APCTP IACS IITB 2019

Abstract: The Chern insulator on a lattice without an external magnetic field realizes so-called the quantum anomalous Hall effect. So far, the experimental realizations of Chern insulators have been devoted to magnetically doped topological-insulator thin films but demonstrated the result at low temperatures due to small band gaps. Here we predict that a two-dimensional metal-organic framework of a single layer of transition-metal bis-dithiolene complex, M3C12S12 (M = Mn, Fe, Co), can become a ferromagnetic insulator with a nontrivial Chern number. Among various synthetic pathways leading to metal bis-dithiolenes, a particular choice of ligand, Benzene-hexathiol, connects metal cations to form a Kagome lattice. From the results of our first-principles density-functional-theory calculations together with a tight-binding analysis, we show that the sulfur and carbon-based ligands in a transition-metal bis-dithiolene complex are playing a pivotal role in making the band structure of the complex topologically nontrivial. An unusual band inversion brings an almost flat band as the highest occupied band. The nontrivial topology of the flat band is confirmed to have a nonzero Chern number, quantized Hall conductivity, and the chiral edge states by using the Wannier function basis and Green’s function approach. Our study shows that transition metal bis-dithiolene complexes are an essential playground for obtaining novel electronic properties in a two-dimensional metal-organic framework. *Work done in collaboration with Santu Baidya (Rutgers U.), Seungjin Kang (SNU), Sung Mo Kang (SNU), and Choong H. Kim (CCES-IBS) and supported by the National Research Foundation of Korea (no. 2017R1A2B4007100).

Abstract: In this talk, I will show the occurrence of Dirac, triple-point, Weyl semimetal, and topological insulating phases in a single ternary compound using specific symmetry-preserving perturbations. Based on first-principles calculations, the k.p model, and symmetry analysis, we show that alloying-induced precise symmetry breaking in SrAgAs (space group P6_3/mmc) leads us to tune various low-energy excitonic phases transforming from Dirac to topological insulating via the intermediate triple-point and Weyl semimetal phases. We also consider the effect of external magnetic field, causing time reversal symmetry (TRS) breaking and analyze the effect of TRS on the realization of the Weyl state. Importantly, in this material, the Fermi level lies extremely close to the nodal point with no extra Fermi pockets, which further makes this compound an ideal platform for topological study. The multifold band degeneracies in these topological phases are analyzed based on point group representation theory. The topological insulating phase is further confirmed by calculating the Z_2 index. Furthermore, the topologically protected surface states and Fermi arcs are investigated in some detail.

Abstract:Most two-dimensional (2D) transition metal dichalcogenides (TMDs) exhibit more than one structural phases with remarkable physical properties. Moreover the transition metal dichalcogenides, MoTe2 and WTe2 are predicted as candidates of type-II Weyl semimetals and host superconductivity under certain conditions, that opens up the possibility of having topological superconductivity. In this talk, I will present the detailed magneto-transport and magnetization study of MoTe2 and WTe2 to understand their novel electronic phases.

Abstract:I will describe a real space theory of the voltage bias driven transition from a Mott insulator to a correlated metal. At zero temperature our approach is a Keldysh mean field scheme for the self-consistent charge and spin profiles in the biased open system. In two dimensions, an increase in bias (V) leads to a suppression of the magnetic moment near the edges, and an increase in double occupancy, over a lengthscale that grows with V, `diverging' at a critical voltage V_c. The current through the system grows from an exponentially small value at small V, through a sharp increase near V_c, to a `saturated' value for V >> V_c. While the current is uniform through the system the local density of states and the magnetic moment profile becomes strongly inhomogeneous near V_c. I will also describe a Langevin scheme that generalises the mean field theory to finite temperature to take amplitude and phase fluctuation of the magnetic moments into account.

