ICTS

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 mix spin-singlet and spin-triplet channels. Moreover, the presence of surface-only superconductivity facilitates an intrinsic superconductor-semimetal-superconductor Josephson junction, with the semimetallic phase sandwiched between the two superconducting surfaces. For a phase difference of \pi, zero-energy Andreev bound states develop between the two terminations.

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 angle, exhibiting unique properties at a 45° twist [6-8]. Notably, these junctions display fractional Shapiro steps and Fraunhofer patterns, indicating two degenerate Josephson ground states with time-reversal symmetry (TRS). The ability to control the junction current bias sequence to selectively break TRS allows the junction to enter either ground state. This discovery has prompted the proposal of a novel capacitively shunted qubit, termed 'flowermon,' characterized by its d-wave order parameter that offers inherent protection against charge-noise-induced relaxation and quasiparticle-induced dissipation [9]. The flowermon signifies a step towards high-coherence, hybrid superconducting quantum devices using unconventional superconductors. To fabricate complex circuits like those needed for the flowermon, a new technique involving cryogenic dry transfer of printable circuits embedded in a silicon nitride membrane has been developed [10]. This technique separates the circuit fabrication process, which involves chemical and physical stresses, from the creation of thin superconducting structures, thereby providing electrical contacts in a single step and protecting the superconducting surface from environmental damage.

References
[1] S. Y. F. Zhao, et al., Physical Review Letters 122, 247001 (2019).
[2] N. Poccia, et al., Physical Review Materials 4, 114007 (2020).
[3] N. Poccia, et al. Proceedings of the National Academy of Sciences 109, 15685-15690 (2012).
[4] N. Poccia, et al., Nature Materials 10, 733 (2011).
[5] M. Fratini, et al. Nature 466, 841 (2010).
[6] S. Y. F. Zhao, et al., Science 382, 1422 (2023).
[7] Y. Lee, et al., Advanced Materials 35, 2209135 (2023).
[8] M. Martini, et al., Materials Today 67, 106 (2023).
[9] V. Brosco, et al., Physical Review Letters 132, 017003 (2024).
[10] C. N. Saggau, et al., ACS Applied Materials & Interfaces 15, 51558-51564 (2023).
 

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.

The fast-paced improvements in ultrafast radiation from X-ray, ultraviolet, visible, and infrared to terahertz frequencies is enabling simultaneous probing of electron, phonon, and spin dynamics on the ps-to-ns time scales, as well as sub-micrometer length scales. In this talk, I will present brief introduction to nonlinear optics, followed by examples of the discovery of new low symmetry phases with large property enhancements in decades old ferroelectrics using nonlinear optical microscopy. In a second example, I will show how an ultrafast laser pulse can create a complex polar supertextures with modulation periodicities of tens of nanometers in oxide thin film heterostructures that have built-in frustration by design.

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 the η/s ratio for our devices from independent measurements of the shear viscosity and entropy density, using electrical conductance and electron noise thermometry respectively and found it to be within a factor of two of the holographic bound for the cleanest channels. Our experimental observations are also quantitatively consistent with that of a non-Galilean invariant Dirac fluid.

References

[1] S. A. Hartnoll et al., Phys. Rev. B 76, 144502 (2007).

[2] M. Muller et al., Phys. Rev. Lett. 103, 025301 (2009).

[3] P. K. Kovtun et al., Phys. Rev. Lett. 94, 11, 111601 (2005).

[4] A. Majumdar et al., Manuscript under preparation

With a rapidly growing family of vdW materials, the role of dielectric and metals have become more important than ever. In this talk, I will present challenges associated with the synthesis of atomically-precise three-dimensional (3D) perovskite nanomembranes followed by our group’s effort to address them. Using hybrid molecular beam epitaxy that employs a metal-organic precursor, titanium isopropoxide (TTIP), to supply both Ti and oxygen (without the need for additional oxygen), epitaxial SrTiO3 (STO) films were grown directly on a graphene layer transferred on to bulk STO substrate. Films were then successfully exfoliated and transferred onto other substrates. Using Raman spectroscopy and high-resolution X-ray diffraction, we show that the transferred STO membrane is single-crystalline and can be integrated with other vdW materials. I will also present sacrificial layer route to create oxide membranes resulting in room temperature dielectric constant of ~ 300. Finally, I will present several opportunities for materials physics and devices engineering using 3D nanomembranes.

