arXiv subjects

Somesh Chandra Ganguli

Publications and source records attributed to Somesh Chandra Ganguli.

17 recordsLinked to original sources

arXiv2024

Doped Mott phase and charge correlations in monolayer 1T-NbSe$_2$

The doped Hubbard model is one of the paradigmatic platforms to engineer exotic quantum many-body states, including charge-ordered states, strange metals and unconventional superconductors. While undoped and doped correlated phases have been experimentally realized in a variety twisted van der Waals materials, experiments in monolayer materials, and in particular 1T transition metal dichalcogenides, have solely reached the conventional insulating undoped regime. Correlated phases in monolayer two-dimensional materials have much higher associated energy scales than their twisted counterparts, making doped correlated monolayers an attractive platform for high temperature correlated quantum matter. Here, we demonstrate the realization of a doped Mott phase in a van der Waals dichalcogenide 1T-NbSe$_2$ monolayer. The system is electron doped due to electron transfer to a monolayer van der Waals substrate via proximity, leading to a correlated triangular lattice with both half-filled and fully-filled sites. We analyze the distribution of the half-filled and filled sites and show the arrangement is unlikely to be controlled by disorder alone, and we show that the presence of competing non-local many-body correlations would account for the charge correlations found experimentally. Our results establish 1T-NbSe$_2$ as a potential monolayer platform to explore correlated doped Mott physics in a frustrated lattice.

cond-mat.str-el↗

arXiv2023

Evidence of nodal superconductivity in monolayer 1H-TaS$_2$ with hidden order fluctuations

Unconventional superconductors represent one of the fundamental directions in modern quantum materials research. In particular, nodal superconductors are known to appear naturally in strongly correlated systems, including cuprate superconductors and heavy-fermion systems. Van der Waals materials hosting superconducting states are well known, yet nodal monolayer van der Waals superconductors have remained elusive. Here, using low-temperature scanning tunneling microscopy (STM) and spectroscopy (STS) experiments, we show that pristine monolayer 1H-TaS$_2$ realizes a nodal superconducting state. By including non-magnetic disorder, we drive the nodal superconducting state to a conventional gapped s-wave state. Furthermore, we observe the emergence of many-body excitations close to the gap edge, signalling a potential unconventional pairing mechanism. Our results demonstrate the emergence of nodal superconductivity in a van der Waals monolayer, providing a building block for van der Waals heterostructures exploiting unconventional superconducting states.

cond-mat.supr-con↗

arXiv2023

Visualization of moiré magnons in monolayer ferromagnet

Two-dimensional magnetic materials provide an ideal platform to explore collective many-body excitations associated with spin fluctuations. In particular, it should be feasible to explore, manipulate and ultimately design magnonic excitations in two-dimensional van der Waals magnets in a controllable way. Here we demonstrate the emergence of moiré magnon excitations, stemming from the interplay of spin-excitations in monolayer CrBr$_3$ and the moiré pattern stemming from the lattice mismatch with the underlying substrate. The existence of moiré magnons is further confirmed via inelastic quasiparticle interference, showing the appearance of a dispersion pattern correlated with the moiré length scale. Our results provide a direct visualization in real-space of the dispersion of moiré magnons, demonstrating the versatility of moiré patterns in creating emerging many-body excitations.

cond-mat.mes-hall↗

arXiv2022

Confinement-engineered superconductor to correlated-insulator transition in a van der Waals monolayer

Transition metal dichalcogenides (TMDC) are a rich family of two-dimensional materials displaying a multitude of different quantum ground states. In particular, d$^3$ TMDCs are paradigmatic materials hosting a variety of symmetry broken states, including charge density waves, superconductivity, and magnetism. Among this family, NbSe$_2$ is one of the best-studied superconducting materials down to the monolayer limit. Despite its superconducting nature, a variety of results point towards strong electronic repulsions in NbSe$_2$. Here, we control the strength of the interactions experimentally via quantum confinement and use low-temperature scanning tunneling microscopy (STM) and spectroscopy (STS) to demonstrate that NbSe$_2$ is in close proximity to a correlated insulating state. This reveals the coexistence of competing interactions in NbSe$_2$, creating a transition from a superconducting to an insulating quantum correlated state by confinement-controlled interactions. Our results demonstrate the dramatic role of interactions in NbSe$_2$, establishing NbSe$_2$ as a correlated superconductor with competing interactions.

