Showing 1–50 of 179 results for author: Heikkilä, T

Symmetry Classification of Non-Reciprocal Responses in Multiterminal Ring Devices

Authors: Chen-How Huang, Tero T. Heikkilä

Abstract: We present a symmetry-based framework to classify the non-reciprocal responses of multiterminal ring quantum devices. The device is modeled as a ring of $n$ vertices, where a binary variable $e_k\in\{+1,-1\}$ on each bond encodes the preferred direction of signal flow between terminals. Non-reciprocity corresponds to a preferred current configuration on the ring, and the symmetry group of the devi… ▽ More We present a symmetry-based framework to classify the non-reciprocal responses of multiterminal ring quantum devices. The device is modeled as a ring of $n$ vertices, where a binary variable $e_k\in\{+1,-1\}$ on each bond encodes the preferred direction of signal flow between terminals. Non-reciprocity corresponds to a preferred current configuration on the ring, and the symmetry group of the device partitions all $2^n$ configurations into equivalence classes(orbits) characterized by a topological winding number $W$. Using the minimal non-trivial case $n=3$, we establish two results independent of microscopic details. First, lifting the degeneracy within an orbit generates non-reciprocal responses. For $n=3$ this requires simultaneous breaking of both time-reversal $T$ and spatial inversion $I$. Breaking either alone is insufficient. Second, the residual geometry symmetry after $T$ and $I$ are broken determines which responses are observable. For an isosceles triangular geometry, only two types of response are allowed: uniform circulation (all bonds carrying current in the same direction) and semi-circulation with the reversed bond on the geometrically distinct base. Semi-circulation with the reversed bond on either equal leg is symmetry-forbidden. Both predictions are validated using a minimal toy model of three quantum dots coupled to superconducting baths, which demonstrates a reactive quantum circulator response. △ Less

Submitted 1 July, 2026; originally announced July 2026.

Comments: 13 pages, 10 Figures

Controlling magnetic domain walls with supercurrents

Authors: Tim Kokkeler, Risto Ojajärvi, F. Sebastian Bergeret, Tero T. Heikkilä

Abstract: Establishing a versatile, fast and reliable magnetic memory technology is a giant bottleneck for cryogenic computing since present-day room-temperature solutions either cease to work or consume too much power. The long-term goal of superconducting spintronics has been to overcome this bottleneck by generating magnetic memories with equal-spin triplet supercurrent driven through them to control the… ▽ More Establishing a versatile, fast and reliable magnetic memory technology is a giant bottleneck for cryogenic computing since present-day room-temperature solutions either cease to work or consume too much power. The long-term goal of superconducting spintronics has been to overcome this bottleneck by generating magnetic memories with equal-spin triplet supercurrent driven through them to control their magnetization direction. This path has been hampered by the short spin relaxation length and strong anisotropy in ferromagnets. Here we show how the supercurrent driven generation of spin accumulation in a superconductor/magnetic insulator bilayer, together with Gilbert damping of magnetization lead to a motion of magnetic domain walls. This manifests as a local voltage across the wall, which allows its position to be identified. Associated with this voltage and the current, there is Joule power which is dissipated via the Gilbert damping. The power required to maintain domain wall motion is orders of magnitude smaller than in the normal state, where most of the power is wasted in producing the current. △ Less

Submitted 17 June, 2026; originally announced June 2026.

Comments: 6 pages, 3 figures

Magnetic Field Walls in Flat-band Superconductors

Authors: Guodong Jiang, Aaron Dunbrack, Tero T. Heikkilä, Päivi Törmä

Abstract: Superconductors of different types have distinct magnetic properties; for example, they can form Abrikosov vortices or alternating normal-superconducting domains. We predict that, in flat bands, a superconducting phase exhibiting walls of magnetic flux is stable in an applied magnetic field. This phase relies on the lack of a single particle energy penalty for forming condensates of any momentum i… ▽ More Superconductors of different types have distinct magnetic properties; for example, they can form Abrikosov vortices or alternating normal-superconducting domains. We predict that, in flat bands, a superconducting phase exhibiting walls of magnetic flux is stable in an applied magnetic field. This phase relies on the lack of a single particle energy penalty for forming condensates of any momentum in flat bands and, consequently, their superconducting free energy being a negative and periodic function of the vector potential at low temperatures. Using a minimal lattice-periodic model of free energy, we study two types of soliton modes of the wall phase: the kink and breather solitons. They determine the lower critical field and the high-field behavior of the wall phase, respectively. The competition between the walls and vortices in flat bands is also discussed. Our results suggest that flat bands help sustain superconductivity in the presence of large magnetic fields. △ Less

Submitted 11 July, 2026; v1 submitted 4 June, 2026; originally announced June 2026.

Robust Model-Based Iteration for Passive Gamma Emission Tomography

Authors: Tommi Heikkilä, Sara Heikkinen, Riina Rimppi, Tapio Helin

Abstract: Passive Gamma Emission Tomography (PGET) is an IAEA-approved technique for verifying spent nuclear fuel assemblies prior to geological disposal. Reconstructing the emission and attenuation maps from PGET measurements is a nonlinear ill-posed inverse problem, currently solved with a Levenberg-Marquardt (LM) scheme that requires 10-20 iterations to achieve sufficient accuracy. We propose an accelera… ▽ More Passive Gamma Emission Tomography (PGET) is an IAEA-approved technique for verifying spent nuclear fuel assemblies prior to geological disposal. Reconstructing the emission and attenuation maps from PGET measurements is a nonlinear ill-posed inverse problem, currently solved with a Levenberg-Marquardt (LM) scheme that requires 10-20 iterations to achieve sufficient accuracy. We propose an accelerated iterative solver that combines the LM algorithm with a Deep Gauss-Newton step, in which a learned operator refines the update proposed by the deterministic algorithm at each iteration. A safeguard condition based on the trust-region model ensures that the accelerated iterates perform no worse than LM and retain convergence to a critical point of the regularized objective. Within this framework we compare three architectures for the learned component: an encoder-decoder-style convolutional neural network, Fourier Neural Operators, and Wavelet Neural Operators. Each is trained on a small set of coarsely simulated 9x9 assemblies. Experiments on simulated and real measurements from Finnish nuclear power plants show that the proposed scheme reaches LM-quality reconstructions in roughly one third of the iterations, while revealing architecture-dependent trade-offs in robustness against out-of-distribution inputs. △ Less

Submitted 29 April, 2026; originally announced April 2026.

Comments: 12 pages

Squeezing and measurement of a mechanical quadrature via PID feedback

Authors: Alberto Hijano, Tero T. Heikkilä

Abstract: Proportional-Integral-Derivative (PID) control is used for automatically regulating a measurable quantity to a desired setpoint. It is widely used in different types of classical control electronics. Here, we show how extending the feedback theory in quantum systems to include the derivative and integral parts influences both the transient and steady-state behavior of the amplitude and squeezing o… ▽ More Proportional-Integral-Derivative (PID) control is used for automatically regulating a measurable quantity to a desired setpoint. It is widely used in different types of classical control electronics. Here, we show how extending the feedback theory in quantum systems to include the derivative and integral parts influences both the transient and steady-state behavior of the amplitude and squeezing of a mechanical quadrature in an optomechanical system. We show that, in contrast to standard proportional feedback, derivative feedback affects both the conditional and unconditional squeezing. Furthermore, we demonstrate how feedback may be employed to drive a mechanical quadrature to track a desired reference signal. Our findings offer new routes for an improved quantum state control and measurement precision. △ Less

Submitted 17 April, 2026; originally announced April 2026.

