Browsing by Author "Ojanen, Teemu"

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  • Amorphous topological superconductivity in a Shiba glass
    (2018-12-01) Pöyhönen, Kim; Sahlberg, Isac; Westström, Alex; Ojanen, Teemu
     A1 Alkuperäisartikkeli tieteellisessä aikakauslehdessä
    Topological states of matter support quantised nondissipative responses and exotic quantum particles that cannot be accessed in common materials. The exceptional properties and application potential of topological materials have triggered a large-scale search for new realisations. Breaking away from the popular trend focusing almost exclusively on crystalline symmetries, we introduce the Shiba glass as a platform for amorphous topological quantum matter. This system consists of an ensemble of randomly distributed magnetic atoms on a superconducting surface. We show that subgap Yu-Shiba-Rusinov states on the magnetic moments form a topological superconducting phase at critical density despite a complete absence of spatial order. Experimental signatures of the amorphous topological state can be obtained by scanning tunnelling microscopy measurements probing the topological edge mode. Our discovery demonstrates the physical feasibility of amorphous topological quantum matter, presenting a concrete route to fabricating new topological systems from nontopological materials with random dopants.
  • Chern mosaic: Topology of chiral superconductivity on ferromagnetic adatom lattices
    (2016-03-24) Röntynen, Joel; Ojanen, Teemu
     A1 Alkuperäisartikkeli tieteellisessä aikakauslehdessä
    In this work, we will explore the properties of superconducting surfaces decorated by two-dimensional ferromagnetic adatom lattices. As discovered recently [Röntynen and Ojanen, Phys. Rev. Lett. 114, 236803 (2015)PRLTAO0031-900710.1103/PhysRevLett.114.236803], in the presence of a Rashba spin-orbit coupling these systems may support topological superconductivity with complex phase diagrams and high Chern numbers. We show how the long-range hopping nature of the effective low-energy theory generically gives rise to a phase diagram covered by a Chern mosaic, a rich pattern of topological phases with large Chern numbers. We study different lattice geometries and the dependence of energy gaps and abundance of different phases as a function of system parameters. Our findings establish the studied system as one of the richest platforms for topological matter known to date.
  • Coupled Yu-Shiba-Rusinov States in Molecular Dimers on NbSe2
    (2018-04-11) Kezilebieke, Shawulienu; Dvorak, Marc; Ojanen, Teemu; Liljeroth, Peter
     A1 Alkuperäisartikkeli tieteellisessä aikakauslehdessä
    Magnetic impurities have a dramatic effect on superconductivity by breaking the time-reversal symmetry and inducing so-called Yu-Shiba-Rusinov (YSR) low energy bound states within the superconducting gap. The spatial extent of YSR states is greatly enhanced in two-dimensional (2D) systems, which should facilitate the formation of coupled states. Here, we observe YSR states on single cobalt phthalocyanine (CoPC) molecules on a 2D superconductor NbSe2 using low-temperature scanning tunneling microscopy (STM) and spectroscopy. We use STM lateral manipulation to create controlled CoPc dimers and demonstrate the formation of coupled YSR states. The experimental results are corroborated by theoretical analysis of the coupled states in lattice and continuum models.
  • Curved spacetime theory of inhomogeneous Weyl materials
    (2019-10-16) Liang, Long; Ojanen, Teemu
     A1 Alkuperäisartikkeli tieteellisessä aikakauslehdessä
    We show how the universal low-energy properties of Weyl semimetals with spatially varying time-reversal (TR) or inversion (I) symmetry breaking are described in terms of chiral fermions experiencing curved-spacetime geometry and synthetic gauge fields. By employing Clifford representations and Schrieffer-Wolff transformations, we present a systematic derivation of an effective curved-space Weyl theory with rich geometric and gauge structure. To illustrate the utility of the formalism, we give a concrete prescription of how to fabricate nontrivial curved spacetimes and event horizons in topological insulators with magnetic textures. Our theory can also account for strain-induced effects, providing a powerful unified framework for studying and designing inhomogeneous Weyl materials.