Observation of magnetic ordering in 2D layered materials at finite temperatures have drawn significant interest in recent years. Though Mermin-Wagner-Hohenburg theorem forbids long range ordering in 2D systems, anisotropy can lead to spin ordering at finite temperatures. One of such classes of 2D magnets explored recently is the transition metal phosphorus trisulphides (MPS3, M = Mn, Fe, and Ni) that hosts antiferromagnetic (AFM) ground state at low temperatures [1]. The AFM ground state exhibits different spin dimensionalities, (viz., n=1,2, and 3) due to the presence of an axial or planar anisotropy or in the absence of any anisotropic element [2] which may be described by the Ising (e.g., FePS3), XY (e.g.,
NiPS3), and Heisenberg (e.g., MnPS3) Hamiltonians, respectively [1]. Engineered heterostructures of magnetic layered materials with high spin-orbit coupled systems like topological materials has the potential to control the quantum interactions unravelling a variety of exotic phenomena at their interfaces [3,4,5]. MPS3 systems exhibit extremely robust magnetic ground state, almost insensitive to their layer thickness [6-8]. We have demonstrated [9] the possibility to control the otherwise robust magnetic properties of transition-metal phosphorus trisulfides (Mn/Fe/NiPS3) in their heterostructures with Weyl semimetallic MoTe2 which can be attributed to the Dzyaloshinskii-Moriya (DM) interactions at the interface of the two materials. While the DM interaction is known to scale with the strength of the spin-orbit coupling (SOC), we demonstrate using Raman scattering on heterostructures with a variety of substrates (underlayers) hosting variable SOC and electronic density of states (DOS) that the effect of DM interaction strongly varies with the electronic DOS of the SOC-hosting layer as well as the spin orientation and degree of anisotropy associated with the magnetic layer. Furthermore, among the three trisulfides, NiPS3 exhibits XY type AFM state where the spins are oriented in-plane (while in the other two, the spins are oriented out-of- plane). Our extended study on this 2D magnet reveals intriguing magnetic excitation-phonon coupling being present even at much higher temperatures (T > 4TN) [10].

References:
[1] P. A. Joy and S. Vasudevan, Magnetism in the Layered Transition-Metal Thiophosphates M PS 3 (M= Mn, Fe, and Ni), Phys. Rev. B 46, 5425 (1992).
[2] M. Gibertini, M. Koperski, A. F. Morpurgo, and K. S. Novoselov, Magnetic 2D Materials and Heterostructures, Nat. Nanotechnol. 14, 408 (2019).