Abstract:In strongly correlated electronic systems, an interplay among electron’s degrees of freedom can induce versatile phase transitions among various ordered phases. To investigate such a electron correlation effect on the phase transitions, research on intermediate phase is crucial. In spite of its importance, experimental results have been limited in macroscopic scale due to inevitable instability of such intermediate phases. 1T-TaS2, one of well-known transition metal dichalcogenides, is an appropriate model system by its metastable intermediate phases between coexisting charge-density-wave (CDW) phases and a Mott insulating phase, which compete with superconductivity. Earlier scanning tunneling microscopy studies on metastable phases of bulk 1T-TaS2 revealed the correlation between smaller CDW domains and Mott gap closing. However, it was very difficult to control such metastable phases in bulk 1T-TaS2, which has been limiting further studies on the correlated phases. In this work, we have prepared thin and clean 1T-TaS2 in order to manipulate metastable CDW phases by electric pulses between an STM tip and thin 1T-TaS2 flakes. Among various metastable CDW phases, we found gradual Mott gap closing due to wider domain boundaries and the unusual distribution of CDW domains. These findings will provide fresh insight into the interplay among strongly correlated intermediate CDW phases.

Abstract:I will describe projected "colouring" wavefunctions that are useful for bipartite and tripartite XXZ quantum magnets. The colours refer to quantum states in the XY plane of the Bloch sphere such that no bond shares the same colour. For bipartite case, the colours may be chosen such that "red" is |+Sx> and "blue" is |-Sx>, while for tripartite case, they may be chosen as three non-orthogonal "120 degree" states in the XY plane. These wavefunctions can be used to represent exact eigenstates at special solvable points in the XXZ phase diagram, which also happen to be transition points between AFM and FM order. By computations of correlation functions, energies and fidelities, we show adiabaticity from Heisenberg point to these solvable points, following which there is a transition to a maximally polarized FM multiplet. This gives an unified perspective on frustrated and unfrustrated XXZ models with ordered phases. The two-coloring solution also applies to bipartite lattices in presence of site/bond dilution, which gives a novel perspective on the long-range Néel order in these systems at percolation established numerically in literature as being “inherited" from the exactly solvable point.

Abstract:A novel route to achieve two dimensional (2D) carrier confinement in a wedge-shaped wall structure made of a polar semiconductor has been demonstrated theoretically. Tapering of the wall along the direction of the spontaneous polarization leads to the development of charges of equal polarity on the two inclined facades of the wall. Negative/positive polarization charges on the facades can push the electrons/holes inward for an n-/p-type material leading to the formation of 2D electron/hole gas at the central plane and depletion regions at the outer edges of the wall. Schrodinger and Poisson equations are solved self-consistently to obtain the potential and charge density distribution in n-type GaN nanowalls tapered along c-direction. Momentum and spin scattering lifetimes in this 2D carrier gas is estimated to be significantly higher than that of bulk. Recent experimental findings in wedge-shaped n-type GaN nanowall networks indeed suggest long momentum scattering lifetime for the electrons in this system. Properties of high mobility and the vertical orientation of the 2D confinement can be exploited in fabricating highly conducting transparent interconnects and high mobility field effect transistors, which can lead to a large-scale integration of 2D devices in future. On the other hand, long spin scattering lifetime of the carriers could make this system potentially attractive for spintronics. Reference: S. Deb et al. Sci Rep 6, 26429 (2016) and S. Deb and S. Dhar, SPIN 8, 1840003 (2018)

Abstract:In this talk, I will show some intriguing results on the effect of magnetic and non-magnetic impurities on the spin excitation spectrum of 1d, spin 1/2 Heisenberg antiferromagnets Sr2CuO3 and SrCuO2. Later, the effect of impurities on the low-energy quasi-acoustic sliding modes will be discussed in the structurally incommensurate composite chain-ladder system Sr14Cu24O41.

Abstract:We have discovered a new spin- 1/2 quasi-one dimensional compound NaVOPO4, which is well described by an alternating spin chain model with a dominant antiferromagnetic (AFM) exchange coupling of J/kB ≈ 39 K and a tiny spin gap of Δ0/kB ≈ 2 K. No magnetic long-range order (LRO) is observed down to 50 mK in zero field. External magnetic field of Hc1 ≈ 1.6 T closes the spin gap and triggers magnetic LRO. A weak alternation of the exchange couplings with the alternation ratio α ≈ 0.98/small spin gap and the onset of magnetic LRO already in low magnetic fields place NaVOPO4 in the vicinity of the quantum critical point separating the spin-gap and AFM LRO states in the phase diagram. The field-induced magnetic LRO is indeed confirmed from the magnetic susceptibility, heat capacity, and 31P NMR measurements and found to move toward high temperatures with the field. A power-law [TN ∝ (H − Hc1)1/φ] fit to the H-T phase boundary yields an exponent φ ≈ 1.8, a possible signature of Bose-Einstein Condensation of triplon. The unusual spin model and the low energy scale of the spin-gap make NaVOPO4 a promising candidate for further experimental investigations in high magnetic fields to elucidate microscopic nature of the field-induced ordered phase.

