### Browsing by Department "Correlated Quantum Materials (CQM)"

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Item Andreev Reflection and Klein Tunneling in High-Temperature Superconductor-Graphene Junctions(American Physical Society, 2023-04-12) Jois, Sharadh; Lado, Jose; Gu, Genda; Li, Qiang; Lee, Ji Ung; SUNY Polytechnic Institute; Correlated Quantum Materials (CQM); Brookhaven National Laboratory; Department of Applied PhysicsScattering processes in quantum materials emerge as resonances in electronic transport, including confined modes, Andreev states, and Yu-Shiba-Rusinov states. However, in most instances, these resonances are driven by a single scattering mechanism. Here, we show the appearance of resonances due to the combination of two simultaneous scattering mechanisms, one from superconductivity and the other from graphene p−n junctions. These resonances stem from Andreev reflection and Klein tunneling that occur at two different interfaces of a hole-doped region of graphene formed at the boundary with superconducting graphene due to proximity effects from Bi2Sr2Ca1Cu2O8+δ. The resonances persist with gating from p+−p and p−n configurations. The suppression of the oscillation amplitude above the bias energy which is comparable to the induced superconducting gap indicates the contribution from Andreev reflection. Our experimental measurements are supported by quantum transport calculations in such interfaces, leading to analogous resonances. Our results put forward a hybrid scattering mechanism in graphene–high-temperature superconductor heterojunctions of potential impact for graphene-based Josephson junctions.Item Antichiral states in twisted graphene multilayers(American Physical Society, 2020-11-05) Denner, M. Michael; Lado, Jose; Zilberberg, Oded; ETH Zurich; Correlated Quantum Materials (CQM); Department of Applied PhysicsThe advent of topological phases of matter revealed a variety of observed boundary phenomena, such as chiral and helical modes found at the edges of two-dimensional (2D) topological insulators. Antichiral states in 2D semimetals, i.e., copropagating edge modes on opposite edges compensated by a counterpropagating bulk current, are also predicted, but, to date, no realization of such states in a solid-state system has been found. Here, we put forward a procedure to realize antichiral states in twisted van der Waals multilayers, by combining the electronic Dirac-cone spectra of each layer through the combination of the orbital moiré superstructure, an in-plane magnetic field, and interlayer bias voltage. In particular, we demonstrate that a twisted van der Waals heterostructure consisting of graphene/two layers of hexagonal boron nitride [(hBN)2]/graphene will show antichiral states at in-plane magnetic fields of 8 T, for a rotation angle of 0.2∘ between the graphene layers. Our findings engender a controllableprocedure to engineer antichiral states in solid-state systems, as well as in quantum engineered metamaterials.Item Atomic-Scale Andreev Probe of Unconventional Superconductivity(American Chemical Society, 2023-09-13) Ko, Wonhee; Song, Sang Yong; Yan, Jiaqiang; Lado, Jose; Maksymovych, Petro; Oak Ridge National Laboratory; Correlated Quantum Materials (CQM); Department of Applied PhysicsRecent emergence of low-dimensional unconventional superconductors and their exotic interface properties calls for new approaches to probe the pairing symmetry, a fundamental and frequently elusive property of the superconducting condensate. Here, we introduce the unique capability of tunneling Andreev reflection (TAR) to probe unconventional pairing symmetry, utilizing the sensitivity of this technique to specific Andreev reflections. Specifically, suppression of the lowest-order Andreev reflection due to quantum interference but emergence of the higher-order Andreev processes provides direct evidence of the sign-changing order parameter in the paradigmatic FeSe superconductor. TAR spectroscopy also reveals two superconducting gaps, points to a possibility of a nodal gap structure, and directly confirms that superconductivity is locally suppressed along the nematic twin boundary, with preferential and near-complete suppression of the larger energy gap. Our findings therefore enable new, atomic-scale insight into microscopic, inhomogeneous, and interfacial properties of emerging quantum materials.Item Conductance oscillation in surface junctions of Weyl semimetals(American Physical Society, 2021-11-08) Chen, Xi