  • Designer curved-space geometry for relativistic fermions in weyl metamaterials
    (2017-10-30) Westström, Alex; Ojanen, Teemu
     A1 Alkuperäisartikkeli tieteellisessä aikakauslehdessä
    Weyl semimetals are recently discovered materials supporting emergent relativistic fermions in the vicinity of band-crossing points known as Weyl nodes. The positions of the nodes and the low-energy spectrum depend sensitively on the time-reversal and inversion symmetry breaking in the system. We introduce the concept of Weyl metamaterials where the particles experience a 3D curved geometry and gauge fields emerging from smooth spatially varying time-reversal-And inversion-breaking fields. The Weyl metamaterials can be fabricated from semimetal or insulator parent states where the geometry can be tuned, for example, through inhomogeneous magnetization. We derive an explicit connection between the effective geometry and the local symmetry-breaking configuration. This result opens the door for a systematic study of 3D designer geometries and gauge fields for relativistic carriers. The Weyl metamaterials provide a route to novel electronic devices as highlighted by a remarkable 3D electron lens effect.
  • Determination of Dynamical Quantum Phase Transitions in Strongly Correlated Many-Body Systems Using Loschmidt Cumulants
    (2021-10-26) Peotta, Sebastiano; Brange, Fredrik; Deger, Aydin; Ojanen, Teemu; Flindt, Christian
     A1 Alkuperäisartikkeli tieteellisessä aikakauslehdessä
    Dynamical phase transitions extend the notion of criticality to nonstationary settings and are characterized by sudden changes in the macroscopic properties of time-evolving quantum systems. Investigations of dynamical phase transitions combine aspects of symmetry, topology, and nonequilibrium physics; however, progress has been hindered by the notorious difficulties of predicting the time evolution of large, interacting quantum systems. Here, we tackle this outstanding problem by determining the critical times of interacting many-body systems after a quench using Loschmidt cumulants. Specifically, we investigate dynamical topological phase transitions in the interacting Kitaev chain and in the spin-1 Heisenberg chain. To this end, we map out the thermodynamic lines of complex times, where the Loschmidt amplitude vanishes, and identify the intersections with the imaginary axis, which yield the real critical times after a quench. For the Kitaev chain, we can accurately predict how the critical behavior is affected by strong interactions, which gradually shift the time at which a dynamical phase transition occurs. We also discuss the experimental perspectives of predicting the first critical time of a quantum many-body system by measuring the energy fluctuations in the initial state, and we describe the prospects of implementing our method on a near-term quantum computer with a limited number of qubits. Our work demonstrates that Loschmidt cumulants are a powerful tool to unravel the far-from-equilibrium dynamics of strongly correlated many-body systems, and our approach can immediately be applied in higher dimensions.
  • Dynamical quantum phase transitions in strongly correlated two-dimensional spin lattices following a quench
    (2022-07-13) Brange, Fredrik; Peotta, Sebastiano; Flindt, Christian; Ojanen, Teemu
     A1 Alkuperäisartikkeli tieteellisessä aikakauslehdessä
    Dynamical quantum phase transitions are at the forefront of current efforts to understand quantum matter out of equilibrium. Except for a few exactly solvable models, predictions of these critical phenomena typically rely on advanced numerical methods. However, those approaches are mostly restricted to one dimension, making investigations of two-dimensional systems highly challenging. Here, we present evidence of dynamical quantum phase transitions in strongly correlated spin lattices in two dimensions. To this end, we apply our recently developed cumulant method [Phys. Rev. X11, 041018 (2021)] to determine the zeros of the Loschmidt amplitude in the complex plane of time, and we predict the crossing points of the thermodynamic lines of zeros with the real-time axis, where dynamical quantum phase transitions occur. We find the critical times of a two-dimensional quantum Ising lattice and the XYZ model with ferromagnetic or antiferromagnetic couplings. We also show how dynamical quantum phase transitions can be predicted by measuring the initial energy fluctuations, for example in quantum simulators or other engineered quantum systems.
  • Effects of electron-electron interactions in the Yu-Shiba-Rusinov lattice model
    (2023-05-25) Kachin, Valerii; Ojanen, Teemu; Lado, Jose; Hyart, Timo
     A1 Alkuperäisartikkeli tieteellisessä aikakauslehdessä
    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.