Abstract:Interlayer interactions in stacked 2D crystals are one of key ingredients in exhibiting their qualitative different physical properties [1]. For example, a well-known 2D quantum spin Hall insulating transition metal dichalcogenide can show either topological Weyl metallic phase or trivial one depending on a minute stacking order difference [1,2]. Recent advances in fabricating stacked 2D crystals enable us to perform controlled studies on interesting electronic, magnetic and topological properties in low dimensional heterostructures. In this talk, I will first discuss interplay between intralayer correlation, interlayer interaction, and magnetism in layered materials [3]. To understand magnetic properties, I will introduce a new computationally efficient method to calculate various correlation effects in layered materials [3]. Another interesting example revealing intriguing interlayer interactions will be discussed for graphene bilayer systems [4]. The system has a quasicrystalline order through a perfect incommensurate interlayer interaction between 30 degrees rotated two graphene layers and shows localized 12 fold resonant states with fractal scaling [5]. In addition to existing 2D materials, I will also introduce a new computational scheme [6] to search a new family of 2D crystals [7] that will expand both material and property spaces of layered crystals. References 1) Y.-W. Son, Nature 565, 32 (2019). 2) H.-J. Kim, S.-H. Kang, I. Hamada, and Y.-W. Son, Phys. Rev. B 95, 180101 (R) (2017). 3) S. Lee and Y.-W. Son, in preparation (2019). 4) S. J. Ahn, P. Moon et al, Science 361, 782 (2018). 5) P. Moon, M. Koshino, Y.-W. Son, Phys. Rev. B 99, 165430 (2019). 6) K. Chae, D. Y. Kim and Y.-W. Son, 2D Mater. 5, 025013 (2018). 7) K. Chae and Y.-W. Son, Nano Lett. 19, 2694 (2019).

Abstract:We will present a tight-binding (TB) model which is appropriate for exploring non-trivial quantum states in the perovskite family ABX3 (oxides and halides). This TB Hamiltonian takes into account the electron hopping, atomistic spin-orbit coupling, and Rashba coupling. As an output, it provides a set of descriptors which can be utilized to define conditionalites for creating trivial and non-trivial quantum states (e.g. normal insulator, topological insulator, Dirac semimetal, Weyl semimetal, etc.). Finally, we will discuss the role of external stimuli such as pressure, epitaxial strain and doping to form the desired quantum states.

Abstract:In usual magnetic systems exhibiting transitions from paramagnetic to ordered (ferro- or antiferro-) magnetic states with the lowering of temperature, one may obtain an estimate of the net interaction strength from the temperature-intercept (Ɵ CW ) of the inverse-susceptibility plot as a function of the temperature via the Curie-Weiss Law. This magnitude of Ɵ CW is often a good indication of the transition temperature. However, there are many systems where the magnetic ordering temperature is significantly suppressed compared to Ɵ CW due to frustrations in the magnetic interactions. In extreme cases, no magnetic ordering is found down to the lowest temperature probed despite a sizable value of Ɵ CW , prompting one to believe that such systems have an exotic ground state, known as quantum spin liquid, characterised, among other things, by an absence of ordering despite sizable interactions between various magnetic sites. We have been probing several new systems that are improbable candidates of this class of compounds but finding experimental features to suggest unusual ground states suggestive of a highly entangled dynamic ground state. I shall discuss some of these systems in my presentation. Some of our past work leading to these ideas and these results are based on several collaborations with many people, including most prominently P. S. Anil Kumar, M. Baenitz, Peter J. Baker, S. V. Bhat, S. Bhattacharjee, Subhro Bhattacharjee, D. Choudhury, Kedar Damle, Ranjan Das, O. Eriksson, Akmal Hossain, Olof Karis, Pranava Keerthi S, S. Kundu, P. Lazor, A. V. Mahajan, P. Mandal, Roland Mathieu, J. Matsuno, S. Mukherjee, Per Nordblad, Jean-Christophe Orain, B. Pal, Sumiran Pujari, B. Sanyal, A. Sharma, A. Sundaresan, A. Thamizavel, and U. Waghmare.