[3] H. Wang, Y. Liu, P. Wu, W. Hou, Y. Jiang, X. Li, C. Pandey, D. Chen, Q. Yang, and H. Wang, Above Room- Temperature Ferromagnetism in Wafer-Scale Two-Dimensional van Der Waals Fe3GeTe2 Tailored by a Topological Insulator, ACS Nano 14, 10045 (2020).
[4] C.-Z. Chang, Marriage of Topology and Magnetism, Nat. Mater. 19, 484 (2020).
[5] W. Zhao, Z. Fei, T. Song, H. K. Choi, T. Palomaki, B. Sun, P. Malinowski, M. A. McGuire, J.-H. Chu, and X. Xu,Magnetic Proximity and Nonreciprocal Current Switching in a Monolayer WTe2 Helical Edge, Nat. Mater. 19, 503 (2020).
[6] J.-U. Lee, S. Lee, J. H. Ryoo, S. Kang, T. Y. Kim, P. Kim, C.-H. Park, J.-G. Park, and H. Cheong, Ising-Type Magnetic Ordering in Atomically Thin FePS3, Nano Lett. 16, 7433 (2016).
[7] K. Kim, S. Y. Lim, J.-U. Lee, S. Lee, T. Y. Kim, K. Park, G. S. Jeon, C.-H. Park, J.-G. Park, and H. Cheong, Suppression of Magnetic Ordering in XXZ-Type Antiferromagnetic Monolayer NiPS3, Nat. Commun. 10, 1 (2019).
[8] Y.-J. Sun, Q.-H. Tan, X.-L. Liu, Y.-F. Gao, and J. Zhang, Probing the Magnetic Ordering of Antiferromagnetic MnPS3 by Raman Spectroscopy, J. Phys. Chem. Lett. 10, 3087 (2019).
[9] Suvodeep Paul, et al. Tuning the robust magnetic properties in MPS3 (M = Mn, Fe, and Ni) by proximity-induced Dzyaloshinskii-Moriya interactions, Phys. Rev. B 109, 085136 (2024).
[10] Devesh Negi, et al. Magnetic excitation-phonon coupling in NiPS3 at high temperatures: Possible signature of novel ground state (under review).

In recent years, the experimental realization of magnetic long-range order in atomically thin 2D materials has shown a big potential in spintronic applications in ultrathin magnets due to the possibility of manipulation of magnetism by external fields, strain or proximity effects in van der Waals heterostructures. Specifically, the family of metallic magnets FenGeTe2 (n=3, 4, 5) has attracted a huge attention due to their high Curie temperatures and intriguing properties. In this talk, I will review the status of this research field, highlighting our own research by ab initio density functional theory, calculations of interatomic exchange interaction parameters and Monte Carlo simulations. A particular emphasis will be given on the systematic study of the electronic structure and magnetism of FenGeTe2 magnets along with some critical discussions on the importance of electron correlation with the aid of dynamical mean field theory, spin-orbit coupling and effects of transition metal doping. Finally, some results on the spin-polarized quantum transport will be shown for PtTe2/Fe4GeTe2/PtTe2 van der Waals heterostructures.

[1] S. Ghosh, S. Ershadrad, V. Borisov, B. Sanyal, npj comp. mat. 9, 86 (2023).

[2] S. Ershadrad, S. Ghosh, D. Wang, Y. Kvashnin, B. Sanyal, J. Phys. Chem. Lett. 13, 4877 (2022).

[3] S. Ghosh, S. Ershadrad, B. Sanyal, 2D Mater. 11, 035002 (2024)

[4] M. Davoudiniya, B. Sanyal (submitted)

[5] R. Ngaloy et al., ACS Nano 18, 5240 (2024)

Complex oxides are endowed with a rich phase space that enables the study of emergent phenomena, exploiting strong correlation effects and topology. We study these using electrical transport schemes that allows us to manipulate competing ground states and exploiting spin orbit interactions. With rapid advances made in the palette of materials and devices available in Spintronics, our work shows the importance of complex oxide interfaces for the design and study of energy efficient devices for unconventional computing applications. In this talk, I will discuss a few examples of these in different complex oxide materials, ranging from skyrmion bubbles, coexisting magnetic order in antiferromagnets and energy-efficient hardware for computing applications beyond von Neumann.

Fluctuations and disorder effects are substantially enhanced in reduced dimensionalities. While they are mostly considered as the foe for long-range orders, fluctuations and disorders can also stimulate the emergence of novel phases of matter, for example, vestigial orders. Taking 2D magnetism as a platform, existing efforts have been focused on maintaining 2D long-range magnetic orders by suppressing fluctuations, whereas the other side, exploiting fluctuations for realizing new 2D magnetic phases, remains as an uncharted territory. We will use a suite of optical spectroscopy techniques to demonstrate the magnetic phase evolution of NiPS3 as functions of temperature and dimensionality and demonstrate the crossover from the 3D zigzag AFM to the 2D Potts nematic phase.