  • Engineering of Chern insulators and circuits of topological edge states
    (2019-04-10) Minarelli, Emma L.; Poÿhönen, Kim; Van Dalum, Gerwin A.R.; Ojanen, Teemu; Fritz, Lars
     A1 Alkuperäisartikkeli tieteellisessä aikakauslehdessä
    Impurities embedded in electronic systems induce bound states which under certain circumstances can hybridize and lead to impurity bands. Doping of insulators with impurities has been identified as a promising route toward engineering electronic topological states of matter. In this paper we show how to realize tuneable Chern insulators starting from a three-dimensional topological insulator whose surface is gapped and intentionally doped with magnetic impurities. The main advantage of the protocol is that it is robust and in particular not very sensitive to the impurity configuration. We explicitly demonstrate this for a square lattice of impurities as well as a random lattice. In both cases we show that it is possible to change the Chern number of the system by one through manipulating its topological state. We also discuss how this can be used to engineer circuits of edge channels.
  • Engineering one-dimensional topological phases on p -wave superconductors
    (2017-05-18) Sahlberg, Isac; Westström, Alex; Pöyhönen, Kim; Ojanen, Teemu
     A1 Alkuperäisartikkeli tieteellisessä aikakauslehdessä
    In this paper, we study how, with the aid of impurity engineering, two-dimensional p-wave superconductors can be employed as a platform for one-dimensional topological phases. We discover that, while chiral and helical parent states themselves are topologically nontrivial, a chain of scalar impurities on both systems supports multiple topological phases and Majorana end states. We develop an approach which allows us to extract the topological invariants and subgap spectrum, even away from the center of the gap, for the representative cases of spinless, chiral, and helical superconductors. We find that the magnitude of the topological gaps protecting the nontrivial phases may be a significant fraction of the gap of the underlying superconductor.
  • Extracting electronic many-body correlations from local measurements with artificial neural networks
    (2023-04-17) Aikebaier, Faluke; Ojanen, Teemu; Lado, Jose
     A1 Alkuperäisartikkeli tieteellisessä aikakauslehdessä
    The characterization of many-body correlations provides a powerful tool for analyzing correlated quantum materials. However, experimental extraction of quantum entanglement in correlated electronic systems remains an open problem in practice. In particular, the correlation entropy quantifies the strength of quantum correlations in interacting electronic systems, yet it requires measuring all the single-particle correlators of a macroscopic sample. To circumvent this bottleneck, we introduce a strategy to obtain the correlation entropy of electronic systems solely from a set of local measurements. We show that, by combining local particle-particle and density-density correlations with a neural-network algorithm, the correlation entropy can be predicted accurately. Specifically, we show that for a generalized interacting fermionic model, our algorithm yields an accurate prediction of the correlation entropy from a set of noisy local correlators. Our work shows that the correlation entropy in interacting electron systems can be reconstructed from local measurements, providing a starting point to experimentally extract many-body correlations with local probes.
  • Ground-state angular momentum, spectral asymmetry, and topology in chiral superfluids and superconductors
    (2016-05-10) Ojanen, Teemu
     A1 Alkuperäisartikkeli tieteellisessä aikakauslehdessä
    Recently, it was discovered that the ground-state orbital angular momentum in two-dimensional chiral superfluids with pairing symmetry (px+ipy)ν depends on the winding number ν in a striking manner. The ground-state value for the ν=1 case is Lz=N/2 as expected by counting the Cooper pairs, while a dramatic cancellation takes place for ν>1. The origin of the cancellation is associated with the topological edge states that appear in a finite geometry and give rise to a spectral asymmetry. Here, we study the reduction of orbital angular momentum for different potential profiles and pairing strengths, showing that the result Lz=N/2 is robust for ν=1 under all studied circumstances. We study how angular momentum depends on the gap size Δ/EF and obtain the result Lz=ν2N(1-μEF) for ν=2,3. Thus, the gap dependence of Lz for ν
  • Identifying Chern numbers of superconductors from local measurements
    (2023) Baireuther, Paul; Płodzién, Marcin; Ojanen, Teemu; Tworzydło, Jakub; Hyart, Timo
     A1 Alkuperäisartikkeli tieteellisessä aikakauslehdessä
    Fascination in topological materials originates from their remarkable response properties and exotic quasiparticles which can be utilized in quantum technologies. In particular, large-scale efforts are currently focused on realizing topological superconductors and their Majorana excitations. However, determining the topological nature of superconductors with current experimental probes is an outstanding challenge. This shortcoming has become increasingly pressing due to rapidly developing designer platforms which are theorized to display very rich topology and are better accessed by local probes rather than transport experiments. We introduce a robust machine learning protocol for classifying the topological states of two-dimensional (2D) chiral superconductors and insulators from local density of states (LDOS) data. Since the LDOS can be measured with standard experimental techniques, our protocol contributes to overcoming the almost three decades standing problem of identifying the topological phase of 2D superconductors with broken time-reversal symmetry.