Comments: 3 figures, 11 pages

Detection of spin- and valley-polarized states in van der Waals materials via thermoelectric and non-reciprocal transport

Authors: Oladunjoye A. Awoga, Pauli Virtanen, Tero T. Heikkilä, Stefan Ilić

Abstract: We predict thermoelectric and current rectification effects in hybrid junctions formed by Ising superconductors and materials hosting valley-polarized states. Both effects originate from the interplay of intrinsic Ising spin-orbit coupling, spin-splitting from an exchange or Zeeman field, and valley polarization. The resulting transport signatures provide experimentally accessible probes of valley… ▽ More We predict thermoelectric and current rectification effects in hybrid junctions formed by Ising superconductors and materials hosting valley-polarized states. Both effects originate from the interplay of intrinsic Ising spin-orbit coupling, spin-splitting from an exchange or Zeeman field, and valley polarization. The resulting transport signatures provide experimentally accessible probes of valley-polarized states in van der Waals heterostructures, such as junctions of few-layer transition metal dichalcogenides and twisted bilayer or rhombohedral graphene. △ Less

Submitted 2 April, 2026; originally announced April 2026.

Nonequilibrium spin-splitter effect in altermagnet superconductor hybrids

Authors: Tim Kokkeler, Tero T. Heikkilä, F. Sebastian Bergeret

Abstract: We study the nonequilibrium spin-splitter effect in superconducting altermagnets and superconductor altermagnet hybrids by computing the alternating spin current and edge the spin density in the presence of an alternating electric field. We show that while in the normal state the effect is not sensitive to the field frequency, in the superconducting state, there is a strong effect for frequencies… ▽ More We study the nonequilibrium spin-splitter effect in superconducting altermagnets and superconductor altermagnet hybrids by computing the alternating spin current and edge the spin density in the presence of an alternating electric field. We show that while in the normal state the effect is not sensitive to the field frequency, in the superconducting state, there is a strong effect for frequencies on the scale of $Δ_0$ or lower. We contrast the effect to the spin accumulation induced by the spin-Hall effect, by showing that for the altermagnet spin-splitter effect the out-of-phase spin density does not diverge in the adiabatic limit. This difference is attributed to the absence of any equilibrium spin-splitter effect in altermagnets. In fact, the out-of-phase component vanishes below the gap excitation frequency $2Δ_0$, because below this frequency the absence of dissipation and the behavior of the system under time-reversal directly determine the relative phase between the charge current, spin current, and spin accumulation. The nonequilibrium effect can be tuned by external parameters like temperature. In fact, it has a nonmonotonic temperature dependence, taking its largest value for temperatures around $0.8T_{c}$. The value at this temperature can be significantly larger than the normal state spin density or the low temperature spin density. Thus, besides using the nonequilibrium spin-splitter effect to identify altermagnets, its tunability makes it also suitable for applications. △ Less

Submitted 13 October, 2025; originally announced October 2025.

Comments: 9 pages, 6 figures

Superfluid weight in disordered flat-band superconductors as a competition between localization functionals

Authors: Kryštof Kolář, Tero T. Heikkilä, Päivi Törmä

Abstract: According to Anderson's theorem, the gap of a time-reversal symmetric weak-coupling superconductor is unaffected by non-magnetic disorder. However, the superfluid weight (stiffness) is reduced in the disordered limit by a factor of $Δτ$, a product of the scattering time $τ$ and the superconducting order parameter $Δ$. Here we show that the opposite holds true in flat-band superconductors. While no… ▽ More According to Anderson's theorem, the gap of a time-reversal symmetric weak-coupling superconductor is unaffected by non-magnetic disorder. However, the superfluid weight (stiffness) is reduced in the disordered limit by a factor of $Δτ$, a product of the scattering time $τ$ and the superconducting order parameter $Δ$. Here we show that the opposite holds true in flat-band superconductors. While non-magnetic disorder does reduce the order parameter, we find that its direct effect on superfluid weight is mostly negligible. We show analytically that the effect of disorder is to lowest order given in terms of the difference between the intraband and interband parts of the localization functional of impurity wavefunctions, finding it to be typically vanishing. △ Less

Submitted 21 August, 2026; v1 submitted 6 October, 2025; originally announced October 2025.

Comments: 5 pages + Supplementary. Published version

Journal ref: Phys. Rev. B 114, L020503, 2026

Gaussian path model library for intuitive robot motion programming by demonstration

Authors: Samuli Soutukorva, Markku Suomalainen, Martin Kollingbaum, Tapio Heikkilä

Abstract: This paper presents a system for generating Gaussian path models from teaching data representing the path shape. In addition, methods for using these path models to classify human demonstrations of paths are introduced. By generating a library of multiple Gaussian path models of various shapes, human demonstrations can be used for intuitive robot motion programming. A method for modifying existing… ▽ More This paper presents a system for generating Gaussian path models from teaching data representing the path shape. In addition, methods for using these path models to classify human demonstrations of paths are introduced. By generating a library of multiple Gaussian path models of various shapes, human demonstrations can be used for intuitive robot motion programming. A method for modifying existing Gaussian path models by demonstration through geometric analysis is also presented. △ Less

Submitted 15 September, 2025; v1 submitted 12 September, 2025; originally announced September 2025.

Robot and Overhead Crane Collaboration Scheme to Enhance Payload Manipulation

Authors: Antonio Rosales, Alaa Abderrahim, Markku Suomalainen, Mikael Haag, Tapio Heikkilä

Abstract: This paper presents a scheme to enhance payload manipulation using a robot collaborating with an overhead crane. In the current industrial practice, when the crane's payload has to be accurately manipulated and located in a desired position, the task becomes laborious and risky since the operators have to guide the fine motions of the payload by hand. In the proposed collaborative scheme, the cran… ▽ More This paper presents a scheme to enhance payload manipulation using a robot collaborating with an overhead crane. In the current industrial practice, when the crane's payload has to be accurately manipulated and located in a desired position, the task becomes laborious and risky since the operators have to guide the fine motions of the payload by hand. In the proposed collaborative scheme, the crane lifts the payload while the robot's end-effector guides it toward the desired position. The only link between the robot and the crane is the interaction force produced during the guiding of the payload. Two admittance transfer functions are considered to accomplish harmless and smooth contact with the payload. The first is used in a position-based admittance control integrated with the robot. The second one adds compliance to the crane by processing the interaction force through the admittance transfer function to generate a crane's velocity command that makes the crane follow the payload. Then the robot's end-effector and the crane move collaboratively to guide the payload to the desired location. A method is presented to design the admittance controllers that accomplish a fluent robot-crane collaboration. Simulations and experiments validating the scheme potential are shown. △ Less

Submitted 11 August, 2025; originally announced August 2025.

Josephson anomalous vortices

Authors: Dan Crawford, Stefan Ilić, Pauli Virtanen, Tero T. Heikkilä

Abstract: We show that vortices with circulating current, related with odd-frequency triplet pairing, appear in Josephson junctions where the barrier is a weak ferromagnet with strong spin-orbit coupling. By symmetry analysis we show that there is an additional term - a rotary invariant - in the superconducting free energy which allows for magnetoelectric effects even when the previously considered Lifshitz… ▽ More We show that vortices with circulating current, related with odd-frequency triplet pairing, appear in Josephson junctions where the barrier is a weak ferromagnet with strong spin-orbit coupling. By symmetry analysis we show that there is an additional term - a rotary invariant - in the superconducting free energy which allows for magnetoelectric effects even when the previously considered Lifshitz invariant vanishes. Using a microscopic model based on a modified Usadel equation incorporating those effects, we show that the size, shape, and position of these vortices can be controlled by manipulating Rashba spin-orbit coupling in the weak link, via gates, and we suggest that these vortices could be detected via scanning magnetometry techniques. We also show that the transverse triplet components of the pairing amplitudes can form a texture. △ Less

Submitted 10 October, 2025; v1 submitted 24 April, 2025; originally announced April 2025.