Graphene moiré superlattices host van Hove singularities appear at low energies, which are malleable with progressive band filling, leading to a sequence of Lifshitz transitions and resets observable in Hall measurements. However, at zero magnetic fields, transport measurements in the linear response regime have limited sensitivity to the band's topology. Here, we probe these unique features in twisted bilayer graphene at zero magnetic field using second-order transport measurements. We demonstrate that the nonlinear responses, induced by the Berry curvature dipole and extrinsic scattering processes, intricately map the Fermi surface reconstructions at various partial fillings of the band. Importantly, our study confirms that the applied magnetic field does not induce or stabilize the probed transitions, highlighting these features as intrinsic to the moiré bands. Additionally, we show the tunability of the Berry curvature dipole and extrinsic scattering process with an out-of-plane electric field near the conduction band edge. Beyond corroborating the insights from linear Hall measurements, our findings establish nonlinear transport as a pivotal tool for probing band topology and correlated phenomena.

Akshay Singh1, Mainak Mondal1, Soumadeep Saha1, Sushma Gali2, Kenji Watanabe3, Takashi Taniguchi4, Shankar Kumar Selvaraja2
1Department of Physics, Indian Institute of Science, Bengaluru, Karnataka 560012, India
2Centre for Nanoscience and Engineering, Indian Institute of Science, Bengaluru, India
3Research Center for Functional Materials, National Institute for Materials Science, Japan
4International Center for Materials Nanoarchitectonics, National Institute for Materials Science, Japan
*Email: aksy@iisc.ac.in

Abstract: In this talk I will first give a broad overview of Moire excitons in twisted 2D material heterostructures. I will then present our group’s recent work on localized excitations in near 0-degree twisted MoSe2/WSe2 heterostructures, where we observe several sub-meV peaks in photoluminescence (PL) spectra. Power-dependent PL suggests that certain moiré-interlayer exciton emission peaks start saturating after just 10 nW of excitation power, suggesting deep localization. Remarkably, the time-resolved PL measurements from two closely spaced emissions (separated by < 4 meV) show possible optical cascade nature between these states. We will speculate on the origin of these peaks. If time permits, I will discuss the concept of optical transition dipole (OTD), and our efforts to measure the orientation of OTD in these heterostructures.
 

Although d+id’ superconductors are topological, the Majorana zero modes therein occur in pairs. This talk will present the Fu-Kane setup between a three-dimensional topological insulator and a d+id’ superconductor to show that an unpaired Majorana Zero mode can be realized in the core of the Abrikosov vortex. The d+id superconducting order can be realized in High-Tc superconductors, thus these Majorana modes can persist up to unprecedented high temperatures.

Inspired by the observation of crystallization of the vortices in the intermediate field range in a Kitaev model [Phys. Rev B 108, 165118 (2023)], we delve into obtaining different superlattice formations of the vortices and the corresponding Majorana dispersions. This gives a novel possibility of governing flat bands for Majoranas and other possible excitations that may arise in analogous settings. We find several superlattice configurations of vortices where Majorana states are weakly dispersive or even fully flat with or without a gap. The gapped flat bands have a non-zero Chern number with a higher quantum metric value (bandwidth) tunable by varying applied magnetic fields. Having satisfied the ‘vortexibility condition,’ these bands can harbor fractional Chern insulator phase of the emergent Majorana fermions if appropriate interactions are present. Interestingly, we have used the projection method and maximally localized Wannier functions algorithm to identify the localizing centers of the nearly flat-band Majorana states to be around the fluxes.