  • Inverse Problem for Quantum Mechanical Scattering in One Dimension
    (2014-09-30) Hujanen, Jaakko
     Perustieteiden korkeakoulu | Master's thesis
    The field of inverse problem considers mathematical methods for reconstructing unknown parameters of some model from given measurements. Such problems arise in different branches of science, e.g, in medical imaging. In this thesis, inverse mathematics is applied to a one-dimensional scattering problem in a quantum mechanical setting: the aim is to create an algorithm to determine the potential function of a scattering interval when transmission of electrons is observed. Since the scattering problem includes non-linear phenomena, an iterative reconstruction scheme is employed. The implemented numerical method combines finite element method (FEM) and the Gauss-Newton optimization algorithm in a manner which utilizes the FEM solver for numerical evaluations of the Jacobian matrix. Furthermore, the forward problem is shown to have a unique solution to reinforce credibility of the procedure. The resulting program optimally gives good reconstructions with simulated test data, but it is sensitive to the choice of regularization.
  • Machine learning the Kondo entanglement cloud from local measurements
    (2024-05-08) Aikebaier, Faluke; Ojanen, Teemu; Lado, Jose
     A1 Alkuperäisartikkeli tieteellisessä aikakauslehdessä
    A quantum coherent screening cloud around a magnetic impurity in metallic systems is the hallmark of the antiferromagnetic Kondo effect. Despite the central role of the Kondo effect in quantum materials, the structure of quantum correlations of the screening cloud has defied direct observations. In this work, we introduce a machine-learning algorithm that allows one to spatially map the entangled electronic modes in the vicinity of the impurity site from experimentally accessible data. We demonstrate that local correlators allow reconstruction of the local many-body correlation entropy in real space in a double Kondo system with overlapping entanglement clouds. Our machine-learning methodology allows bypassing the typical requirement of measuring long-range nonlocal correlators with conventional methods. We show that our machine-learning algorithm is transferable between different Kondo system sizes, and we show its robustness in the presence of noisy correlators. Our work establishes the potential machine-learning methods to map many-body entanglement from real-space measurements.
  • Majorana states in helical Shiba chains and ladders
    (2014) Pöyhönen, Kim; Westström, Alex; Röntynen, Joel; Ojanen, Teemu
     A1 Alkuperäisartikkeli tieteellisessä aikakauslehdessä
    Motivated by recent proposals to realize Majorana bound states in chains and arrays of magnetic atoms deposited on top of a superconductor, we study the topological properties of various chain structures, ladders, and two-dimensional arrangements exhibiting magnetic helices. We show that magnetic domain walls where the chirality of a magnetic helix is inverted support two protected Majorana states giving rise to a tunneling conductance peak twice the height of a single Majorana state. The topological properties of coupled chains exhibit nontrivial behavior as a function of the number of chains beyond the even-odd dichotomy expected from the simple Z2 nature of coupled Majorana states. In addition, it is possible that a ladder of two or more coupled chains exhibit Majorana edge states even when decoupled chains are trivial. We formulate a general criterion for the number of Majorana edge states in multichain ladders and discuss some experimental consequences of our findings.