Rong; Chen, Guangze; Zheng, Yue; Chen, Wei; Xing, D. Y.; Nanjing University; Correlated Quantum Materials (CQM); Department of Applied PhysicsFermi arc surface states, the manifestation of the bulk-edge correspondence in Weyl semimetals, have attracted much research interest. In contrast to the conventional Fermi loop, the disconnected Fermi arcs provide an exotic two-dimensional (2D) system for exploration of novel physical effects on the surface of Weyl semimetals. Here, we propose that visible conductance oscillation can be achieved in planar junctions fabricated on the surface of a Weyl semimetal with a pair of Fermi arcs. It is shown that Fabry-Pérot-type interference inside the 2D junction can generate conductance oscillation with its visibility strongly relying on the shape of the Fermi arcs and their orientation relative to the strip electrodes, the latter clearly revealing the anisotropy of the Fermi arcs. Moreover, we show that the visibility of the oscillating pattern can be significantly enhanced by a magnetic field perpendicular to the surface taking advantage of the bulk-surface connected Weyl orbits. Our work offers an effective way to identify Fermi arc surface states through transport measurement and predicts the surface of Weyl semimetals as a novel platform for the implementation of 2D conductance oscillations.Item Controlling magnetism through Ising superconductivity in magnetic van der Waals heterostructures(American Physical Society, 2022-02-07) Aikebaier, Faluke; Heikkilä, Tero T.; Lado, Jose; Department of Applied Physics; University of Jyväskylä; Correlated Quantum Materials (CQM)Van der Waals heterostructures have risen as a tunable platform to combine different electronic orders, due to the flexibility in stacking different materials with competing symmetry broken states. Among them, van der Waals ferromagnets such as CrI3, CrBr3, or CrCl3 and superconductors as NbSe2 provide a natural platform to engineer novel phenomena at ferromagnet-superconductor interfaces. In particular, NbSe2 is well known for hosting strong spin-orbit coupling effects that influence the properties of the superconducting state. Here we put forward a ferromagnet/NbSe2/ferromagnet heterostructure where the interplay between Ising superconductivity in NbSe2 and magnetism controls the magnetic alignment of the heterostructure. In particular, we show that the interplay between spin-orbit coupling and superconductivity provides a new mechanism to control magnetic ordering in van der Waals materials. We show that this coupling allows creating heterostructures featuring a magnetic phase transition from in-plane to out-of-plane associated with the onset of superconductivity. Our results show how a hybrid van der Waals ferromagnet/superconductor heterostructure can be used as a tunable materials platform for superconducting spin-orbitronics.Item Dynamical topological excitations in parafermion chains(American Physical Society, 2021-01-29) Kaskela, Vilja; Lado, Jose; Department of Applied Physics; Correlated Quantum Materials (CQM)Parafermions are elusive fractional excitations potentially emerging in fractional quantum Hall-superconductor junctions and represent one of the major milestones in fractional quantum matter. However, generic models of parafermions are not analytically solvable, and understanding their topological modes is a bigger challenge than conventional Majorana modes. Here, by using a combination of tensor network and kernel polynomial techniques, we demonstrate the emergence of zero modes and finite energy excitations in many-body parafermion chains. We show the appearance of zero-energy modes in the many-body spectral function at the edge of a topological parafermion chain, their relation with the topological degeneracy of the system, and we compare their physics with the Majorana bound states of topological superconductors. We demonstrate the robustness of parafermion topological modes with respect to a variety of perturbations, and we show how weakly coupled parafermion chains give rise to in-gap excitations. Our results exemplify the versatility of tensor network methods for studying dynamical excitations of interacting parafermion chains and highlight the robustness of topological modes in parafermion models.Item Effects of electron-electron interactions in the Yu-Shiba-Rusinov lattice model(American Physical Society, 2023-05-25) Kachin, Valerii; Ojanen, Teemu; Lado, Jose; Hyart, Timo; Department of Applied Physics; Tampere