurate (NC) CDW at 225 K, to an incommensurate (IC) CDW at 355 K, and finally to a metallic phase at 545 K. Each transition involves both conduction electron and lattice degrees of freedom-large changes in electronic transport properties occur, concomitant with structural changes to the crystal. By either chemical doping or applying high pressures, it is possible to suppress the CDWs and induce superconductivity (1-3). For device applications, it is desirable to control these phases by electrical means, but this capability is difficult to achieve in bulk crystals due to the high conduction electron density. Recent efforts to produce thin samples by mechanical exfoliation provide a new avenue for manipulating the CDWs in 1T-TaS 2 (4-8). These studies have demonstrated the suppression of CDW phase transitions using polar electrolytes, as well as resistive switching between the different phases. As the material approaches the 2D limit, however, significant changes have been observed in the transport properties (4,5,8). However, the microscopic nature of the 2D state remains unclear. In this work, we use transmission electron microscopy (TEM) together with transport measurements to develop a systematic understanding of the CDW phases and phase transitions in ultrathin 1T-TaS 2 . We find that charge ordering disappears in flakes with few atomic layers due to surface oxidation. When samples are instead environmentally protected, the CDWs persist and their transitions can be carefully tuned by electric currents.Both the atomic and CDW structure of 1T-TaS 2 can be visualized in reciprocal space by TEM electron diffraction (9, 10). In Fig. 1A, we show diffraction images taken from a bulk-like, 50-nm-thick crystal at low and room tem...

Unconventional superconductivity in the cuprates coexists with other types of electronic order. However, some of these orders are invisible to most experimental probes because of their symmetry. For example, the possible existence of superfluid stripes is not easily validated with linear optics, because the stripe alignment causes interlayer superconducting tunneling to vanish on average. Here we show that this frustration is removed in the nonlinear optical response. A giant terahertz third harmonic, characteristic of nonlinear Josephson tunneling, is observed in LaBaCuO above the transition temperature = 13 kelvin and up to the charge-ordering temperature = 55 kelvin. We model these results by hypothesizing the presence of a pair density wave condensate, in which nonlinear mixing of optically silent tunneling modes drives large dipole-carrying supercurrents.

Motivated by recent pump-probe experiments indicating enhanced coherent c-axis transport in underdoped YBCO, we study Josephson junctions periodically driven by optical pulses. We propose a mechanism for this observation by demonstrating that a parametrically driven Josephson junction shows an enhanced imaginary part of the low-frequency conductivity when the driving frequency is above the plasma frequency, implying an effectively enhanced Josephson coupling. We generalize this analysis to a bilayer system of Josephson junctions modeling YBCO. Again, the Josephson coupling is enhanced when the pump frequency is blue detuned to either of the two plasma frequencies of the material. We show that the emergent driven state is a genuine, nonequilibrium superconducting state, in which equilibrium relations between the Josephson coupling, current fluctuations, and the critical current no longer hold. DOI: 10.1103/PhysRevLett.117.227001 Recent pump-probe experiments on high-temperature superconductors such as YBCO revealed transiently enhanced superconducting-like states both below and above their critical temperatures T c [1][2][3][4]. The origin of these transient superconducting states has not been identified yet. Several ideas have been proposed: nonlinear phononic effects [5], parametric cooling [6,7], competing orders [8,9], and redistribution of phase fluctuations [10]. In this Letter, we model the layered structure of YBCO as a Josephson junction chain, and demonstrate that below the critical temperature the parametric driving of these junctions enhances the Josephson coupling in the steady state [11]. We extract this quantity from the 1=ω divergence of the imaginary part of the conductivity σðωÞ as the frequency ω approaches zero.So far, most of the theoretical studies have considered quantities such as the power spectrum of currents [10], or the dc current response [6]. While these quantities give important insight into the system, here we discuss the optical conductivity itself by including the probing field in our calculation to obtain the actual nonequilibrium response. The Kubo formula or other equilibrium methods are not used. This achievement is crucially important because the conclusions of Refs. [1][2][3][4] are based precisely on this quantity. We note that while experiments indicate stiffening of the superfluid density both below and above T c , here we focus on cases below T c , in which condensed Cooper pairs are safely assumed. We will discuss cases with fluctuating Cooper pairs just above T c (below the pseudogap temperature) elsewhere. Complete understanding how the interlayer coherence is dynamically enhanced may provide new theoretical and experimental means to investigate fluctuating orders above T c .In this Letter, we first study a single Josephson junction. We show that a driving frequency just above the plasma frequency leads to strong enhancement of the Josephson coupling, derived from the low-frequency conductivity. This arises because the nonlinear driving term couples the probe puls...