1T-TaS2 exhibits several resistivity phases due to the modulation of charge density wave (CDW). The fact that such phase transition can be driven electrically has attracted a lot of attention in the recent past toward active-metal based electronics. However, the bias-driven resistivity switching is not very large (less than five-fold), and an enhancement in the same will highly impact such phase transition devices. One aspect that is often overlooked is that such phase transition is also accompanied by a significant change in the local temperature due to the low thermal conductivity of 1T-TaS2. In this work, such electrically driven phase transition induced temperature change is exploited to promote carriers over a thermionic barrier in a 1T-TaS2/2H-TaSe2/2H-MoS2 T-Junction, achieving a 964-fold abrupt switching in the current through the MoS2 channel. The device is highly reconfigurable and exhibits an abrupt reduction in current as well when the biasing configuration changes. The results are promising for several electronic applications, including neuromorphic chips, switching, nonlinear devices, and industrial electronics such as current and temperature sensing.

Gaining control over the electronic and magnetic phenomena in solids is a critical step to advance our understanding that can have technological implications. Thin film interfaces and layered materials offer a wide range of possibilities to engineer new phases of electron matter. In the first part of this talk, I will discuss about realizing topological electron transport in SrCuO2 (001)/SrIrO3 (001) epitaxial thin films [1]. When SrIrO3 is proximitized with an antiferromagnetic SrCuO2 layer, we observe an enhancement of effective phase coherence length (lφ) and the chiral anomaly induced topological response in longitudinal magnetoconductance (B‖E) (which is absent in bare SrIrO3 film). Both the above effects is discussed in view of possible quenching of undesired magnetic impurity scattering through antiferromagnetic proximity effect. In the last part of the talk, I will discuss about notable halide ligand tunability of non-collinear magnetism in Cu-based quasi 2D hybrid perovskites. This non-collinearity is argued to stem from the

interplay of ferromagnetic intra-layer exchange, magneto-crystalline anisotropy and Dzyaloshinskii- Moriya interaction (DMI) [2].

[1] S. Jana,T. Senapati, Shwetha G. Bhat, S. N. Sarangi, K. Senapati, and D. Samal, Phys. Rev. B
107, 134415 (2023)
[2] P. Biswal et al., (unpublished)

Computer Engineered 2D Materials: Host for Unconventional Properties

Tanusri Saha-Dasgupta
S. N. Bose National Centre for Basic Sciences, Kolkata 700106, INDIA

In this talk, we will discuss two computer-engineered 2D materials, which are predicted to host unconventional topological properties. The first problem to discuss is robust half-metallicity and topological properties in square-net potassium manganese chalcogenides, paving the way to design topological half-metals and application possibilities in topological quantum spintronics.[1] The second problem deals with the giant Rashba effect and nonlinear anomalous Hall conductivity in a two-dimensional molybdenum-based Janus structure. With strong spin-orbit coupling and inversion symmetry broken by asymmetric surface passivation in these 2D MXene compounds, a giant Rasbha effect and a simultaneous appearance of nonlinear anomalous Hall conductivity.[2]

[1] Koushik Pradhan, Prabuddha Sanyal, and Tanusri Saha-Dasgupta, Phys. Rev. B 107, 155115 (2023).
[2] Shiladitya Karmakar, Rajdeep Biswas, and Tanusri Saha-Dasgupta, Phys. Rev. B 107, 075403 (2023).
 