Abstract:Elementary quasiparticles in solids such as phonons and magnons occasionally have nontrivial interactions between them, as well as among themselves. As a result, their energy eigenvalues are renormalized, the quasiparticles spontaneously decay into a multi-particle continuum state, or they are hybridized with each other when their energies are close. As discussed in this review, such anomalous features can appear dominantly in quantum magnets but are not, a priori, negligible for magnetic systems with larger spin values and noncollinear magnetic structures. We review the unconventional magnetic excitations in two-dimensional triangular lattice antiferromagnets and discuss their implications on related issues.

Abstract:The antiferromagnetic kagome lattice decorated with quantum spins has been considered since the late 90’s as the best candidate for stabilizing a quantum spin liquid in dimension higher than one. From a theoretical point of view, the simple Heisenberg case remains a standing problem, not solved yet. In my talk, I will present results on one of the materials which has been at the center of our activity on spin liquids in Orsay, herbertsmithite, ZnCu3(OH)6Cl2, which has been known since 2005 as one of the best representative of that spin liquid physics for the Heisenberg model. Recently, working on high quality single crystals considerably improved the accuracy of NMR measurements and its ability to address fundamental issues such as the existence of a gap and the class of models relevant to describe the ground state. Despite the existence of defects out-of-kagome planes, we could then isolate the 17O NMR spectral signature of kagome Cu's. Neither in our shift measurements nor in our relaxation studies, do we find any hint of a gap. On the contrary, we conclude that the susceptibility is finite and independent of the field in the range 2.6 - 12 Teslas. I'll discuss the relation to models, especially the Dirac spin liquid one which seems to be favoured in various thoretical approaches.

Abstract:A variety of condensed matter systems can be used to engineer topological superconductivity and Majorana zero modes (MZMs). To date, evidence for MZMs has come from experiments that have detected their zero-energy excitation signature in various spectroscopic measurements when the parameters of the system make it most likely to be in a topological superconducting phase. However, the question as to whether a MZM could be distinguished from a finely tuned or accidental trivial zero-energy edge state in these experiments has been unresolved. In this presentation, I will introduce newly discovered platforms to host MZMs and the signatures of MZMs. Using spin-polarized and spin un-polarized spectroscopic measurements studied on these platforms, we experimentally confirmed all the ingredients to host MZMs and found spectroscopic signature of MZMs. Moreover, observation of the spin signature of MZMs that exceeds the spin-polarization of the normal-state background of MZM platform display the topological properties of measured zero energy end states. This feature, captured by a model calculation, is a direct consequence of the nonlocality of the Hilbert space of MZMs emerging from a topological band structure. This study establishes spin-polarization measurement as a diagnostic tool to distinguish topological MZMs from trivial in-gap states of a superconductor.

Abstract:Considerable theoretical and experimental debate currently surrounds the possibility of observing Kitaev quasiparticle effects in the honeycomb Kitaev materials, the alkali iridate Na2IrO3 and α-RuCl3, where, due to the presence of small competing interactions, the ground state is antiferromagnetically ordered and not a quantum spin liquid. Motivated thus, we study numerous physical situations where Kitaev quasiparticle effects can survive and may be observed in the presence of perturbations. In Kitaev models perturbed by competing Heisenberg interactions, we find that over a large parameter window where magnetic long range order is present, the low-lying excitations resemble better the Kitaev quasiparticles rather than the usual magnon-like excitations of an antiferromagnet. We cast the problem of stability of Kitaev quasiparticles to such perturbations as one of many-body localization. References [1] Aman Kumar and V. Tripathi, arXiv:1910.00030 [2] Sitikantha D. Das et al., Phys. Rev. B 99, 081101(R) (2019) [3] S. D. Das, K. Dhochak, and V. Tripathi, Phys. Rev. B 94, 024411 (2016) [4] K. Dhochak, R. Shankar and V. Tripathi, Phys. Rev. Lett. 105, 117201 (2010)