Recent pump-probe experiments reported an enhancement of superconducting transport along the c axis of underdoped YBa2Cu3O 6+δ (YBCO), induced by a midinfrared optical pump pulse tuned to a specific lattice vibration. To understand this transient nonequilibrium state, we develop a pump-probe formalism for a stack of Josephson junctions, and we consider the tunneling strengths in the presence of modulation with an ultrashort optical pulse. We demonstrate that a transient enhancement of the Josephson coupling can be obtained for pulsed excitation and that this can be even larger than in a continuously driven steady state. Especially interesting is the conclusion that the effect is largest when the material is parametrically driven at a frequency immediately above the plasma frequency, in agreement with what is found experimentally. For bilayer Josephson junctions, an enhancement similar to that experimentally is predicted below the critical temperature Tc. This model reproduces the essential features of the enhancement measured below Tc. To reproduce the experimental results above Tc, we will explore extensions of this model, such as in-plane and amplitude fluctuations, elsewhere.

We employ low-frequency Raman spectroscopy to study the nearly commensurate (NC) to commensurate (C) charge density wave (CDW) transition in 1T-TaS2 ultrathin flakes protected from oxidation. We identify new modes originating from C phase CDW phonons that are distinct from those seen in bulk 1T-TaS2. We attribute these to CDW modes from the surface layers. By monitoring individual modes with temperature, we find that surfaces undergo a separate, low-hysteresis NC-C phase transition that is decoupled from the transition in the bulk layers. This indicates the activation of a secondary phase nucleation process in the limit of weak interlayer interaction, which can be understood from energy considerations.

Transport studies of atomically thin 1T-TaS 2 have demonstrated the presence of intermediate resistance states across the nearly commensurate (NC) to commensurate (C) charge density wave (CDW) transition, which can be further switched electrically. While this presents exciting opportunities for memristor applications, the switching mechanism could be potentially attributed to the formation of inhomogeneous C and NC domains. Here, we present combined electrical driving and photocurrent imaging of ultrathin 1T-TaS 2 in a heterostructure geometry. While micron-sized CDW domains are seen upon cooling, electrically driven transitions are largely uniform, indicating that the latter likely induces true metastable CDW states, which we then explain by a free energy analysis. Additionally, we are able to perform repeatable and bidirectional switching across the intermediate states without changing sample temperature, demonstrating that atomically thin 1T-TaS 2 can be further used as a robust and reversible multimemristor material for the first time.

Analysis of the spatial dependence of current-voltage characteristics obtained from scanning tunneling microscopy experiments indicates that the charge density wave (CDW) occurring in NbSe2 is subject to locally strong pinning by a non-negligible density of defects, but that on the length scales accessible in this experiment the material is in a "Bragg glass" phase where dislocations and antidislocations occur in bound pairs and free dislocations are not observed. A Landau theory-based analysis is presented showing how a strong local modulation may produce only a weak long range effect on the CDW phase. The effect of disorder on the properties of condensed matter systems is important both in terms of fundamental physics and of technological applications. In charge density wave (CDW) systems, randomly positioned impurities provide a random field which couples linearly to the order parameter [1]. Theory dating back to the 1970s indicates that if the impurity potential is strong enough, the random field destroys the charge density wave completely, leading to a phase with exponentially decaying correlations and a correlation length of the order of the mean distance between impurities [2, 3]. Subsequent work revised this picture, showing that in spatial dimensions d = 3, weak impurity pinning may lead instead to a topologically ordered "Bragg glass" phase with powerlaw density correlations [4][5][6][7][8][9][10][11][12].While the physics of random field systems has been of intense theoretical interest, experimental information has mainly come from transport and scattering measurements which average over large sample volumes [1,[13][14][15][16][17]. An important exception is the flux lattice decoration experiments which provided important early support to the Bragg glass picture for vortices in superconductors [18,19]. The development of stable scanning tunneling spectroscopy (STS) techniques which provide atomic-resolution imaging of local electronic density over wide fields of view has opened up new avenues for investigation of fundamental electronic physics, in particular providing real-space information on the effects of disorder on electronically ordered states [20,21]. In this paper, we present an analysis of scanning tunneling spectroscopy measurements carried out on NbSe 2 , a representative charge density wave system. The analysis motivates a Landau theory which provides insights into the effects of strong pinning in charge density wave systems.NbSe 2 is a quasi-two dimensional material. Its unit cell consists of two blocks of Se-Nb-Se layers; the Nb atoms in each layer form a triangular lattice and the electrical conductivity is strongly anisotropic, being much larger for in-plane currents than for currents flowing perpendicular to the layers [22]. Scattering measurements [23] indicate that a second order phase transition occurs at

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