University; Correlated Quantum Materials (CQM)In two-dimensional superconductors, Yu-Shiba-Rusinov bound states, induced by the magnetic impurities, extend over long distances giving rise to a long-range hopping model supporting a large number of topological phases with distinct Chern numbers. Here, we study how the electron-electron interactions affect, on a mean-field level, the selection of the realized Chern numbers and the magnitudes of the topological energy gaps in this model. We find that, in the case of an individual choice of the model parameters, the interactions can enhance or reduce the topological gap as well as cause topological phase transitions because of the complex interplay of superconductivity, magnetism, and the large spatial extent of the Yu-Shiba-Rusinov states. By sampling a large number of realizations of Yu-Shiba-Rusinov lattice models with different model parameters, we show that, statistically, the interactions have no effect on the realized Chern numbers and typical magnitudes of the topological gaps. However, the interactions substantially increase the likelihood of the largest topological gaps in the tails of the energy gap distribution in comparison to the noninteracting case.Item Impurity-induced resonant spinon zero modes in Dirac quantum spin liquids(American Physical Society, 2020-09-22) Chen, Guangze; Lado, Jose; Correlated Quantum Materials (CQM); Department of Applied PhysicsQuantum spin liquids are strongly correlated phases of matter displaying a highly entangled ground state. Because of their unconventional nature, finding experimental signatures of these states has proven to be a remarkable challenge. Here we show that the effects of local impurities can provide strong signatures of a Dirac quantum spin-liquid state. Focusing on a gapless Dirac quantum spin-liquid state as realized in NaYbO2, we show that a single magnetic impurity coupled to the quantum spin-liquid state creates a resonant spinon peak at zero frequency, coexisting with the original Dirac spinons. We explore the spatial dependence of this zero-bias resonance and show how different zero modes stemming from several impurities interfere. We finally address how such spinon zero-mode resonances can be experimentally probed with inelastic spectroscopy and electrically driven paramagnetic resonance with scanning tunnel microscopy. Our results put forward impurity engineering as a means of identifying Dirac quantum spin liquids with scanning probe techniques, highlighting the dramatic impact of magnetic impurities in a macroscopically entangled many-body ground state.Item Interplay of quantum spin Hall effect and spontaneous time-reversal symmetry breaking in electron-hole bilayers. I. Transport properties(American Physical Society, 2022-12-15) Paul, Tania; Becerra, V. Fernández; Hyart, Timo; Institute of Physics of the Polish Academy of Sciences; Correlated Quantum Materials (CQM); Department of Applied PhysicsThe band-inverted electron-hole bilayers, such as InAs/GaSb, are an interesting playground for the interplay of quantum spin Hall effect and correlation effects because of the small density of electrons and holes and the relatively small hybridization between the electron and hole bands. It has been proposed that Coulomb interactions lead to a time-reversal symmetry broken phase when the electron and hole densities are tuned from the trivial to the quantum spin Hall insulator regime. We show that the transport properties of the system in the time-reversal symmetry broken phase are consistent with recent experimental observations in InAs/GaSb. Moreover, we carry out a quantum transport study on a Corbino disk where the bulk and edge contributions to the conductance can be separated. We show that the edge becomes smoothly conducting and the bulk is always insulating when one tunes the system from the trivial to the quantum spin Hall insulator phase, providing unambiguous transport signatures of the time-reversal symmetry broken phase.Item Interplay of quantum spin Hall effect and spontaneous time-reversal symmetry breaking in electron-hole bilayers. II. Zero-field topological superconductivity(American Physical Society, 2022-12-15) Paul, Tania; Becerra, V. Fernández; Hyart, Timo; Institute of Physics of the Polish Academy of Sciences; Correlated Quantum Materials (CQM); Department of Applied PhysicsIt has been proposed that band-inverted electron-hole bilayers support a phase transition from an insulating phase with spontaneously broken time-reversal