Abstract:Semiconductor nanowire-superconductor hybrid systems (Fig.1) provide a promising platform for hosting unpaired Majorana fermions and thus realizing fault-tolerant topological qubits. In this talk, starting from the basic tenets of quantum transport theory, we demonstrate how to adapt the Non-Equilibrium Green’s Function (NEGF) formalism to model quantum transport in normal (N)-superconductor (S) junctions. We analyze Josephson junctions based on semiconductor nanowires and derive the Andreev bound state spectrum and current-phase relations. Recently, [1], and [2] have reported oscillations in the critical supercurrent with an axial magnetic field. Our simulations indicate that this phenomenon arises from the interference of orbital angular momentum modes [3,4] of the cylindrical nanowire. We also add disorder and study its effect on the critical current oscillations, with an aim to gain a thoroughgoing understanding of the experiments. Finally, we also comment on the Majorana modes in a finite topological superconductor [5], and most importantly the transport signatures that detect them. [1] K. Gharavi et.al., ArXiv:1405.7455v2, (2014). [2] Zuo, et. al. Phys. Rev. Lett. 119, 187704 (2017) [3] P. Sriram et. al., Phys. Rev B, 100, 155431, (2019). [4] A. Lahiri et.al., Phys Rev B, 98, 125417, (2018). [5] N. Leumer et.al., ArXiv:1909.10971, (2019).

Abstract:Starting with a brief summary of the progresses in understanding on 2D chalcogen compounds PdxTaSe2, in which a strong evidence of a new CDW quantum critical point is found, in this talk, I’ll mainly discuss the recent progresses in new forms of multiferroic materials. Indeed, large direct and converse magnetoeletric (ME) coupling with the type II multiferroicity has been achieved in bulk hexaferrite single crystals; modulation and reversal of magnetization (M) by E were realized in either in the Z-type and Y-type hexaferrites, in which a transverse conical spin state plays a major role in exhibiting remanent M and electric polarization. However, realization of thin film forms of such hexaferrite materials have been quite challenging. In this work, we present the observation of large ME coupling in the Co2Z type thin films grown on SrTiO3(111). Strikingly, their ME coupling turns out to be much larger than those of single crystals by more than one order of magnitude. In the second part, we’ll also report the observation of the new type II multiferroicity in a layered Van der Waals system CuCrP2S6. In both systems, we discuss that the origin of the ME coupling is closely associated with the pd hybridization mechanism. In close collaboration with Aga Shahee, Changbae Park Kwangwoo Shin (Seoul) and Josef Buršík (Czech). Reference: [1] K. W. Shin et al., preprint; [2] Y. S. Chai et al., Nature comm. 5, 4208 (2014); [3] Sae Hwan Chun et al., Phys. Rev. Lett. 108, 177201 (2012); ibid, 104, 037204 (2010)

Abstract:The story of stereochemically active cationic lone pairs, arising due to s-p mixing in metal assisted by covalency with the ligand p-orbitals and finally driving ferroelectricity within a polar unit cell, has always been an exciting point of discussion in condensed matter physics. The excitement increases many-fold when such systems also accommodate magnetic cations and possibility of multiferroicity arises. In this context we have explored rarely studied langasites (Pb3TeMn3P2O14), having lone pair bearing Pb2+ ions along with 3d transition metal (TM) ions within tetrahedral unit, which is more conducive for TM-O covalent interaction and consequent lone pair activity of Pb. We find that lone pairs of Pb2+ (ns2) distort the general non-polar structure into a polar one, causing a shifting of magnetic Mn/P planes (M-P planes) towards the Pb/Te containing plane (P-T plane) in the direction of positive c-axis. This results in formation of stripe-like distribution of pairwise closely placed M-P / P-T planes within the structure which in turn enhances the covalency as well as magnetic moments on otherwise nonmagnetic entities such as Pb2+, Te6+, P5+, and O2-. This brings forth spontaneous polarization in the system at around ~320 K, and most interestingly the above mentioned redistribution of magnetic moments and shortened relative distances among them affect the local magnetism too which gets manifested in a clear signal of magnetoelectric coupling within the apparent paramagnetic phase, i.e. far above the long range magnetic transition (~7 K) of the system.