Comments: 8 pages, 4 figures

Ideal quantum geometry of the surface states of rhombohedral graphite and its effects on the surface superconductivity

Authors: Guodong Jiang, Tero Heikkilä, Päivi Törmä

Abstract: The interplay of quantum geometry and interactions determines the correlated state properties of flat bands. Here, we investigate the ideal quantum geometry (IQG), i.e., the property that the trace of quantum metric equals the Berry curvature, in the surface flat bands of rhombohedrally stacked graphene (RG) multilayers. We show that RG represents a class of semimetals with IQG, among which only R… ▽ More The interplay of quantum geometry and interactions determines the correlated state properties of flat bands. Here, we investigate the ideal quantum geometry (IQG), i.e., the property that the trace of quantum metric equals the Berry curvature, in the surface flat bands of rhombohedrally stacked graphene (RG) multilayers. We show that RG represents a class of semimetals with IQG, among which only RG has a nonvanishing IQG at the center of the surface bands. In the presence of long-range hoppings, a displacement field polarizes the electron density to one of the two surfaces, stabilizing the IQG and boosting transitions to correlated phases like superconductivity. Analyzing the superfluid stiffness of the superconducting state in a many-layer RG, we reveal a heavy-fermion picture where the "heavy electrons" carry a nonzero supercurrent. This unusual behavior arises from the IQG of the surface states, which suggests a large fraction of supercurrent running on the surface of RG. △ Less

Submitted 28 September, 2025; v1 submitted 4 April, 2025; originally announced April 2025.

Characterization of a quantum bus between two driven qubits

Authors: Alberto Hijano, Henri Lyyra, Juha T. Muhonen, Tero T. Heikkilä

Abstract: We investigate the use of driven qubits coupled to a harmonic oscillator to implement a $\sqrt{i\mathrm{SWAP}}$-gate. By dressing the qubits through an external driving field, the qubits and the harmonic oscillator can be selectively coupled, allowing for the measurement of individual qubit states, as well as leading to effective qubit-qubit interactions. We compare the qubit readout on bare and d… ▽ More We investigate the use of driven qubits coupled to a harmonic oscillator to implement a $\sqrt{i\mathrm{SWAP}}$-gate. By dressing the qubits through an external driving field, the qubits and the harmonic oscillator can be selectively coupled, allowing for the measurement of individual qubit states, as well as leading to effective qubit-qubit interactions. We compare the qubit readout on bare and dressed qubits, and demonstrate that when coupled to low-frequency resonators, dressed qubits provide a more robust readout than bare qubits in the presence of damping and thermal effects. Furthermore, we study the impact of various system parameters on the fidelity of the two-qubit gate, identifying an optimal range for quantum computation. Our findings guide the implementation of high-fidelity quantum gates in experimental setups, for example those employing nanoscale mechanical resonators. △ Less

Submitted 26 January, 2026; v1 submitted 24 March, 2025; originally announced March 2025.

Comments: 14 pages, 5 figures

Journal ref: Phys. Rev. Research 7, 043339 (2025)

Optomechanical quantum bus for donor spins in silicon

Authors: Henri Lyyra, Cliona Shakespeare, Simeoni Ahopelto, Teemu Loippo, Alberto Hijano, Reetu Inkilä, Pyry Runko, Tero T. Heikkilä, Juha T. Muhonen

Abstract: Silicon is the foundation of current information technology, and a promising platform for future quantum information technology as silicon-based qubits exhibit some of the longest coherence times in solid-state. At the same time, silicon is the underlying material for advanced photonics activity, and photonics structures in silicon can be used to define optomechanical cavities where the vibrations… ▽ More Silicon is the foundation of current information technology, and a promising platform for future quantum information technology as silicon-based qubits exhibit some of the longest coherence times in solid-state. At the same time, silicon is the underlying material for advanced photonics activity, and photonics structures in silicon can be used to define optomechanical cavities where the vibrations of nanoscale mechanical resonators can be probed down to the quantum level with laser light. Here, we propose to bring all these developments together by coupling silicon donor spins into optomechanical structures. We show theoretically and numerically that this allows telecom wavelength optical readout of the spin-qubits and implementing high-fidelity entangling two-qubit gates between donor spins that are spatially separated by tens of micrometers. We present an optimized geometry of the proposed device and discuss with the help of numerical simulations the predicted performance of the proposed quantum bus. We analyze the optomechanical spin readout fidelity and find the optimal donor species for different coupling mechanisms. △ Less

Submitted 15 January, 2026; v1 submitted 24 March, 2025; originally announced March 2025.

Comments: 15 pages, 11 figures. Added discussion and a new figure Fig. 10. Updated figures Fig. 3, Fig. 4, and Fig. 7 (in this version it is Fig. 6). Updated figures Fig. 5 and Fig. 6 and merged them into new figure Fig. 5

Quantum Geometric Helical Superconductivity

Authors: Aaron Dunbrack, Pauli Virtanen, Tero T. Heikkilä

Abstract: Several physical phenomena in superconductors, such as helical superconductivity and the diode effect, rely on breaking time-reversal symmetry. This symmetry-breaking is usually accounted for via the Lifshitz invariant, a contribution to the free energy which is linear in the phase gradient of the order parameter. In dispersive single-band superconductors with conventional pairing, the Lifshitz in… ▽ More Several physical phenomena in superconductors, such as helical superconductivity and the diode effect, rely on breaking time-reversal symmetry. This symmetry-breaking is usually accounted for via the Lifshitz invariant, a contribution to the free energy which is linear in the phase gradient of the order parameter. In dispersive single-band superconductors with conventional pairing, the Lifshitz invariant can be computed from the asymmetries of the spectrum near the Fermi surface. We show that in multi-band superconductors, the quantum geometry also contributes to the Lifshitz invariant, and this is the dominant contribution when the low-energy bands are flat. We also analogously demonstrate quantum-geometry-driven commensurate-incommensurate transitions in charge and pair density waves. △ Less

Submitted 22 May, 2026; v1 submitted 18 March, 2025; originally announced March 2025.

MultiResolution Low-Rank Regularization of Dynamic Imaging Problems

Authors: Tommi Heikkilä

Abstract: MultiResolution Low-Rank decomposition is formulated for regularization of dynamic image sequences. The decomposition applies a local low-rank decomposition on a sequence of discrete wavelet transforms. Its effective formulation as a regularization functional is discussed and numerically tested for dynamic X-ray tomography in comparison to other low-rank methods. The results suggest it is similar… ▽ More MultiResolution Low-Rank decomposition is formulated for regularization of dynamic image sequences. The decomposition applies a local low-rank decomposition on a sequence of discrete wavelet transforms. Its effective formulation as a regularization functional is discussed and numerically tested for dynamic X-ray tomography in comparison to other low-rank methods. The results suggest it is similar to traditional locally low-rank decomposition but produces less severe block artifacts. △ Less

Submitted 28 February, 2025; originally announced February 2025.

