Engineered 2D Quantum Materials

Hydrodynamic flow of electrons in graphene has garnered significant attention over the past decade, emerging as a solid-state platform which can be used to probe the physics associated with relativistic plasma, black holes, and quantum gravity [1,2]. Particularly near the charge neutrality point, graphene is expected to behave like a “Dirac fluid” [3], with its shear viscosity per unit entropy density (η/s) reaching a universal holographic lower bound, ħ/4πk_B where ħ is the reduced Planck’s constant and kB is the Boltzmann’s constant. However, direct experimental evidence of this is still lacking. In this work [4], we have fabricated hBN-encapsulated ultraclean graphene devices with exceptionally high electron mobilities (~ 10^6 cm^2 V^-1 s^-1) and performed electrical and thermal transport from room temperature down to 20 K. We observed a giant violation of the Wiedemann-Franz Law near the charge neutrality point across a range of temperatures T >> T_F. We also computed t...

Recent experiments have revealed the possibility of achieving sizeable Coulomb interaction driven gapped phases in dual gated rhombohedral multilayer graphene devices where carrier densities and perpendicular electric fields can be simultaneously controlled. Of particular importance are the pentalayer and tetralayer devices whose spontaneous gaps of the order of a few tens of meV are attributable to layer pseudospin polarization of the states near the Dirac point in chiral 2DEG systems. The layer pseudospin polarization can take place in a variety of ways leading to different Hall conductivities depending on the signs of the mass terms for each one of the spin-valley flavors. By means of mean-field Hartree-Fock approach we examine and compare the self-consistent solutions corresponding to the different pseudospin magnetic phases. Addition of a spin-orbit coupling term or moire potentials are expected to sensitively alter the ground-state of the system, allowing for example to split the...

The ground state of a one-dimensional spin-1/2 uniform antiferromagnetic Heisenberg chain (AfHc) is a Tomonaga-Luttinger liquid which is quantum-critical with respect to applied magnetic fields upto a saturation field Hs beyond which it transforms to a fully polarised state. Wilson ratio has been predicted to be a good indicator for demarcating these phases [Phys. Rev. B 96, 220401 (2017)]. From detailed temperature and magnetic field dependent magnetisation, magnetic susceptibility and specific heat measurements in a metalorganic complex and comparisons with field theory and quantum transfer matrix method calculations, the complex was found to be an excellent realisation of a spin-1/2 AfHc. Wilson ratio obtained from experimentally obtained magnetic susceptibility and magnetic contribution of specific heat values was used to map the magnetic phase diagram of the uniform spin- 1/2 AfHc over large regions of phase space demarcating Tomonaga-Luttinger liquid, saturation field quantum cri...

Trigonal PtBi2 is a layered Van der Waals semimetal without inversion symmetry, featuring 12 Weyl points in the vicinity of the Fermi energy. We present and discuss the experimental evidence that its topological Fermi arcs superconduct at low temperatures where bulk superconductivity is absent. With first-principles calculations we investigate in detail the bulk and surface electronic structure of PtBi2, and discuss the spin texture as well as the momentum-dependent localization of the arcs. Motivated by the experimentally observed recovery of inversion symmetry under pressure or upon doping, we interpolate between the two structures and determine the energy and momentum dependence of the Weyl nodes. For deeper insights into the surface superconductivity of PtBi2, we present a symmetry-adapted effective four-band model that accurately reproduces the Weyl points of PtBi2. We supplement this model with an analysis of the symmetry-allowed pairings between the Fermi arcs, which naturally m...

Golam Haider
Leibniz Institute for Solid State and Materials Research Dresden, Helmholtzstraße 20, 01069 Dresden, Germany
The physicochemical properties of van der Waals (vdW) heterostructures are governed by the delicate interactions between the individual layers in a multilayer stack. While addressing monolayer components of different compositions within the stack is feasible, exploring the intrinsic properties of a layer with the same composition presents a significant challenge. It becomes particularly important for determining the electron-phonon interaction and charge distribution on individual layers and disentangling their behavior. We have investigated the intrinsic strain associated with the coupling of twisted MoS2/MoSe2 heterobilayers by combining experiments and molecular dynamics simulations. The study reveals that small twist angles (between 0 and 2°) give rise to considerable atomic reconstructions, large moiré periodicities, and high levels of local strain (with an a...

Composite fermion (CF) is a topological quasiparticle that emerges from a non-perturbative attachment of vortices to electrons in strongly correlated two-dimensional materials. Similar to non-interacting fermions that form Landau levels in a magnetic field, CFs can fill analogous ``Lambda'' levels, giving rise to the fractional quantum Hall (FQH) effect of electrons. Here, we show that Lambda levels can be directly visualized through the characteristic peak structure in the signal obtained via spectroscopy with the scanning tunneling microscopy (STM) on a FQH state. Complementary to transport, which probes low-energy properties of CFs, we show that \emph{high-energy} features in STM spectra can be interpreted in terms of Lambda levels. We numerically demonstrate that STM spectra can be accurately modeled using Jain's CF theory. Our results show that STM provides a powerful tool for revealing the anatomy of FQH states and identifying physics beyond the non-interacting CF paradigm.

Interaction between the electrons and phonons determines some of the most fundamental properties of solids, including superconductivity, thermal and thermoelectric transport, polaronic effects, and electrical resistance in metal at high temperatures. In good metals, particularly noble metals such as gold (Au), silver (Ag), or copper (Cu), the coupling of electrons and phonons is rather weak [1], and the electron-phonon coupling constant () is small. The corresponding electron-
phonon scattering rate provides an excellent quantitative description of the metallic resistivity when the temperature () exceeds the Debye temperature (). The regime of however, remained experimentally inaccessible so far, raising questions on possible universal Planckian bound on scattering rate [2], polaronic deformation, or indeed, even the stability of a metallic state itself [3]. In this work [6], we demonstrate how coulomb interactions at the nanoscale interface of Au and Ag (of
radius nm) [4] that allow...

Recent technological advancements have enabled the preservation of near-perfect superconductivity and lattice structure in isolated, atomically thin Bi2Sr2CuCa2O8+δ (Bi-2212) crystals, facilitating the development of Bi-2212-based junctions [1,2]. These advancements focus on controlling the diffusion of oxygen interstitials, a key factor causing disorder in Bi-2212 cuprates. While intrinsic local lattice distortions in pristine cuprates [3] may contribute minimally without affecting the d-wave nature of the dominant order parameter, lattice distortions due to oxygen interstitials diffusion above 200 K [4,5] are detrimental. To counter this, a cryogenic stacking protocol has been developed, freezing oxygen interstitial motion at temperatures well below 200 K and rapidly establishing the interface in an ultra-low moisture environment [6-8]. This method has led to the creation of artificial intrinsic Josephson junctions, which show a strong dependence of Josephson energy on the twist angl...