  • Observation of coexistence of Yu-Shiba-Rusinov states and spin-flip excitations
    (2019-07-10) Shawulienu, Kezilebieke; Žitko, Rok; Dvorak, Marc; Ojanen, Teemu; Liljeroth, Peter
     A1 Alkuperäisartikkeli tieteellisessä aikakauslehdessä
    We investigate the spectral evolution in different metal phthalocyanine molecules on NbSe2 surface using scanning tunnelling microscopy (STM) as a function of the coupling with the substrate. For manganese phthalocyanine (MnPc), we demonstrate a smooth spectral crossover from Yu-Shiba-Rusinov (YSR) bound states to spin-flip excitations. This has not been observed previously and it is in contrast to simple theoretical expectations. We corroborate the experimental findings using numerical renormalization group calculations. Our results provide fundamental new insight on the behavior of atomic scale magnetic/SC hybrid systems, which is important, for example, for engineered topological superconductors and spin logic devices.
  • Quantum fluctuations and transport in mesoscopic physics
    (2007-11-30) Ojanen, Teemu
     Doctoral dissertation (article-based)
    Mesoscopic physics and nanoelectronics concentrate on systems with dimensions somewhere between atomic and everyday macroscopic scale. Modern technology enables construction of submicron nanostructures where modeling based on classical physics has proven inadequate. It is possible to design electric circuits where dynamics of single electrons and photons are controlled using state-of-the-art experimental methods. For quantitative understanding of these systems it is necessary to resort to a quantum-mechanical description. Quantum phenomena, such as tunneling and a wave-like interference of particles, are essential ingredients of physics in mesoscopic systems. Field of mesoscopic physics contains a rich variety of topics ranging from fundamental condensed matter physics to quantum information processing and possible future technological applications. This thesis presents theoretical studies of mesoscopic quantum phenomena in nanostructures and small electronic devices. We have focused on effects of environment fluctuations and investigated connections between fluctuations and transport phenomena. Decoherence in quantum bits (qubits) and quantum state engineering in superconducting circuits are also studied. The theoretical analysis in each case requires an open-system treatment. Effects of current fluctuations on quantum probe systems have been studied in detail. We have calculated transitions induced by current noise and discussed how these could be used for characterization of fluctuations. We have also shown that electric fluctuations play a key role in radiation and photon heat transport in nanostructures. Motivated by recent advances in mesoscopic electron-photon systems, we have studied a response of a coupled resonator-qubit system, squeezing of quantum fluctuations in small superconducting circuits and investigated decoherence in Josephson flux qubits.
  • Quantum walks on random lattices : Diffusion, localization, and the absence of parametric quantum speedup
    (2023-04) Duda, Rostislav; Ivaki, Moein N.; Sahlberg, Isac; Pöyhönen, Kim; Ojanen, Teemu
     A1 Alkuperäisartikkeli tieteellisessä aikakauslehdessä
    Discrete-time quantum walks, quantum generalizations of classical random walks, provide a framework for quantum information processing, quantum algorithms, and quantum simulation of condensed-matter systems. The key property of quantum walks, which lies at the heart of their quantum information applications, is the possibility for a parametric quantum speedup in propagation compared to classical random walks. In this work we study propagation of quantum walks on percolation-generated two-dimensional random lattices. In large-scale simulations of topological and trivial split-step walks, we identify distinct prediffusive and diffusive behaviors at different timescales. Importantly, we show that even arbitrarily weak concentrations of randomly removed lattice sites give rise to a complete breakdown of the superdiffusive quantum speedup, reducing the motion to ordinary diffusion. By increasing the randomness, quantum walks eventually stop spreading due to Anderson localization. Near the localization threshold, we find that the quantum walks become subdiffusive. The fragility of quantum speedup implies dramatic limitations for quantum information applications of quantum walks on random geometries and graphs.
  • Reply to: “Topological and trivial domain wall states in engineered atomic chains”
    (2022-02-16) Huda, Md Nurul; Kezilebieke, Shawulienu; Ojanen, Teemu; Drost, Robert; Liljeroth, Peter
     Letter