Comments: 13 pages, 5 figures, SSVM 2025 conference

MSC Class: 15A29; 65F55; 65T60; 65F22

Journal ref: Scale Space and Variational Methods in Computer Vision. SSVM 2025. Lecture Notes in Computer Science, vol 15667. Springer

Probing the flat-band limit of the superconducting proximity effect in Twisted Bilayer Graphene Josephson junctions

Authors: A. Diez-Carlon, J. Diez-Merida, P. Rout, D. Sedov, P. Virtanen, S. Banerjee, R. P. S. Penttila, P. Altpeter, K. Watanabe, T. Taniguchi, S. -Y. Yang, K. T. Law, T. T. Heikkila, P. Torma, M. S. Scheurer, D. K. Efetov

Abstract: While extensively studied in normal metals, semimetals and semiconductors, the superconducting (SC) proximity effect remains elusive in the emerging field of flat-band systems. In this study we probe proximity-induced superconductivity in Josephson junctions (JJs) formed between superconducting NbTiN electrodes and twisted bilayer graphene (TBG) weak links. Here the TBG acts as a highly tunable to… ▽ More While extensively studied in normal metals, semimetals and semiconductors, the superconducting (SC) proximity effect remains elusive in the emerging field of flat-band systems. In this study we probe proximity-induced superconductivity in Josephson junctions (JJs) formed between superconducting NbTiN electrodes and twisted bilayer graphene (TBG) weak links. Here the TBG acts as a highly tunable topological flat-band system, which due to its twist-angle dependent bandwidth, allows to probe the SC proximity effect at the crossover from the dispersive to the flat-band limit. Contrary to our original expectations, we find that the SC remains strong even in the flat-band limit, and gives rise to broad, dome shaped SC regions, in the filling dependent phase diagram. In addition, we find that unlike in conventional JJs, the critical current Ic strongly deviates from a scaling with the normal state conductance GN. We attribute these findings to the onset of strong electron interactions, which can give rise to an excess critical current, and also work out the potential importance of quantum geometric terms as well as multiband pairing mechanisms. Our results present the first detailed study of the SC proximity effect in the flat-band limit and shed new light on the mechanisms that drive the formation of SC domes in flat-band systems. △ Less

Submitted 7 February, 2025; originally announced February 2025.

Superconducting junctions with flat bands

Authors: P. Virtanen, R. P. S. Penttilä, P. Törmä, A. Díez-Carlón, D. K. Efetov, T. T. Heikkilä

Abstract: We analyze the properties of flat-band superconductor junctions that behave differently from ordinary junctions containing only metals with Fermi surfaces. In particular, we show how in the tunneling limit the critical Josephson current between flat-band superconductors is inversely proportional to the pair potential, how the quantum geometric contribution to the supercurrent contributes even in t… ▽ More We analyze the properties of flat-band superconductor junctions that behave differently from ordinary junctions containing only metals with Fermi surfaces. In particular, we show how in the tunneling limit the critical Josephson current between flat-band superconductors is inversely proportional to the pair potential, how the quantum geometric contribution to the supercurrent contributes even in the normal state of a flat-band weak link, and how Andreev reflection is strongly affected by the presence of bound states. Our results are relevant for analyzing the superconducting properties of junctions involving electronic systems with flat bands. △ Less

Submitted 5 June, 2025; v1 submitted 30 October, 2024; originally announced October 2024.

Comments: 17 pages

Journal ref: Phys. Rev. B 112, L100502 (2025)

Programming of Skill-based Robots

Authors: Taneli Lohi, Samuli Soutukorva, Tapio Heikkilä

Abstract: Manufacturing is facing ever changing market demands, with faster innovation cycles resulting to growing agility and flexibility requirements. Industry 4.0 has been transforming the manufacturing world towards digital automation and the importance of software has increased drastically. Easy and fast task programming and execution in robot - sensor systems become a prerequisite for agile and flexib… ▽ More Manufacturing is facing ever changing market demands, with faster innovation cycles resulting to growing agility and flexibility requirements. Industry 4.0 has been transforming the manufacturing world towards digital automation and the importance of software has increased drastically. Easy and fast task programming and execution in robot - sensor systems become a prerequisite for agile and flexible automation and in this paper, we propose such a system. Our solution relies on a robot skill library, which provides the user with high level and parametrized operations, i.e., robot skills, for task programming and execution. Programming actions results to a control recipe in a neutral product context and is based on use of product CAD models or alternatively collaborative use of pointers and tracking sensor with real parts. Practical tests are also reported to show the feasibility of our approach. △ Less

Submitted 25 September, 2024; originally announced September 2024.

Comments: IEEE ICIEA 2024

A study in scarlet -- II. Spectroscopic properties of a sample of Intermediate Luminosity Red Transients

Authors: G. Valerin, A. Pastorello, E. Mason, A. Reguitti, S. Benetti, Y. -Z. Cai, T. -W. Chen, D. Eappachen, N. Elias-Rosa, M. Fraser, A. Gangopadhyay, E. Y. Hsiao, D. A. Howell, C. Inserra, L. Izzo, J. Jencson, E. Kankare, R. Kotak, P. Lundqvist, P. A. Mazzali, K. Misra, G. Pignata, S. J. Prentice, D. J. Sand, S. J. Smartt , et al. (43 additional authors not shown)

Abstract: We investigate the spectroscopic characteristics of Intermediate Luminosity Red Transients (ILRTs), a class of elusive objects with peak luminosity between that of classical novae and standard supernovae. We present the extensive optical and near-infrared (NIR) spectroscopic monitoring of four ILRTs, namely NGC 300 2008OT-1, AT 2019abn, AT 2019ahd and AT 2019udc. First we focus on the evolution of… ▽ More We investigate the spectroscopic characteristics of Intermediate Luminosity Red Transients (ILRTs), a class of elusive objects with peak luminosity between that of classical novae and standard supernovae. We present the extensive optical and near-infrared (NIR) spectroscopic monitoring of four ILRTs, namely NGC 300 2008OT-1, AT 2019abn, AT 2019ahd and AT 2019udc. First we focus on the evolution of the most prominent spectral features observed in the low resolution spectra, then we discuss more in detail the high resolution spectrum collected for NGC 300 2008OT-1 with the Very Large Telescope equipped with UVES. Finally we analyse late time spectra of NGC 300 2008OT-1 and AT 2019ahd through comparisons with both synthetic and observed spectra. Balmer and Ca lines dominate the optical spectra, revealing the presence of slowly moving circumstellar medium (CSM) around the objects. The line luminosity of H$α$, H$β$ and Ca II NIR triplet presents a double peaked evolution with time, possibly indicative of interaction between fast ejecta and the slow CSM. The high resolution spectrum of NGC 300 2008OT-1 reveals a complex circumstellar environment, with the transient being surrounded by a slow ($\sim$30 km s$^{-1}$) progenitor wind. At late epochs, optical spectra of NGC 300 2008OT-1 and AT 2019ahd show broad ($\sim$2500 km s$^{-1}$) emission features at $\sim$6170 A and $\sim$7000 A which are unprecedented for ILRTs. We find that these lines originate most likely from the blending of several narrow lines, possibly of iron-peak elements. △ Less

Submitted 31 July, 2024; originally announced July 2024.