A two dimensional electron or hole gas (2DEG or 2DHG) can be formed at the interface of modulation doped Si/Si1−xGex heterostructures [1], [2]. The 2DEG or 2DHG so formed offers a way to make semiconducting quantum dot qubits [3]. Experimentally, these heterostructure in- terfaces have also been investigated extensively to make electronic devices like MOSFETs [4]. However, the experiments performed on these heterostructures for application to various de- vices and qubit technologies seem to have been primarily guided by an accumulation of a large amount of empirical knowledge, with some amount of trial and error, rather than a process of rational design. To understand the factors responsible for formation of a two-dimensional elec- tron and/or hole gas at Si/Si1−xGex interfaces, we have performed accurate Density Functional Theory (DFT) calculations using HSE06 hybrid functional [5]. Our calculations have been aimed to design the heterostructures for making electron or hole based quantum dot qubits. Band alignment between Si and SiGe alloy is an essential property to know if a 2DEG or 2DHG forms, the direction of charge transfer at the interface, and the position of 2DEG or 2DHG. Type of Band alignment and band offsets depend on the amount of Ge concentration in SiGe alloy, and strain condition (due to lattice mismatch between Si and SiGe) in the het-erostructure layers. We have studied these factors by calculating band alignment between bulk Si (strained) and bulk SiGe for several concentrations of Ge (x) in the alloy. Our calculations have shown that the type of alignment changes with x. We have found optimum concentrations of Ge to obtain a 2DEG or 2DHG based on the type of alignment and offsets. We have found that for very low values of x, the type of alignment does not fall into the usual classification of band alignment into types 1, 2 or 3. Our results have successfully explained the experimental reports on formation of 2DEG and 2DHG in these heterostructures for different values of x. It could also be identified by the calculations, whether the electrons (or holes) will be confined on the Si side or on the SiGe side. Results from DFT calculations will be presented and the physical implications of the type of band alignment and the type of dopant, leading to the formation of a 2DEG or 2DHG will be discussed.
[1] G. Abstreiter, H. Brugger, et al., Phys. Rev. Lett. 54, 2441 (1985)
[2] R. People, J. C. Bean, et al., J. Appl. Phys. 97, 011101 (2005)
[3] D. Loss & D. P. DiVincenzo, Phys. Rev. A 57, 120 (1998)
[4] L. Yang, J. R. Watling, et al., Semicond. Sci. Technol. 19, 1174 (2004)
[5] J. Heyd, G. E. Scuseria & M. Ernzerhof, Chem. Phys. 118, 8207 (2003)

Spontaneous synchronization is at the core of many natural phenomena. Your heartbeat is maintained because cells contract in a synchronous wave; some bird species synchronize their motion into flocks; quantum synchronization is responsible for laser action and superconductivity. The transition to synchrony, or between states of different patterns of synchrony, is a dynamical phase transition that has much in common with conventional phase transitions of state – for example solid to liquid, or magnetism – but the striking feature of driven dynamical systems is that the components are “active”. Consequently quantum systems with dissipation and decay are described by non-Hermitian Hamiltonians, and active matter can abandon Newton’s third law and have non-reciprocal interactions. This substantially changes the character of many-degree-of-freedom dynamical phase transitions between steady states and the critical phenomena in their vicinity, since the critical point is an “exceptional point” where eigenvalues become degenerate and eigenvectors coalesce.
We will illustrate this in several different systems – a Bose-Einstein condensate of polaritons, models of multicomponent active matter such as flocks of birds, generalized Kuramoto models, and others. We argue that there is a systematic theory and generalized phase diagram, and corresponding universality behaviors determined by the symmetry of the models.
 

In this talk, we demonstrate the intriguing properties of three distinct materials: Sr2FeMoO6 thin films, tungsten (W)-doped Sr2FeMoO6 thin films, and the conducting interface of LaFeO3 and SrTiO3.
Leveraging pulsed laser deposition (PLD) as a versatile technique, we demonstrate precise control over the stoichiometry of Sr2FeMoO6 thin films, thereby achieving the highest spin-polarization within the system. Additionally, for W-doped Sr2FeMoO6 thin films, we observe fascinating transitions in both conductivity and magnetism, alongside the emergence of a substantial topological Hall effect, indicative of a non-trivial spin-texture arising from intricate ferro-antiferro interactions.
Transitioning from thin films to interfaces, we explore the crystallographic transition occurring at the LaFeO3-SrTiO3 interface, which induces a magnetic transition leading to a remarkable shift from non-spin polarized to spin-polarized conductivity. This novel phenomenon manifests as a room temperature spin-polarized conducting interface with approximately 90% optical transparency, paving the way for the paradigm of "Room Temperature Transparent Oxide Spin-Electronics." Our findings not only expand the understanding of oxide-based materials but also hold promise for future advancements in spintronics and transparent electronics at ambient conditions.
These works can be find in
1. PHYSICAL REVIEW B 109, L201114 (2024)
2. ACS Appl. Electron. Mater. 2021, 3, 597−604
 