cond-mat.mes-hall↗

arXiv2022

Moiré-enabled topological superconductivity

The search for artificial topological superconductivity has been limited by the stringent conditions required for its emergence. As exemplified by the recent discoveries of various correlated electronic states in twisted van der Waals materials, moiré patterns can act as a powerful knob to create artificial electronic structures. Here, we demonstrate that a moiré pattern between a van der Waals superconductor and a monolayer ferromagnet creates a periodic potential modulation that enables the realization of a topological superconducting state that would not be accessible in the absence of the moiré. The magnetic moiré pattern gives rise to Yu-Shiba-Rusinov minibands and periodic modulation of the Majorana edge modes that we detect using low-temperature scanning tunneling microscopy (STM) and spectroscopy (STS). Moiré patterns and, more broadly, periodic potential modulations are powerful tools to overcome the conventional constrains for realizing and controlling topological superconductivity.

cond-mat.mes-hall↗

arXiv2021

Artificial heavy fermions in a van der Waals heterostructure

Heavy fermion systems represent one of the paradigmatic strongly correlated states of matter. They have been used as a platform for investigating exotic behavior ranging from quantum criticality and non-Fermi liquid behavior to unconventional topological superconductivity. Heavy fermions arise from the exchange interaction between localized magnetic moments and conduction electrons that leads to the well-known Kondo effect. In a Kondo lattice, the interaction between the localized moments gives rise to a band with heavy effective mass. This intriguing phenomenology has so far only been realized in compounds containing rare-earth elements with 4f or 5f electrons. Here, we realize a designer van der Waals heterostructure where artificial heavy fermions emerge from the Kondo coupling between a lattice of localized magnetic moments and itinerant electrons in a 1T/1H-TaS$_2$ heterostructure. We study the heterostructure using scanning tunneling microscopy (STM) and spectroscopy (STS) and show that depending on the stacking order of the monolayers, we can either reveal the localized magnetic moments and the associated Kondo effect, or the conduction electrons with a heavy-fermion hybridization gap. Our experiments realize an ultimately tuneable platform for future experiments probing enhanced many-body correlations, dimensional tuning of quantum criticality, and unconventional superconductivity in two-dimensional artificial heavy-fermion systems.

cond-mat.mes-hall↗

arXiv2020

Electronic and magnetic characterization of epitaxial CrBr$_3$ monolayers

The ability to imprint a given material property to another through proximity effect in layered two-dimensional materials has opened the way to the creation of designer materials. Here, we use molecular-beam epitaxy (MBE) for a direct synthesis of a superconductor-magnet hybrid heterostructure by combining superconducting niobium diselenide (NbSe$_2$) with the monolayer ferromagnetic chromium tribromide (CrBr$_3$). Using different characterization techniques and density-functional theory (DFT) calculations, we have confirmed that the CrBr$_3$ monolayer retains its ferromagnetic ordering with a magnetocrystalline anisotropy favoring an out-of-plane spin orientation. Low-temperature scanning tunneling microscopy (STM) measurements show a slight reduction of the superconducting gap of NbSe$_2$ and the formation of a vortex lattice on the CrBr$_3$ layer in experiments under an external magnetic field. Our results contribute to the broader framework of exploiting proximity effects to realize novel phenomena in 2D heterostructures.

cond-mat.mtrl-sci↗

arXiv2018

Signatures of two-step impurity mediated vortex lattice melting in Bose-Einstein Condensates