Comments: 28 pages, 19 figures. Submitted to A&A

Journal ref: A&A 695, A43 (2025)

A study in scarlet -- I. Photometric properties of a sample of Intermediate Luminosity Red Transients

Authors: G. Valerin, A. Pastorello, A. Reguitti, S. Benetti, Y. -Z. Cai, T. -W. Chen, D. Eappachen, N. Elias-Rosa, M. Fraser, A. Gangopadhyay, E. Y. Hsiao, D. A. Howell, C. Inserra, L. Izzo, J. Jencson, E. Kankare, R. Kotak, P. A. Mazzali, K. Misra, G. Pignata, S. J. Prentice, D. J. Sand, S. J. Smartt, M. D. Stritzinger, L. Tartaglia , et al. (35 additional authors not shown)

Abstract: We investigate the photometric characteristics of a sample of Intermediate Luminosity Red Transients (ILRTs), a class of elusive objects with peak luminosity between that of classical novae and standard supernovae. We present the multi-wavelength photometric follow-up of four ILRTs, namely NGC 300 2008OT-1, AT 2019abn, AT 2019ahd and AT 2019udc. Through the analysis and modelling of their spectral… ▽ More We investigate the photometric characteristics of a sample of Intermediate Luminosity Red Transients (ILRTs), a class of elusive objects with peak luminosity between that of classical novae and standard supernovae. We present the multi-wavelength photometric follow-up of four ILRTs, namely NGC 300 2008OT-1, AT 2019abn, AT 2019ahd and AT 2019udc. Through the analysis and modelling of their spectral energy distribution and bolometric light curves we infer the physical parameters associated with these transients. All four objects display a single peaked light curve which ends in a linear decline in magnitudes at late phases. A flux excess with respect to a single black body emission is detected in the infrared domain for three objects in our sample, a few months after maximum. This feature, commonly found in ILRTs, is interpreted as a sign of dust formation. Mid infrared monitoring of NGC 300 2008OT-1 761 days after maximum allows us to infer the presence of $\sim$10$^{-3}$-10$^{-5}$ M$_{\odot}$ of dust, depending on the chemical composition and the grain size adopted. The late time decline of the bolometric light curves of the considered ILRTs is shallower than expected for $^{56}$Ni decay, hence requiring an additional powering mechanism. James Webb Space Telescope observations of AT 2019abn prove that the object has faded below its progenitor luminosity in the mid-infrared domain, five years after its peak. Together with the disappearance of NGC 300 2008OT-1 in Spitzer images seven years after its discovery, this supports the terminal explosion scenario for ILRTs. With a simple semi-analytical model we try to reproduce the observed bolometric light curves in the context of few M$_{\odot}$ of material ejected at few 10$^{3}$ km s$^{-1}$ and enshrouded in an optically thick circumstellar medium. △ Less

Submitted 31 July, 2024; originally announced July 2024.

Comments: 21 pages, 15 figures plus 20 additional pages of data in appendix. Submitted to A&A

Journal ref: A&A 695, A42 (2025)

Superconducting diode effect in diffusive superconductors and Josephson junctions with Rashba spin-orbit coupling

Authors: Stefan Ilic, Pauli Virtanen, Daniel Crawford, Tero T. Heikkilä, F. Sebastian Bergeret

Abstract: We characterize the superconducting diode effect (SDE) in two-dimensional diffusive structures with Rashba spin-orbit coupling using the quasiclassical formalism. We consider both homogeneous superconductors and Josephson junctions. In the first case, the diode effect monotonically increases as the magnetic field is increased and the temperature is reduced, in contrast to the non-monotonic behavio… ▽ More We characterize the superconducting diode effect (SDE) in two-dimensional diffusive structures with Rashba spin-orbit coupling using the quasiclassical formalism. We consider both homogeneous superconductors and Josephson junctions. In the first case, the diode effect monotonically increases as the magnetic field is increased and the temperature is reduced, in contrast to the non-monotonic behavior found in clean structures. In Josephson junctions, SDE dramatically increases and changes its sign close to the $0-π$ transition of the junction, which occurs at specific junction lengths and strengths of the magnetic field. We show that the SDE is strongly suppressed in narrow junctions. Our results are relevant for understanding recent experiments that measure SDE in mesoscopic nanostructures, where significant disorder is unavoidable. △ Less

Submitted 24 June, 2024; originally announced June 2024.

Comments: 9 pages, 6 figures

Journal ref: Phys. Rev. B 110, L140501 (2024)

Visual Servoing Based on 3D Features: Design and Implementation for Robotic Insertion Tasks

Authors: Antonio Rosales, Tapio Heikkilä, Markku Suomalainen

Abstract: This paper proposes a feature-based Visual Servoing (VS) method for insertion task skills. A camera mounted on the robot's end-effector provides the pose relative to a cylinder (hole), allowing a contact-free and damage-free search of the hole and avoiding uncertainties emerging when the pose is computed via robot kinematics. Two points located on the hole's principal axis and three mutually ortho… ▽ More This paper proposes a feature-based Visual Servoing (VS) method for insertion task skills. A camera mounted on the robot's end-effector provides the pose relative to a cylinder (hole), allowing a contact-free and damage-free search of the hole and avoiding uncertainties emerging when the pose is computed via robot kinematics. Two points located on the hole's principal axis and three mutually orthogonal planes defining the flange's reference frame are associated with the pose of the hole and the flange, respectively. The proposed VS drives to zero the distance between the two points and the three planes aligning the robot's flange with the hole's direction. Compared with conventional VS where the Jacobian is difficult to compute in practice, the proposed featured-based uses a Jacobian easily calculated from the measured hole pose. Furthermore, the feature-based VS design considers the robot's maximum cartesian velocity. The VS method is implemented in an industrial robot and the experimental results support its usefulness. △ Less

Submitted 29 May, 2024; originally announced May 2024.

Comments: To be published in proceedings of The 19th IEEE Conference on Industrial Electronics and Applications (ICIEA 2024)

A Weeding Robot for Seedling Removal

Authors: Jarkko Kotaniemi, Niko Känsäkoski, Tapio Heikkilä

Abstract: Automatic weeding technologies have attained a lot of attention lately, because of the harms and challenges weeds are causing for livestock farming, in addition to that weeds reduce yields. We are targeting automatic and mechanical Rumex weeding in open pasture fields using light weight mobile field robot technologies. We describe a mobile weeding robot with GNSS navigation, 3D computer vision for… ▽ More Automatic weeding technologies have attained a lot of attention lately, because of the harms and challenges weeds are causing for livestock farming, in addition to that weeds reduce yields. We are targeting automatic and mechanical Rumex weeding in open pasture fields using light weight mobile field robot technologies. We describe a mobile weeding robot with GNSS navigation, 3D computer vision for weed detection, and a robot arm with a mechanical weeding tool. Our main contribution is showing the feasibility of light weight robot, sensor, and tool technologies in mechanical removal of weed seedlings. △ Less

Submitted 21 May, 2024; originally announced May 2024.

Comments: 4 pages, 18 figures

Journal ref: 2024 20th IEEE/ASME International Conference on Mechatronic and Embedded Systems and Applications (MESA), Genova, Italy, 2024, pp. 1-8

Regularization with optimal space-time priors

Authors: Tatiana A. Bubba, Tommi Heikkilä, Demetrio Labate, Luca Ratti

Abstract: We propose a variational regularization approach based on a multiscale representation called cylindrical shearlets aimed at dynamic imaging problems, especially dynamic tomography. The intuitive idea of our approach is to integrate a sequence of separable static problems in the mismatch term of the cost function, while the regularization term handles the nonstationary target as a spatio-temporal o… ▽ More We propose a variational regularization approach based on a multiscale representation called cylindrical shearlets aimed at dynamic imaging problems, especially dynamic tomography. The intuitive idea of our approach is to integrate a sequence of separable static problems in the mismatch term of the cost function, while the regularization term handles the nonstationary target as a spatio-temporal object. This approach is motivated by the fact that cylindrical shearlets provide (nearly) optimally sparse approximations on an idealized class of functions modeling spatio-temportal data and the numerical observation that they provide highly sparse approximations even for more general spatio-temporal image sequences found in dynamic tomography applications. To formulate our regularization model, we introduce cylindrical shearlet smoothness spaces, which are instrumental for defining suitable embeddings in functional spaces. We prove that the proposed regularization strategy is well-defined, and the minimization problem has a unique solution (for $ p > 1$). Furthermore, we provide convergence rates (in terms of the symmetric Bregman distance) under deterministic and random noise conditions, within the context of statistical inverse learning. We numerically validate our theoretical results using both simulated and measured dynamic tomography data, showing that our approach leads to an efficient and robust reconstruction strategy. △ Less

Submitted 4 August, 2025; v1 submitted 10 May, 2024; originally announced May 2024.