Abstract:The spin state crossover at 80-90 K and its crucial role in understanding the unusual magnetism and electronic transport in LaCoO3 are most famous long-standing problems in solid-state physics. In recent years, double perovskite oxides (A2BB'O6) have attracted great attention due to an extra degree of freedom of tuning the B-site ordering. The rock salt like B-site ordering is governed by two major factors: difference in the valence state (ΔV) of two B-site cations and their ionic mismatch (ΔrB). It is important to note that the large difference in ionic radii, and ΔV (≥3) of the B-site cations favor the ordering in the crystals due to reduction in the coulombic repulsion energy and lattice strain, respectively. The degree of B-site ordering in these materials governs most of their magnetic, transport, and electronic properties and consequently their technological aspects. Interestingly, due to the combined effect of the interesting spin states of Co and flexibility in the B-site cationic ordering, Co-based double perovskites oxides are particularly important to understand their unusual physical properties with aliovalent substitution at A-site. In this talk, I will present our recent results on the evolution of structural, magnetic and spin state transitions in Sr2-xLaxCoNbO6 (x= 0-1) double perovskite. The structural transition from tetragonal to monoclinic phase at x≥0.6 and an evolution of (101)/(103) superlattice reflections indicate the enhancement in the B-site ordering with x. The magnetic susceptibility data reveal the transition from weak ferromagnetism (FM) to antiferromagnetic (AFM) ordering x≥0.6 with TN~10-15K. Interestingly, the La substitution drives the system towards more insulating state with increase in Co2+, which establish in high spin (HS) state, whereas a spin-state crossover is observed in Co3+ from HS to intermediate spin (IS) state with x. We discuss the correlation between complex magnetic interactions and the presence of various Co spin states in the system. Moreover, a crossover from second order to first order magnetic phase transition and competition between FM and AFM interactions have been investigated by detailed analysis of temperature and field dependent magnetic entropy measurements. [1] Rishabh Shukla, and R. S. Dhaka `Anomalous magnetic and spin glass behavior in Nb substituted LaCo(1-x)NbxO3' Physical Review B, 97, 024430 (2018). [2] Rishabh Shukla, A. Jain, M. Miryala, M. Murakami, K. Ueno, S. M. Yusuf, & R. S. Dhaka `Spin dynamics and unconventional magnetism in insulating La(1-2x)Sr2xCo(1-x)NbxO3' The Journal of Physical Chemistry C, 123, 22457 (2019). [3] Ravi Prakash, Rishabh Shukla, Priyanka, Anita Dhaka, and R. S. Dhaka `Tuning ferromagnetism and spin state in La(1-x)AxCoO3 (A = Sr, Ca) nanoparticles' Journal of Alloys and Compounds, 764, 379, (2018). [4] Ajay Kumar and R. S. Dhaka, Unraveling the magnetic interactions and spin state in insulating Sr2-xLaxCoNbO6 Manuscript under preparation.

Abstract:We exhibit an exactly solvable example of a SU(2) symmetric Majorana spin liquid phase with honeycomb lattice symmetries, in which quenched disorder leads to {\em random singlet phenomenology} of magnetic response. Specifically, we show that a non-zero density $n_v$ of nonmagnetic defects (missing spins) in an exactly solvable SU(2) symmetric $S=1/2$ model on the decorated honeycomb lattice leads to a singular low-temperature susceptibility: $\chi(T) = {\mathcal C}/T+ T^{\alpha(T) - 1}$, where the second term, with $\alpha(T)$ decreasing slowly to zero as $T \rightarrow 0$, represents behaviour characteristic of the physics of the random-singlet phase of random antiferromagnetic spin chains as well as the analogous regime in Si:P. Interestingly, the vacancy-induced Curie tail ${\mathcal C}/T$ does not arise from microscopic free spins, but rather represents the emergent response of effective moments spread over many unit cells. We argue that this physics is controlled by an underlying strong-disorder fixed point and is expected to be robust to weak perturbations that destroy exact solvability.