symmetry to a quantum spin Hall insulator phase as a function of increasing electron and hole densities. Here we show that in the presence of proximity-induced superconductivity, it is possible to realize Majorana zero modes in the time-reversal symmetry broken phase in the absence of magnetic field. We develop an effective low-energy theory for the system in the presence of a time-reversal symmetry-breaking order parameter to obtain analytically the Majorana zero modes and we find good agreement between the numerical and analytical results in the limit of weakly broken time-reversal symmetry. We show that the Majorana zero modes can be detected in superconductor/time-reversal symmetry broken insulator/superconductor Josephson junctions through the measurement of a 4π Josephson current. Finally, we demonstrate that the Majorana fusion-rule detection is feasible by utilizing the gate voltage dependence of the spontaneous time-reversal symmetry breaking order parameter.Item Many-body Majorana-like zero modes without gauge symmetry breaking(American Physical Society, 2021-04-01) Vadimov, V.; Hyart, T.; Lado, J. L.; Mottonen, M.; Ala-Nissila, T.; Department of Applied Physics; Correlated Quantum Materials (CQM); Centre of Excellence in Quantum Technology, QTFTopological superconductors represent one of the key hosts of Majorana-based topological quantum computing. Typical scenarios for one-dimensional (1D) topological superconductivity assume a broken gauge symmetry associated to a superconducting state. However, no interacting 1D many-body system is known to spontaneously break gauge symmetries. Here, we show that zero modes emerge in a many-body system without gauge symmetry breaking and in the absence of superconducting order. In particular, we demonstrate that Majorana zero modes of the symmetry-broken superconducting state are continuously connected to these zero-mode excitations, demonstrating that zero-bias anomalies may emerge in the absence of gauge symmetry breaking. We demonstrate that these many-body zero modes share the robustness features of the Majorana zero modes of symmetry-broken topological superconductors. We further show that the interface between the interacting model and a 1D topological superconductor does not support Majorana modes. We introduce a bosonization formalism to analyze these excitations and show that a ground state analogous to a topological superconducting state can be analytically found in a certain limit. Our results demonstrate that robust Majorana-like zero modes may appear in a many-body system without gauge symmetry breaking, thus introducing a family of protected excitations with no single-particle analogs.Item Neural network enhanced hybrid quantum many-body dynamical distributions(American Physical Society, 2021-07-29) Koch, Rouven; Lado, Jose; Department of Applied Physics; Correlated Quantum Materials (CQM)Computing dynamical distributions in quantum many-body systems represents one of the paradigmatic open problems in theoretical condensed matter physics. Despite the existence of different techniques both in real-time and frequency space, computational limitations often dramatically constrain the physical regimes in which quantum many-body dynamics can be efficiently solved. Here we show that the combination of machine-learning methods and complementary many-body tensor network techniques substantially decreases the computational cost of quantum many-body dynamics. We demonstrate that combining kernel polynomial techniques and real-time evolution, together with deep neural networks, allows to compute dynamical quantities faithfully. Focusing on many-body dynamical distributions, we show that this hybrid neural-network many-body algorithm, trained with single-particle data only, can efficiently extrapolate dynamics for many-body systems without prior knowledge. Importantly, this algorithm is shown to be substantially resilient to numerical noise, a feature of major importance when using this algorithm together with noisy many-body methods. Ultimately, our results provide a starting point towards neural-network powered algorithms to support a variety of quantum many-body dynamical methods, that could potentially solve computationally expensive many-body systems in a more efficient manner.Item Quantized Spin Pumping in Topological Ferromagnetic-Superconducting Nanowires(American Physical Society, 2023-06-09) Becerra, V. Fernández; Trif, Mircea; Hyart, Timo; Institute of Physics of the Polish Academy