Magic-angle twisted bilayer graphene (MATBG) exhibits a wide variety of correlated phases, spanning from insulating to superconducting and magnetic states, favored by the flat bands. The degeneracy among closely competing ground states can be lifted by polarizing spin and valley degrees of freedom; hence, the four-fold degeneracy of the low-energy electrons has a significant impact on the underlying mechanism governing the correlated phases at different band fillings. The overall phase diagram of MATBG is remarkably sensitive to external perturbations such as carrier density, electromagnetic field, pressure, temperature, and dielectric environments. Despite this unprecedented tunability, a complete understanding of the observed phases has remained elusive. In our recent study, we conducted magneto-transport measurements on MATBG proximitized by a layer of tungsten diselenide, thereby introducing finite spin-orbit coupling into the system. Our findings unveiled an anomalous Hall effect in the vicinity of half-filling (ν = 2), accompanied by an abrupt switching of magnetization that can be fine-tuned by varying the carrier density. Such a reversal of hysteresis suggests a ferromagnetic ground state that is orbital in nature. Additionally, near ν = 2, we observed a series of Lifshitz transitions in the zero-magnetic field limit, indicating Fermi surface reconstructions. As we increased the magnetic field, a perfectly quantized Chern insulator was observed exactly at ν = 2. These intriguing results collectively suggest the presence of valley-polarized ground states near ν = 2, which are stabilized by the inclusion of spin-orbit coupling.

The low-frequency fluctuations, or noise, in electrical resistance not only set a performance benchmark in devices, but also form a sensitive tool to probe non-trivial electronic phases and band structure in solids. Here we report the measurement of such noise in the electrical resistance in twisted bilayer graphene (tBLG), where the layers are misoriented close to the magic angle (θ ∼ 1 degree). At high temperatures (T >∼ 60 − 70 K), the power spectral density (PSD) of the fluctuations inside the low-energy moir ́e bands is predominantly ∝ 1/f, where f is the frequency, being generally lowest close to the magic angle, and can be well-explained within the conventional Mc. Whorter model of the ‘1/f noise’ with trap-assisted density-mobility fluctuations. At low T (

Hall effect is one of the most studied phenomena in condensed matter physics. In its linear form, it relates the electrical potential difference perpendicular to the electrical current via a direct proportionality. Beyond the linear response regime, a recent theoretical proposal pointed to a novel form of Hall effect induced by a Berry curvature dipole, where the electric current in the nonlinear regime is always orthogonal to the local electric field and. The proposed nonlinear Hall effect unlocks an outstanding experimental challenge to properly probe, analyze, and understand the mechanism underlying transport response in the nonlinear regime. In this talk, we report a new scheme to measure and analyze nonlinear transport response from Bernal bilayer graphene. Based on angle-resolved transport measurement, we extract the nonlinear conductivity tensor and identify a non-zero component that corresponds to the nonlinear Hall effect. By examining the evolution of the conductivity tensor across the low-temperature phase space, we discuss possible origins of the observed nonlinear transport response. Overall, not only is angle-resolved measurement imperative to understanding the origin of nonlinear transport response, but it also unlocks a new dimension of electronic order, which remained hidden to conventional transport method till now.

We will first present an introduction to quantum geometry of electrons in periodic structures in terms of Berry phases and curvature. We show that the coupling of phonons with electrons can have nontrivial consequences to quantum geometry of electronic structure, which manifests as oscillations in the Berry curvature dipole and hence have observable nonlinear Hall signatures. Using these, we introduce a vibrational spectroscopy based on Geometry of Quantum Electronic Structure (GQuES) making specific predictions for the transport and radiative GQuES spectra of 2D materials. On a related topic, we demonstrate emergence of nontrivial Berry curvature in graphene from its interaction with monolayer of CrTe2, and how their heterostructure can be used to develop an Anomalous Hall Transistor.