We simulate a rotating 2D BEC to study the melting of a vortex lattice in presence of random impurities. Impurities are introduced either through a protocol in which vortex lattice is produced in an impurity potential or first creating the vortex lattice in the absence of random pinning and then cranking up the (co-rotating) impurity potential. We find that for a fixed strength, pinning of vortices at randomly distributed impurities leads to the new states of vortex lattice. It is unearthed that the vortex lattice follow a two-step melting via loss of positional and orientational order. Also, the comparisons between the states obtained in two protocols show that the vortex lattice states are metastable states when impurities are introduced after the formation of an ordered vortex lattice. We also show the existence of metastable states which depend on the history of how the vortex lattice is created.

cond-mat.quant-gas↗

arXiv2016

Real Space Investigation of order-disorder transition of vortex lattice in Co-intercalated 2H-NbSe2

The work presented in my doctoral thesis is an experimental investigation of the nature of order to disorder transition of vortex lattice in a Type-II superconductor namely Co-intercalated 2H-NbSe2 using scanning tunnelling spectroscopy and complimentary ac susceptibility measurements. We have shown by performing real space imaging across the magnetic field driven peak effect that the vortex lattice in a 3-dimensional, weakly pinned Type-II superconductor disorders through two distinct topological phase transitions. Clear evidence of superheating and supercooling with different topological property of the metastable states from the actual ground states indicated the nature of both the phase transformations to be of first order. We have also demonstrated the role of underlying crystalline lattice symmetry by showing that it aligns the vortex lattice along preferential directions which is the lattice vectors of the crystal lattice itself. This underlying coupling also has profound implication in the order disorder transition indicating the significant role of vortex lattice-crystal lattice interaction in vortex phase diagram which was earlier being neglected.

cond-mat.supr-con↗

arXiv2016

Disorder-induced two-step melting of vortex matter in Co-intercalated NbSe$_2$ single crystals

Disorder induced melting, where the increase in positional entropy created by random pinning sites drives the order-disorder transition in a periodic solid, provides an alternate route to the more conventional thermal melting. Here, using real space imaging of the vortex lattice through scanning tunneling spectroscopy, we show that in the presence of weak pinning, the vortex lattice in a type II superconductor disorders through two distinct topological transitions. Across each transition, we separately identify metastable states formed through superheating of the low temperature state or supercooling of the high temperature state. Comparing crystals with different levels of pinning we conclude that the two-step melting is fundamentally associated with the presence of random pinning which generates topological defects in the ordered vortex lattice.

cond-mat.supr-con↗

arXiv2016

Orientational coupling between the vortex lattice and the crystalline lattice in a weakly pinned Co0.0075NbSe2 single crystal

We report experimental evidence of strong orientational coupling between the crystal lattice and the vortex lattice in a weakly pinned Co-doped NbSe2 single crystal through direct imaging using low temperature scanning tunneling microscopy/spectroscopy. At low fields, when the magnetic field is applied along the six-fold symmetric c-axis of the NbSe2 crystal, the equilibrium configuration of the vortex lattice is preferentially aligned along the basis vectors of the crystal lattice. The orientational coupling between the vortex lattice and crystal lattice becomes more pronounced as the magnetic field is increased. We show that this coupling enhances the stability of the orientational order of the vortex lattice, which persists even in the disordered state at high fields where dislocations and disclinations have destroyed the topological order.

cond-mat.supr-con↗

arXiv2015

Real space imaging of metastable Bragg glass states in a weakly pinned Type II superconductor

The breakdown of crystalline order in a disordered background connects to some of the most challenging problems in condensed matter physics. For a superconducting vortex lattice, the equilibrium state in the presence of impurities is predicted to be a "Bragg glass" (BG), where the local crystalline order is maintained everywhere and yet the global positional order decays algebraically. Here, using scanning tunnelling spectroscopy (STS) we image the vortex lattice in a weakly pinned NbSe2 single crystal. We present direct evidence that the ordered state of the VL is a BG, consisting of a large number of degenerate metastable states, which is a hallmark of a glassy state. These results are a significant step towards understanding the disordering of a lattice under the influence of quenched random disorder with a direct impact on various fields, including charge density waves, colloidal crystals and self-organised periodic structures on a substrate.