of Sciences; Correlated Quantum Materials (CQM); Department of Applied PhysicsSemiconducting nanowires with strong spin-orbit coupling in the presence of induced superconductivity and ferromagnetism can support Majorana zero modes. We study the pumping due to the precession of the magnetization in single-subband nanowires and show that spin pumping is robustly quantized when the hybrid nanowire is in the topologically nontrivial phase, whereas charge pumping is not quantized. Moreover, there exists one-to-one correspondence between the quantized conductance, entropy change and spin pumping in long topologically nontrivial nanowires but these observables are uncorrelated in the case of accidental zero-energy Andreev bound states in the trivial phase. Thus, we conclude that observation of correlated and quantized peaks in the conductance, entropy change and spin pumping would provide strong evidence of Majorana zero modes, and we elaborate how topological Majorana zero modes can be distinguished from quasi-Majorana modes potentially created by a smooth tunnel barrier at the lead-nanowireinterface. Finally, we discuss peculiar interference effects affecting the spin pumping in short nanowires at very low energies.Item Quasiperiodic criticality and spin-triplet superconductivity in superconductor-antiferromagnet moiré patterns(American Physical Society, 2021-03-19) Khosravian, Maryam; Lado, Jose; Department of Applied Physics; Correlated Quantum Materials (CQM)Quasiperiodicity has long been known to be a potential platform to explore exotic phenomena, realizing an intricate middle point between ordered solids and disordered matter. In particular, quasiperiodic structures are promising playgrounds to engineer critical wave functions, a powerful starting point to engineer exotic correlated states. Here we show that systems hosting a quasiperiodic modulation of antiferromagnetism and spin-singlet superconductivity, as realized by atomic chains in twisted van der Waals materials, host a localization-delocalization transition as a function of the coupling strength. Associated with this transition, we demonstrate the emergence of a robust quasiperiodic critical point for arbitrary incommensurate potentials, which appears for generic relative weights of the spin-singlet superconductivity and antiferromagnetism. We show that inclusion of residual electronic interactions leads to an emergent spin-triplet superconducting state, which gets dramatically enhanced at the vicinity of the quasiperiodic critical point. Our results put forward quasiperiodicity as a powerful knob to engineer robust superconducting states, providing an alternative pathway towards artificially designed unconventional superconductors.Item Solitonic in-gap modes in a superconductor-quantum antiferromagnet interface(American Physical Society, 2020-06-16) Lado, J. L.; Sigrist, M.; Correlated Quantum Materials (CQM); ETH Zurich; Department of Applied PhysicsBound states at interfaces between superconductors and other materials are a powerful tool to characterize the nature of the involved systems and to engineer elusive quantum excitations. In-gap excitations of conventional s-wave superconductors occur, for instance, at magnetic impurities with net magnetic moment breaking timereversal symmetry. Here we show that interfaces between a superconductor and a quantum antiferromagnet can host robust in-gap excitations, without breaking time-reversal symmetry. We illustrate this phenomenon in a one-dimensional model system with an interface between a conventional s-wave superconductor and a one-dimensional Mott insulator described by a standard Hubbard model. This genuine many-body problem is solved exactly by employing a combination of kernel polynomial and tensor network techniques. We unveil the nature of such zero modes by showing that they can be adiabatically connected to solitonic solutions between a superconductor and a mean-field antiferromagnet. Our results put forward a new class of in-gap excitations between superconductors and a disordered quantum spin phase, including quantum spin-liquids, that can be relevant for a wider range of heterostructures.Item Universal suppression of superfluid weight by non-magnetic disorder in s-wave superconductors independent of quantum geometry and band dispersion(SCIPOST FOUNDATION, 2022-10-07) Lau, Alexander; Peotta, Sebastiano; Pikulin, Dmitry I.; Rossi, Enrico; Hyart, Timo; Institute of Physics of the Polish Academy of Sciences; Quantum