Comments: 44 pages, 7 figures

MSC Class: 47A52; 42C40; 65J20; 65J22

Journal ref: SIAM Journal on Imaging Sciences 18(3), (2025), 1563-1600

Light-Curve Structure and Halpha Line Formation in the Tidal Disruption Event AT 2019azh

Authors: Sara Faris, Iair Arcavi, Lydia Makrygianni, Daichi Hiramatsu, Giacomo Terreran, Joseph Farah, D. Andrew Howell, Curtis McCully, Megan Newsome, Estefania Padilla Gonzalez, Craig Pellegrino, K. Azalee Bostroem, Wiam Abojanb, Marco C. Lam, Lina Tomasella, Thomas G. Brink, Alexei V. Filippenko, K. Decker French, Peter Clark, Or Graur, Giorgos Leloudas, Mariusz Gromadzki, Joseph P. Anderson, Matt Nicholl, Claudia P. Gutierrez , et al. (11 additional authors not shown)

Abstract: AT 2019azh is a H+He tidal disruption event (TDE) with one of the most extensive ultraviolet and optical data sets available to date. We present our photometric and spectroscopic observations of this event starting several weeks before and out to approximately two years after the g-band peak brightness and combine them with public photometric data. This extensive data set robustly reveals a change… ▽ More AT 2019azh is a H+He tidal disruption event (TDE) with one of the most extensive ultraviolet and optical data sets available to date. We present our photometric and spectroscopic observations of this event starting several weeks before and out to approximately two years after the g-band peak brightness and combine them with public photometric data. This extensive data set robustly reveals a change in the light-curve slope and a possible bump in the rising light curve of a TDE for the first time, which may indicate more than one dominant emission mechanism contributing to the pre-peak light curve. Indeed, we find that the MOSFiT-derived parameters of AT 2019azh, which assume reprocessed accretion as the sole source of emission, are not entirely self-consistent. We further confirm the relation seen in previous TDEs whereby the redder emission peaks later than the bluer emission. The post-peak bolometric light curve of AT 2019azh is better described by an exponential decline than by the canonical t^{-5/3} (and in fact any) power-law decline. We find a possible mid-infrared excess around the peak optical luminosity, but cannot determine its origin. In addition, we provide the earliest measurements of the Halpha emission-line evolution and find no significant time delay between the peak of the V-band light curve and that of the Halpha luminosity. These results can be used to constrain future models of TDE line formation and emission mechanisms in general. More pre-peak 1-2 days cadence observations of TDEs are required to determine whether the characteristics observed here are common among TDEs. More importantly, detailed emission models are needed to fully exploit such observations for understanding the emission physics of TDEs. △ Less

Submitted 1 August, 2024; v1 submitted 6 December, 2023; originally announced December 2023.

Comments: Submitted to ApJ

Superconducting Spintronic Heat Engine

Authors: Clodoaldo I. L. de Araujo, Pauli Virtanen, Maria Spies, Carmen González-Orellana, Samuel Kerschbaumer, Maxim Ilyn, Celia Rogero, Tero T. Heikkilä, Francesco Giazotto, E. Strambini

Abstract: Heat engines are key devices that convert thermal energy into usable energy. Strong thermoelectricity, at the basis of electrical heat engines, is present in superconducting spin tunnel barriers at cryogenic temperatures where conventional semiconducting or metallic technologies cease to work. Here we realize a superconducting spintronic heat engine consisting of a ferromagnetic insulator/supercon… ▽ More Heat engines are key devices that convert thermal energy into usable energy. Strong thermoelectricity, at the basis of electrical heat engines, is present in superconducting spin tunnel barriers at cryogenic temperatures where conventional semiconducting or metallic technologies cease to work. Here we realize a superconducting spintronic heat engine consisting of a ferromagnetic insulator/superconductor/insulator/ferromagnet tunnel junction (EuS/Al/AlO$_x$/Co). The efficiency of the engine is quantified for bath temperatures ranging from 25 mK up to 800 mK, and at different load resistances. Moreover, we show that the sign of the generated thermoelectric voltage can be inverted according to the parallel or anti-parallel orientation of the two ferromagnetic layers, EuS and Co. This realizes a thermoelectric spin valve controlling the sign and strength of the Seebeck coefficient, thereby implementing a thermoelectric memory cell. We propose a theoretical model that allows describing the experimental data and predicts the engine efficiency for different device parameters. △ Less

Submitted 27 October, 2023; originally announced October 2023.

Comments: 12 pages, 7 figures

Journal ref: Nature Communications (2024)

Signatures and characterization of dominating Kerr nonlinearity between two driven systems with application to a suspended magnetic beam

Authors: Andrii M Sokolov, Tero T. Heikkilä

Abstract: We consider a model of two harmonically driven damped harmonic oscillators that are coupled linearly and with a cross-Kerr coupling. We show how to distinguish this combination of coupling types from the case where a coupling of optomechanical type is present. This can be useful for the characterization of various nonlinear systems, such as mechanical oscillators, qubits, and hybrid systems. We th… ▽ More We consider a model of two harmonically driven damped harmonic oscillators that are coupled linearly and with a cross-Kerr coupling. We show how to distinguish this combination of coupling types from the case where a coupling of optomechanical type is present. This can be useful for the characterization of various nonlinear systems, such as mechanical oscillators, qubits, and hybrid systems. We then consider a hybrid system with linear and cross-Kerr interactions and a relatively high damping in one of the modes. We derive a quantum Hamiltonian of a doubly clamped magnetic beam, showing that the cross-Kerr coupling is prominent there. We discuss, in the classical limit, measurements of its linear response as well as the specific higher-harmonic responses. These frequency-domain measurements can allow estimating the magnitude of the cross-Kerr coupling or its magnon population. △ Less

Submitted 14 December, 2023; v1 submitted 5 September, 2023; originally announced September 2023.

Comments: 18 pages, 6 figures

Dynamical Hall responses of disordered superconductors

Authors: Alberto Hijano, Sakineh Vosoughi-nia, F. Sebastián Bergeret, Pauli Virtanen, Tero T. Heikkilä

Abstract: We extend the Mattis-Bardeen theory for the dynamical response of superconductors to include different types of Hall responses. This is possible thanks to a recent modification of the quasiclassical Usadel equation, which allows for analyzing Hall effects in disordered superconductors and including the precise frequency dependence of such effects. Our results form a basis for analyzing dynamical e… ▽ More We extend the Mattis-Bardeen theory for the dynamical response of superconductors to include different types of Hall responses. This is possible thanks to a recent modification of the quasiclassical Usadel equation, which allows for analyzing Hall effects in disordered superconductors and including the precise frequency dependence of such effects. Our results form a basis for analyzing dynamical experiments especially on novel thin-film superconductors, where ordinary Hall and spin Hall effects can both show up. △ Less

Submitted 19 September, 2023; v1 submitted 30 June, 2023; originally announced June 2023.

Comments: 12 pages, 4 figures

Journal ref: Phys. Rev. B 108, 104506 (2023)

Nonreciprocal Josephson linear response

Authors: Pauli Virtanen, Tero T. Heikkilä

Abstract: We consider the finite-frequency response of multiterminal Josephson junctions and show how non-reciprocity in them can show up at linear response, in contrast to the static Josephson diodes featuring non-linear non-reciprocity. At finite frequencies, the response contains dynamic contributions to the Josephson admittance, featuring the effects of Andreev bound state transitions along with Berry p… ▽ More We consider the finite-frequency response of multiterminal Josephson junctions and show how non-reciprocity in them can show up at linear response, in contrast to the static Josephson diodes featuring non-linear non-reciprocity. At finite frequencies, the response contains dynamic contributions to the Josephson admittance, featuring the effects of Andreev bound state transitions along with Berry phase effects, and reflecting the breaking of the same symmetries as in Josephson diodes. We show that outside exact Andreev resonances, the junctions feature non-reciprocal reactive response. As a result, the microwave transmission through those systems is non-dissipative, and the electromagnetic scattering can approach complete non-reciprocity. Besides providing information about the nature of the weak link energy levels, the non-reciprocity can be utilized to create non-dissipative and small-scale on-chip circulators whose operation requires only rather small magnetic fields. △ Less

Submitted 17 January, 2024; v1 submitted 21 June, 2023; originally announced June 2023.

