Session V15: 2D Materials: Charge Density Waves

Weakly Bound and Strongly Interacting: NbSe$_{2}$ and 1T-TaS$_{2}$ in the 2D Limit

The layered metallic dichalcogenides are known to exhibit rich collective electron phases such as charge density waves, spin density waves, and superconductivity. In the past, studies on graphene and various semiconducting dichalcogenides have shown that taking layered materials to their physical two-dimensional (2D) limit leads to fundamental changes in band structure, allowing for a powerful experimental knob to tune for electronic functionality. In contrast, due to their instability in the ambient environment, the effect of thickness control over such collective electron phases has been largely unexplored in metallic systems. We have recently demonstrated a new experimental platform for the isolation and assembly of environmentally sensitive 2D materials in inert atmosphere. I will discuss our recent studies of the charge density wave material 1T-TaS$_{2}$ and superconducting NbSe$_{2}$ in the atomically thin limit, made possible using this technique. For 1T-TaS$_{2}$, we find that the lock-in transition to commensurate charge ordering becomes increasingly metastable for reduced thickness, allowing for all-electrical control over this phase transition in the 2D state. In NbSe$_{2}$, a small magnetic field induces a transition to a quantum metallic phase, the resistivity of which obeys a unique field-scaling property. These methods and experiments open new doors for the study of other correlated 2D materials in the immediate future.   

Magnetic Field Studies Near Superconducting Transition in MBE Grown Monolayer NbSe2 on Bilayer Graphene

Following the work by Frindt [1] on the superconductivity of NbSe2 at reduced thicknesses, recent breakthroughs have enabled the study of bilayers and monolayers. Staley et. al. [2], Tsen et. al. [3], Cao et. al. [4] and Xi et. al. [5] have studied superconductivity in bilayers and monolayers of NbSe2 after mechanical exfoliation and encapsulation with another layered material to protect from air. In this work, we have investigated the superconductivity in monolayer NbSe2 prepared by molecular beam epitaxy growth (MBE) on bilayer graphene (BLG) [6]. The superconducting transition has an onset temperature of 1.9K, midpoint temperature of 0.65K and reaches zero resistance at 0.46K. The upper critical field perpendicular to the NbSe2 monolayer is 0.5T at 100mK. We will show the effect of magnetic fields near the superconducting transition and compare with existing theories. [1] R. Frindt, Phys. Rev. Lett. 28, 299 (1972). [2] N. E. Staley, et. al., Phys. Rev. B 80, 184505 (2009). [3] A. W. Tsen, et. al., arXiv:1507.08639 [cond-Mat.supr-Con] 1 (2015). [4] Y. Cao, et. al., Nano Lett. 15, 4914 (2015). [5] X. Xi, et. al., arXiv:1507.08731 [cond-Mat.supr-Con] 1 (2015). [6] M. M. Ugeda, et al., Nat. Phys. 10.1038/nphys3527 (2015).   

The Upper Critical Field of Bilayer NbSe2

We report on the fate of the superconducting state of bilayer NbSe$_2$ in a large parallel magnetic field. Due to strong spin-orbit coupling, the system exhibits an an out-of-plane polarization of electron spins in each valley, which leads to an enhanced upper critical field $H_{c2}^{||}$ as compared to that expected from the Pauli limit $H_p$ [1]. We explore the behavior of $H_{c2}^{||}(T)$ in the low temperature limit, down to $T = 0.3$K (approximately $0.06T_c$), and we find a maximum upper critical field of 28 T which is >3 times the Pauli limit. We find that the measured $H_{c2}^{||}(T)$ deviates significantly from the standard pair-breaking theory as $T \rightarrow 0$, and we compare our results to recent observations of Ising superconductivity in NbSe$_2$ [1] and in ionic-liquid-gated MoS$_2$ [2], as well as to calculations of $H_{c2}$ based on realistic band structure of NbSe$_2$. References: [1] Xi et al. arXiv:1507.08731 (2015) [2] Saito et al. arXiv:1506.04146 (2015)   

Superconductivity and charge density waves in atomically thin NbSe$_{\mathrm{2}}$

Atomically thin van der Waals materials have emerged as a frontier for both fundamental physics and device applications. Although novel single-particle and excitonic properties have been extensively studied, the collective electron phenomena in these materials remain less well understood. In this talk, we will discuss superconductivity and charge-density-wave (CDW) order in atomically thin group-V transition metal dichalcogenide NbSe$_{\mathrm{2}}$ down to the monolayer limit. Electrical transport measurements show that the superconducting transition temperature decreases monotonically with reducing the layer thickness. The temperature dependent Raman scattering, on the other hand, shows enhanced CDW order as the sample thickness reduces. While the former can be understood mainly as the result of reduced interlayer Cooper pairing, the latter arises from the enhanced electron-phonon coupling in atomically thin samples. Magnetotransport measurements further reveal the effect of spin-momentum locking, a consequence of broken inversion symmetry and strong spin-orbit coupling in monolayer NbSe$_{\mathrm{2}}$, on Cooper pairing and the in-plane upper critical fields. These results set the stage for the exploration and control of collective electronic phases in 2D NbSe$_{\mathrm{2}}$ and related systems.   