Dynamics; Microsoft USA; College of William and Mary; Correlated Quantum Materials (CQM); Department of Applied PhysicsMotivated by the experimental progress in controlling the properties of the energy bands in superconductors, significant theoretical efforts have been devoted to study the effect of the quantum geometry and the flatness of the dispersion on the superfluid weight. In conventional superconductors, where the energy bands are wide and the Fermi energy is large, the contribution due to the quantum geometry is negligible, but in the opposite limit of flat-band superconductors the superfluid weight originates purely from the quantum geometry of Bloch wave functions. Here, we study how the energy band dispersion and the quantum geometry affect the disorder-induced suppression of the superfluid weight. In particular, we consider non-magnetic disorder and s-wave superconductivity. Surprisingly, we find that the disorder-dependence of the superfluid weight is universal across a variety of models, and independent of the quantum geometry and the flatness of the dispersion. Our results suggest that a flat-band superconductor is as resilient to disorder as a conventional superconductor.Item Unprotected edge modes in quantum spin Hall insulator candidate materials(American Physical Society, 2023-01-26) Nguyen, Nguyen Minh; Cuono, Giuseppe; Islam, Rajibul; Autieri, Carmine; Hyart, Timo; Brzezicki, Wojciech; Institute of Physics of the Polish Academy of Sciences; Correlated Quantum Materials (CQM); Department of Applied PhysicsThe experiments in quantum spin Hall insulator candidate materials, such as HgTe/CdTe and InAs/GaSb heterostructures, indicate that in addition to the topologically protected helical edge modes, these multilayer heterostructures may also support additional edge states, which can contribute to scattering and transport. We use first-principles calculations to derive an effective tight-binding model for HgTe/CdTe, HgS/CdTe, and InAs/GaSb heterostructures, and we show that all these materials support additional edge states which are sensitive to edge termination. We trace the microscopic origin of these states back to a minimal model supporting flat bands with a nontrivial quantum geometry that gives rise to polarization charges at the edges. We show that the polarization charges transform into additional edge states when the flat bands are coupled to each other and to the other states to form the Hamiltonian describing the full heterostructure. Interestingly, in HgTe/CdTe quantum wells the additional edge statesare far away from the Fermi level so that they do not contribute to the transport, but in the HgS/CdTe and InAs/GaSb heterostructures they appear within the bulk energy gap, giving rise to the possibility of multimode edge transport. Finally, we demonstrate that because these additional edge modes are nontopological it is possible to remove them from the bulk energy gap by modifying the edge potential, for example, with the help of a side gate or chemical doping.Item Yu-Shiba-Rusinov Qubit(American Physical Society, 2021-12-07) Mishra, Archana; Simon, Pascal; Hyart, Timo; Trif, Mircea; Institute of Physics of the Polish Academy of Sciences; Université Paris-Saclay; Correlated Quantum Materials (CQM); Department of Applied PhysicsMagnetic impurities in s-wave superconductors lead to spin-polarized Yu-Shiba-Rusinov (YSR) in-gap states. Chains of magnetic impurities offer one of the most viable routes for the realization of Majorana bound states, which hold promise for topological quantum computing. However, this ambitious goal looks distant, since no quantum coherent degrees of freedom have yet been identified in these systems. To fill this gap, we propose an effective two-level system, a YSR qubit, stemming from two nearby impurities. Using a time-dependent wave-function approach, we derive an effective Hamiltonian describing the YSR-qubit evolution as a function of the distance between the impurity spins, their relative orientations, and their dynamics. We show that the YSR qubit can be controlled and read out using state-of-the-art experimental techniques for manipulation of the spins. Finally, we address the effect of spin noise on the coherence properties of the YSR qubit and show robust behavior for a wide range of experimentallyrelevant parameters. Looking forward, the YSR qubit could facilitate the implementation of a universal set of quantum gates in hybrid systems where they are coupled to topological Majorana qubits.