Comments: 15 pages, 8 figures

Journal ref: Phys. Rev. Lett. 132, 046002 (2024)

Superconductor-ferromagnet hybrids for non-reciprocal electronics and detectors

Authors: Zhuoran Geng, Alberto Hijano, Stefan Ilic, Maxim Ilyn, Ilari J. Maasilta, Alessandro Monfardini, Maria Spies, Elia Strambini, Pauli Virtanen, Martino Calvo, Carmen Gonzalez-Orellana, Ari P. Helenius, Sara Khorshidian, Clodoaldo I. L. de Araujo, Florence Levy-Bertrand, Celia Rogero, Francesco Giazotto, F. Sebastián Bergeret, Tero T. Heikkilä

Abstract: We review the use of hybrid thin films composed of superconductors and ferromagnets for creating non-reciprocal electronic components and self-biased detectors of electromagnetic radiation. We begin by introducing the theory behind these effects, as well as discussing various potential materials that can be used in the fabrication of these components. We then proceed with a detailed discussion on… ▽ More We review the use of hybrid thin films composed of superconductors and ferromagnets for creating non-reciprocal electronic components and self-biased detectors of electromagnetic radiation. We begin by introducing the theory behind these effects, as well as discussing various potential materials that can be used in the fabrication of these components. We then proceed with a detailed discussion on the fabrication and characterization of Al/EuS/Cu and EuS/Al/Co-based detectors, along with their noise analysis. Additionally, we suggest some approaches for multiplexing such self-biased detectors. △ Less

Submitted 23 October, 2023; v1 submitted 24 February, 2023; originally announced February 2023.

Comments: 30 pages, 21 figures

On a fixed-point continuation method for a convex optimization problem

Authors: Jean-Baptiste Fest, Tommi Heikkilä, Ignace Loris, Ségolène Martin, Luca Ratti, Simone Rebegoldi, Gesa Sarnighausen

Abstract: We consider a variation of the classical proximal-gradient algorithm for the iterative minimization of a cost function consisting of a sum of two terms, one smooth and the other prox-simple, and whose relative weight is determined by a penalty parameter. This so-called fixed-point continuation method allows one to approximate the problem's trade-off curve, i.e. to compute the minimizers of the cos… ▽ More We consider a variation of the classical proximal-gradient algorithm for the iterative minimization of a cost function consisting of a sum of two terms, one smooth and the other prox-simple, and whose relative weight is determined by a penalty parameter. This so-called fixed-point continuation method allows one to approximate the problem's trade-off curve, i.e. to compute the minimizers of the cost function for a whole range of values of the penalty parameter at once. The algorithm is shown to converge, and a rate of convergence of the cost function is also derived. Furthermore, it is shown that this method is related to iterative algorithms constructed on the basis of the $ε$-subdifferential of the prox-simple term. Some numerical examples are provided. △ Less

Submitted 23 December, 2022; originally announced December 2022.

Comments: 15 pages, 2 figures. Workshop on Advanced Techniques in Optimization for Machine learning and Imaging

Reducing the frequency of the Higgs mode in a helical superconductor coupled to an LC-circuit

Authors: Yao Lu, Stefan Ilić, Risto Ojajärvi, Tero T. Heikkilä, F. Sebastian Bergeret

Abstract: We show that the amplitude, or Higgs mode of a superconductor with strong spin-orbit coupling and an exchange field, couples linearly to the electromagnetic field. Furthermore, by coupling such a superconductor to an LC resonator, we demonstrate that the Higgs resonance becomes a regular mode at frequencies smaller than the quasiparticle energy threshold $2Δ$. We finally propose and discuss a poss… ▽ More We show that the amplitude, or Higgs mode of a superconductor with strong spin-orbit coupling and an exchange field, couples linearly to the electromagnetic field. Furthermore, by coupling such a superconductor to an LC resonator, we demonstrate that the Higgs resonance becomes a regular mode at frequencies smaller than the quasiparticle energy threshold $2Δ$. We finally propose and discuss a possible experiment based on microwave spectroscopy for an unequivocal detection of the Higgs mode. Our approach may allow visualizing Higgs modes also in more complicated multiband superconductors with a coupling between the charge and other electronic degrees of freedom. △ Less

Submitted 22 December, 2022; originally announced December 2022.

Comments: 6+4 pages, 5 figures

Journal ref: Phys. Rev. B 108, 224517 (2023)

STEMPO -- dynamic X-ray tomography phantom

Authors: Tommi Heikkilä

Abstract: This is the documentation for Spatio-TEmporal Motor-POwered (STEMPO) phantom for dynamic X-ray tomography. Provided are different measurements designed for testing dynamic tomography reconstruction algorithms. The measured data and additional materials are available at www.fips.fi/dataset.php and doi.org/10.5281/zenodo.7081688 for open use by the scientific community, as long as the data and this… ▽ More This is the documentation for Spatio-TEmporal Motor-POwered (STEMPO) phantom for dynamic X-ray tomography. Provided are different measurements designed for testing dynamic tomography reconstruction algorithms. The measured data and additional materials are available at www.fips.fi/dataset.php and doi.org/10.5281/zenodo.7081688 for open use by the scientific community, as long as the data and this documentation are appropriately referenced. The files contain: (1) measurement data (sinograms) from one static scan and two dynamic scans (same movement but different choice of projection directions). All data is provided both for 2D and 3D geometry in varying resolution levels; (2) approximation of the ground truth obtained using the known motion model; and (3) short example codes to showcase how the data could be used to test and validate dynamic tomography algorithms. △ Less

Submitted 19 September, 2023; v1 submitted 26 September, 2022; originally announced September 2022.

Comments: Data available on Zenodo: https://doi.org/10.5281/zenodo.7081688

ACM Class: E.0

Journal ref: Advanced Techniques in Optimization for Machine learning and Imaging (ATOMI), Springer INdAM Series, vol 61, p. 1-14 (2022)

Self-Supervised Clustering on Image-Subtracted Data with Deep-Embedded Self-Organizing Map

Authors: Y. -L. Mong, K. Ackley, T. L. Killestein, D. K. Galloway, M. Dyer, R. Cutter, M. J. I. Brown, J. Lyman, K. Ulaczyk, D. Steeghs, V. Dhillon, P. O'Brien, G. Ramsay, K. Noysena, R. Kotak, R. Breton, L. Nuttall, E. Palle, D. Pollacco, E. Thrane, S. Awiphan, U. Burhanudin, P. Chote, A. Chrimes, E. Daw , et al. (23 additional authors not shown)

Abstract: Developing an effective automatic classifier to separate genuine sources from artifacts is essential for transient follow-ups in wide-field optical surveys. The identification of transient detections from the subtraction artifacts after the image differencing process is a key step in such classifiers, known as real-bogus classification problem. We apply a self-supervised machine learning model, th… ▽ More Developing an effective automatic classifier to separate genuine sources from artifacts is essential for transient follow-ups in wide-field optical surveys. The identification of transient detections from the subtraction artifacts after the image differencing process is a key step in such classifiers, known as real-bogus classification problem. We apply a self-supervised machine learning model, the deep-embedded self-organizing map (DESOM) to this "real-bogus" classification problem. DESOM combines an autoencoder and a self-organizing map to perform clustering in order to distinguish between real and bogus detections, based on their dimensionality-reduced representations. We use 32x32 normalized detection thumbnails as the input of DESOM. We demonstrate different model training approaches, and find that our best DESOM classifier shows a missed detection rate of 6.6% with a false positive rate of 1.5%. DESOM offers a more nuanced way to fine-tune the decision boundary identifying likely real detections when used in combination with other types of classifiers, for example built on neural networks or decision trees. We also discuss other potential usages of DESOM and its limitations. △ Less

Submitted 13 September, 2022; originally announced September 2022.