In this talk, I will present a detailed experimental study of the particle- hole (PH) symmetry of the the abelian and a putative non-abelian Fractional Quantum Hall (FQH) state about half filling in a multiband system. Specifically, we focus on the lowest Landau level of the monolayer-like band of Bernal stacked trilayer graphene (TLG). In pristine TLG, the excitation energy gaps, Lande g factor, effective mass, and disorder broadening of the FQH states is the same as their hole-
conjugate counterpart. This precise PH symmetry stems from the lattice mirror symmetry that precludes Landau-level mixing. Introducing a non- zero displacement field D disrupts this mirror symmetry, facilitating the interaction and hybridization between the NM = 0 of monolayer-like and NB = 2 bilayer-like Landau levels. This band hybridization eventually leads to a violation of the particle-hole symmetry of the FQH states. We find the one-third and two-fifth FQH states to be more robust against Landau level mixing than their hole-conjugate states, which agrees with theoretical predictions [1]. The Landau level mixing parameter  is found to increase sharply with D. Our research identifies an external factor that can be manipulated to adjust the particle-hole symmetry in FQH states.

References
[1] Peterson, Michael R., and Chetan Nayak. Physical Review Letters 113.8 (2014): 086401.

Orbital current, defined as the orbital character of Bloch states in solids, can ballistically travel with larger coherence length through a broader range of materials than its spin counterpart, facilitating a robust, higher density and energy efficient information transmission. Hence, active control of orbital transport plays a pivotal role in propelling the progress of the evolving field of quantum information technology. Unlike spin angular momentum, orbital angular momentum (OAM), couples to phonon angular momentum (PAM) efficiently via orbital-crystal momentum (L-k) coupling, giving us the opportunity to control orbital transport through crystal field potential mediated angular momentum transfer. Here, leveraging the orbital dependant efficient L-k coupling, we have experimentally demonstrated the active control of orbital current velocity using THz emission spectroscopy. Our findings include the identification of a critical energy density required to overcome collisions in orbital transport, enabling a swifter flow of orbital current. The capability to actively control the ballistic orbital transport lays the groundwork for the development of ultrafast devices capable of efficiently transmitting information over extended distance

We have studied 2DEG formed at the interface of two oxide perovskite insulators; one is LaVO3 (LVO) which is a Mott insulator and another one is KTaO3 (KTO) which is a band insulator. Our experimental collaborators in the group of Prof. Suvankar Chakraverty from INST, Mohali have created LVO/KTO interface for the first time and observed metallic behaviour at this interface with one order of magnitude higher electron density and very high mobility of the 2DEG. The reason for 2DEG formation at the interface of two insulators/semiconductors was not clear. Our DFT calculations showed that LVO and KTO bulk materials are insulating, but the LVO/KTO interface is metallic which is an emergent phenomena and existence of parabolic bands crossing the Fermi level indicates source of free electrons at the interface. In this LVO/KTO heterostructure, both the individual parts are polar, consisting of alternating charged layers. Our DFT calculations show, to avoid polar catastrophe, “electronic reconstruction” is the mechanism for the origin of 2DEG at this interface. The calculated interfacial electron density matches very well with the experimental value which is one order of magnitude higher than the polar/non-polar LAO/STO interface. Our calculations show that the 2DEG is highly confined to the interface region and has very high mobility due to smaller effective mass of conduction electrons at the interface. Thus the LVO/KTO interface produces 2DEG with very high electron density and mobility. The 2DEG at oxide interfaces have potential applications in field-effect transistors, sensors, thermoelectrics, solar cells etc.

Quantum paraelectricity is a distinct phase of matter where the onset of ferroelectric order is suppressed by quantum fluctuations. Two well-known examples are SrTiO3 and KTaO3. In this talk, I will discuss our observations of surprising new electronic phases that emerge when these materials are intentionally turned metallic through electron doping.