cond-mat.supr-con↗

arXiv2015

Two step disordering of the vortex lattice across the peak effect in a weakly pinned Type II superconductor, Co0.0075NbSe2

The vortex lattice in a Type II superconductor provides a versatile model system to investigate the order-disorder transition in a periodic medium in the presence of random pinning. Here, using scanning tunnelling spectroscopy in a weakly pinned Co0.0075NbSe2 single crystal, we show that at low temperatures, the vortex lattice in a 3-dimensional superconductor disorders in two steps across the peak effect. At the onset of the peak effect, the equilibrium Bragg glass transforms into an orientational glass through the proliferation of dislocations. At a higher field, the dislocations dissociate into isolated disclination giving rise to an amorphous vortex glass. We also show the existence of a variety of additional non-equilibrium metastable states, which can be accessed through different thermomagnetic cycling.

cond-mat.supr-con↗

arXiv2013

A 350 mK, 9 T scanning tunneling microscope for the study of superconducting thin films and single crystals

We report the construction and performance of a low temperature, high field scanning tunneling microscope (STM) operating down to 350mK and in magnetic fields up to 9T, with thin film deposition and in-situ cleaving capabilities. The main focus lies on the simple design of STM head allowing top loading of the sample and innovative sample holder design that allows us to get spectroscopic data on superconducting thin films grown in-situ on insulating substrates. Other design details on sample transport, chamber and vibration isolation scheme are also described. We demonstrate the capability of our instrument through the atomic resolution imaging and spectroscopy on NbSe2 single crystal and spectroscopic maps obtained on homogeneously disordered NbN thin film.

cond-mat.supr-con↗

arXiv2013

Emergence of nanoscale inhomogeneity in the superconducting state of a homogeneously disordered conventional superconductor, NbN

The notion of spontaneous formation of an inhomogeneous superconducting state is at the heart of most theories attempting to understand the superconducting state in the presence of strong disorder. Using scanning tunneling spectroscopy and high resolution scanning transmission electron microscopy, we experimentally demonstrate that under the competing effects of strong homogeneous disorder and superconducting correlations, the superconducting state of a conventional superconductor, NbN, spontaneously segregates into domains. Tracking these domains as a function of temperature we observe that the superconducting domains persist across the bulk superconducting transition, Tc, and disappear close to the pseudogap temperature, T*, where signatures of superconducting correlations disappear from the tunneling spectrum and the superfluid response of the system.

cond-mat.supr-con↗

arXiv2012

Frequency dependent superfluid stiffness in the pseudogap regime in strongly disordered NbN thin films

We measure the frequency dependence of the complex ac conductivity of NbN films with different levels of disorder in frequency range 0.4-20 GHz. Films with low disorder exhibit a narrow dynamic fluctuation regime above T_c as expected for a conventional superconductor. However, for strongly disordered samples, the fluctuation regime extends well above T_c, with a strongly frequency-dependent superfluid stiffness which disappears only at a temperature T* close to the pseudogap temperature obtained from scanning tunneling measurements. Such a finite-frequency response is associated to a marked slowing down of the superconducting fluctuations already below T*. The corresponding large length-scale fluctuations suggest a scenario of thermal phase fluctuations between superconducting domains in a strongly disordered s-wave superconductor.

cond-mat.supr-con↗

arXiv2012

Observation of Andreev bound state and multiple energy gaps in the non-centrosymmetric superconductor BiPd

We report directional point contact Andreev reflection (PCAR) measurements on high-quality single crystals of the non-centrosymmetric superconductor, BiPd. The PCAR spectra measured on different crystallographic faces of the single crystal clearly show the presence of multiple superconducting energy gaps. For point contacts with low resistance, in addition to the superconducting gap feature, a pronounced zero bias conductance peak is observed. These observations provide strong evidence of the presence of unconventional order parameter in this material.

cond-mat.supr-con↗