Charge Density Waves in the bulk and mono-layer VSe2

Charge density waves (CDWs) are widely observed in the layered transition-metal dichalcogenides (TMDs). With the capability of preparing atomically thin samples in the experiment, the underlying mechanism of the formation of CDWs and the role played by dimensionality in TMDs can now be studied in great detail. We present the first-principles calculations on bulk and mono-layer VSe2. Our results agree with the experimental findings that the dominant CDW phase has a 4x4x3 supercell structure in the bulk system. Electronic structure calculations suggest Fermi-surface nesting is a relevant mechanism. On the other hand, we find a new 3{\$}$\backslash $sqrt\textbraceleft 3\textbraceright $\backslash $times$\backslash $sqrt\textbraceleft 3\textbraceright {\$} CDW phase as the lowest energy structure in the mono-layer case induced by strong electron-phonon interaction. We also find that substantial hole doping leads to a CDW-superconducting (SC) phase transition. The SC transition temperature is predicted to be higher than that of the bulk from our first-principles calculations.   

Scanning Tunneling Microscopy and Spectroscopy of Graphene on NbSe$_2$

A wide range of phenomena can be induced in graphene by creating vertical heterostructures with other two-dimensional materials. NbSe$_2$ is a layered transition metal dichalcogenide that exhibits a charge density wave transition below $T_{cdw} = 33$ K and then becomes superconducting below $T_c = 7.2$ K. By placing monolayer graphene on NbSe$_2$ the interplay between charge density waves, superconductivity and Dirac fermions can be explored. We use low temperature scanning tunneling microscopy and spectroscopy to study the electronic properties of this van der Waals heterostructure. We observe the coexistence of a moiré pattern and charge density wave in the graphene on NbSe$_2$ heterostructure.   

Scanning Tunneling Microscopy Study of Atomic and Electronic Structures of PbTaSe2

The non-centrosymmetric PbTaSe2 becomes superconducting at Tc $=$ 3.7K and is proposed to have a 3D massive Dirac fermions by large spin orbital coupling. The observation of topological nodal line states has been reported by recent ARPES measurements, making this material a great candidate to investigate the coupling between topological states and superconductivity. Here we conduct detail studies on cleaved PbTaSe2 surfaces by spectroscopic imaging-scanning tunneling microscope. Our results reveal several types of cleaved surfaces, within which each exhibits distinct different LDOS from scanning tunneling spectroscopy measurements. We identify different surface terminations from their atomic structures and their corresponding electronic properties both above and below Tc. We will report the impact on superconducting properties of different surfaces, and also discuss the relation between the surface state and superconductivity.   

Zone-center phonons of bulk, few-layer, and monolayer 1T-TaS2: Application to Raman scattering

The transition metal dichalcogenide 1T-TaS$_2$ has attracted attention for decades due to its multiple charge density wave phases. More recently it is being considered as a 2D device material, due to the wide range of electrical conductivities in these phases. The metal-insulator transition that occurs when the commensurate charge density wave forms is particularly attractive. We present first-principles calculations of the vibrational properties of 1T-TaS$_2$ for various thicknesses in the high-temperature (undistorted) phase and the low-temperature commensurate charge density wave phase. We also present measurements of the Raman frequencies for bulk and few-layer samples in the low-T phase. We find strong evidence for the low-T commensurate charge density wave state remaining stable as the crystal is thinned, even down to one layer. We explore the effects of substrate-induced strain on the vibrational spectrum and propose polarized Raman spectroscopy as a method for quickly identifying the c-axis orbital texture in the low-T phase. This orbital texture has recently been identified as playing a role in the metal-insulator transition.   

Structural, electronic and vibrational properties of few-layer 2H- and 1T-TaSe$_2$

Two-dimensional metallic transition metal dichalcogenides (TMDs) are of interest for studying phenomena such as charge-density wave (CDW) and superconductivity. Few-layer tantalum diselenides (TaSe$_2$) are typical metallic TMDs exhibiting rich CDW phase transitions. However, a description of the structural, electronic and vibrational properties for different crystal phases and stacking configurations, essential for interpretation of experiments, is lacking. We present first-principles calculations of structural phase energetics, band dispersion near the Fermi level, phonon properties and vibrational modes at the Brillouin zone center for different layer numbers, crystal phases and stacking geometries. Evolution of the Fermi surfaces as well as the phonon dispersions as a function of layer number reveals dramatic dimensionality effects in this CDW material. Our results indicate strong electronic interlayer coupling, detail energetically possible stacking geometries, and provide a basis for interpretation of Raman spectra.   

Memristive phase switching in two-dimensional 1T-TaS2 crystals

Among 2D materials with correlated electrons, 1T-TaS2 is one of the most attracting systems with charge density wave (CDW) phases [1]. In this presentation, we report an electrical switching between various non-volatile metastable electronic phases in 1T-TaS2 thin flakes. By applying a high lateral electric field, we realized multiple metastable states, where the system shows truly metallic behavior. The emergence of novel ground states, possibly stabilized by the slow kinetics due to the reduced dimensionality [2], reflects the electronic complexity in 2D materials with nanometer thickness. [1] M. Yoshida et al. Sci. Rep. 4, 7302 (2014); [2] M. Yoshida et al. Sci. Adv. 1, e1500606 (2015).