Abstract:Pure liquid 3He has two superfluid phases in zero magnetic field, 3He-A and 3He-B, both of which are topological phases. Despite being one of the oldest material known to be topological, direct experimental signatures of the non-trivial topology are hard to come by in superfluid 3He. Many of the experimental methods available for solid state topological matter, such as transport measurements, ARPES, local microscopy, neutron or x-ray scatterings, etc. are inaccessible to superfluid 3He due to lack of charge and extremely low Tc. Recent advance in micro- and nano-electromechanical systems (MEMS and NEMS), however, provides a mean to detect these. In specific, we will describe an experimental method involving a MEMS gyroscope with which the signature of the chiral current of 3He-A, a superfluid analogue of a quantum Hall system, can be directly measured. The method described in this talk can be applied to chiral topological superconductors as well.

Abstract:Quantum phase transitions out of magnetic orders in quantum spin liquid (QSL) phases have gained much recent attention in context of several spin-orbit coupled magnets. In this talk, I shall report our theoretical calculations about the nature of such unconventional phase transition in a class of experimentally relevant Hamiltonians. In particular, we shall show how such deconfined quantum phase transitions are naturally captured in terms of the condensation of the fractionalised excitations of the QSL. In addition we shall show how to think about such transitions in terms of the domain walls of the magnetically ordered phases.

Abstract:The most commonly seen phase transition is possibly "melting", a transition from ordered crystalline solids to disordered isotropic liquids. While melting in three-dimensions is always a single, first-order phase transition, in two-dimensional systems a scenario of two continuous phase transitions separated by an intermediate \oriented liquid" state, the so-called hexatic phase, has been proposed theoretically and evidenced experimentally in colloidal systems, Wigner solids and liquid crystals. Fundamentally different from these real-matter particles, skyrmions are countable soliton configurations localized in continuous fields with non-trivial topology, and these emergent quasi-particles can form two-dimensional lattices, whose melting dynamics remains unexplored. Here we show, by direct imaging with cryo-Lorentz transmission electron microscopy, that the phase of the skyrmion ensembles in the material Cu2OSeO3 can be tuned by magnetic field from two-dimensional skyrmion solids, through the long-speculated skyrmion hexatic phase, to skyrmion liquids, with the local spin order preserved throughout the whole process. Remarkably, our quantitative analysis demonstrates that this mesoscopic phase transition can be well described as topological-defect-induced crystal melting in two dimensions. By uncovering the novel phase behaviors of skyrmionic quasi-particles, we demonstrate skyrmion ensembles as an ideal platform for exploring novel properties in two-dimensions.

Abstract:The momentum dependent splitting of spin-bands in an electronic system is known as the ”Rashba effect”. Systems with the ”Rashba effect” possess a Dirac point in momentum space. An electron in a cyclotron orbit enclosing that Dirac point in the reciprocal space gains a ”Berry phase”. Among the perovskite oxide family, KTaO3 (KTO) has recently attracted considerable interest as apossible system for the realization of the Rashba effect. In one of the work, we have improvised a novel conducting interface by juxtaposing KTO with another insulator, namely LaVO3 (LVO) and report planar Hall effect (PHE) and anisotropic magnetoresistance (AMR) measurements. This interface exhibits a signature of strong spin-orbit coupling. Our experimental observation of two fold AMR and PHE at low magnetic fields is similar to those obtained for topological systems and can be intuitively understood using a phenomenological theory for a Rashba spin-split system. At high fields (∼8 T), we see a two fold to four fold transition in the AMR that could not be explained using only Rashba spin-split energy spectra. Our experimental data show a B2 dependence of AMR and PHE at low magnetic field that could also be explained based on our model. In another work, we report the Shubnikov-de-Haas oscillations (SdH) at the conducting interface of EuO-KTaO3 (KTO). Observed SdH oscillations suggest the presence of two Fermi surfaces. For both the Fermi surfaces, we have seen the presence of a non-trivial ”Berry phase” suggesting that the surfaces enclose the ”Dirac point”. Thus the Berry phase originates from the inner and outer Fermi surfaces of the Rashba spin-split bands. As in topological insulators, two fold planar Hall and anisotropic magnetoresistance have also been observed in EuO-KTO. Analyzing the SdH, Hall and magnetoresistance data, we have drawn a possible band diagram near the Fermi surface. References: 1. (arxiv identifier:1908. 06636): https://arxiv.org/pdf/1908.06636.pdf 2. (arxiv identifier:1908. 04977): https://arxiv.org/pdf/1908.04977.pdf