Journal of Economic Literature codes classification system (JEL)

Authors: Jussi T. S. Heikkila

Abstract: The Journal of Economic Literature codes classification system (JEL) published by the American Economic Association (AEA) is the de facto standard classification system for research literature in economics. The JEL classification system is used to classify articles, dissertations, books, book reviews, and working papers in EconLit, a database maintained by the AEA. Over time, it has evolved and ex… ▽ More The Journal of Economic Literature codes classification system (JEL) published by the American Economic Association (AEA) is the de facto standard classification system for research literature in economics. The JEL classification system is used to classify articles, dissertations, books, book reviews, and working papers in EconLit, a database maintained by the AEA. Over time, it has evolved and extended to a system with over 850 subclasses. This paper reviews the history and development of the JEL classification system, describes the current version, and provides a selective overview of its uses and applications in research. The JEL codes classification system has been adopted by several publishers, and their instructions are reviewed. There are interesting avenues for future research as the JEL classification system has been surprisingly little used in existing bibliometric and scientometric research as well as in library classification systems. △ Less

Submitted 13 July, 2022; originally announced July 2022.

Journal ref: Heikkila, Jussi T. S. 2022. Journal of Economic Literature codes classification system (JEL). ISKO Encyclopedia of Knowledge Organization, eds. Birger Hjorland and Claudio Gnoli. jel

Cavity-induced bifurcation in classical rate theory

Authors: Kalle S. U. Kansanen, Tero T. Heikkilä

Abstract: We show how coupling an ensemble of bistable systems to a common cavity field affects the collective stochastic behavior of this ensemble. In particular, the cavity provides an effective interaction between the systems, and parametrically modifies the transition rates between the metastable states. We predict that the cavity induces a collective phase transition at a critical temperature which dep… ▽ More We show how coupling an ensemble of bistable systems to a common cavity field affects the collective stochastic behavior of this ensemble. In particular, the cavity provides an effective interaction between the systems, and parametrically modifies the transition rates between the metastable states. We predict that the cavity induces a collective phase transition at a critical temperature which depends linearly on the number of systems. It shows up as a spontaneous symmetry breaking where the stationary states of the bistable system bifurcate. We observe that the transition rates slow down independently of the phase transition, but the rate modification vanishes for alternating signs of the system-cavity couplings, corresponding to a disordered ensemble of dipoles. Our results are of particular relevance in polaritonic chemistry where the presence of a cavity has been suggested to affect chemical reactions. △ Less

Submitted 8 December, 2023; v1 submitted 24 February, 2022; originally announced February 2022.

Comments: Resubmission to SciPost; minor text changes in Secs. 2 and 3; 28 pages and 8 figures

Journal ref: SciPost Phys. 16, 025 (2024)

Directly probing the chirality of Majorana edge states

Authors: Yao Lu, P. Virtanen, Tero T. Heikkilä

Abstract: We propose to directly probe the chirality of Majorana edge states in 2D topological superconductors using polarization selective photon absorption. When shining circularly polarized light on a 2D topological superconductor in disk geometry, the photons can excite quasiparticles only when the polarization of the light matches the chirality of the Majorana edge states required by the angular moment… ▽ More We propose to directly probe the chirality of Majorana edge states in 2D topological superconductors using polarization selective photon absorption. When shining circularly polarized light on a 2D topological superconductor in disk geometry, the photons can excite quasiparticles only when the polarization of the light matches the chirality of the Majorana edge states required by the angular momentum conservation. Hence one can obtain the chirality of the Majorana edge states by measuring the photon absorption rate. We show that the polarization selective photon absorption can also serve as a smoking gun evidence of the chiral Majorana edge mode. Our results pave a new way of detecting, investigating and manipulating the chiral Majorana edge states. △ Less

Submitted 10 November, 2021; originally announced November 2021.

Comments: 7 pages, 6 figures. Comments are welcome

Journal ref: Phys. Rev. B 106, 045139 (2022)

The Gravitational-wave Optical Transient Observer (GOTO): prototype performance and prospects for transient science

Authors: D. Steeghs, D. K. Galloway, K. Ackley, M. J. Dyer, J. Lyman, K. Ulaczyk, R. Cutter, Y. L. Mong, V. Dhillon, P. O'Brien, G. Ramsay, S. Poshyachinda, R. Kotak, L. K. Nuttall, E. Palle, R. P. Breton, D. Pollacco, E. Thrane, S. Aukkaravittayapun, S. Awiphan, U. Burhanudin, P. Chote, A. Chrimes, E. Daw, C. Duffy , et al. (28 additional authors not shown)

Abstract: The Gravitational-wave Optical Transient Observer (GOTO) is an array of wide-field optical telescopes, designed to exploit new discoveries from the next generation of gravitational wave detectors (LIGO, Virgo, KAGRA), study rapidly evolving transients, and exploit multi-messenger opportunities arising from neutrino and very high energy gamma-ray triggers. In addition to a rapid response mode, the… ▽ More The Gravitational-wave Optical Transient Observer (GOTO) is an array of wide-field optical telescopes, designed to exploit new discoveries from the next generation of gravitational wave detectors (LIGO, Virgo, KAGRA), study rapidly evolving transients, and exploit multi-messenger opportunities arising from neutrino and very high energy gamma-ray triggers. In addition to a rapid response mode, the array will also perform a sensitive, all-sky transient survey with few day cadence. The facility features a novel, modular design with multiple 40-cm wide-field reflectors on a single mount. In June 2017 the GOTO collaboration deployed the initial project prototype, with 4 telescope units, at the Roque de los Muchachos Observatory (ORM), La Palma, Canary Islands. Here we describe the deployment, commissioning, and performance of the prototype hardware, and discuss the impact of these findings on the final GOTO design. We also offer an initial assessment of the science prospects for the full GOTO facility that employs 32 telescope units across two sites. △ Less

Submitted 11 October, 2021; originally announced October 2021.

Comments: 19 pages, 16 Figures, accepted for publication in Monthly Notices of the Royal Astronomical Society

Efficient representation of spatio-temporal data using cylindrical shearlets

Authors: Tatiana A. Bubba, Glenn Easley, Tommi Heikkilä, Demetrio Labate, Jose P. Rodriguez Ayllon

Abstract: Efficient representations of multivariate functions are critical for the design of state-of-the-art methods of data restoration and image reconstruction. In this work, we consider the representation of spatio-temporal data such as temporal sequences (videos) of 2- and 3-dimensional images, where conventional separable representations are usually very inefficient, due to their limitations in handli… ▽ More Efficient representations of multivariate functions are critical for the design of state-of-the-art methods of data restoration and image reconstruction. In this work, we consider the representation of spatio-temporal data such as temporal sequences (videos) of 2- and 3-dimensional images, where conventional separable representations are usually very inefficient, due to their limitations in handling the geometry of the data. To address this challenge, we define a class $\mathcal{E}(A) \subset L^2(\mathbb{R}^4)$ of functions of 4 variables dominated by hypersurface singularities in the first three coordinates that we apply to model 4-dimensional data corresponding to temporal sequences (videos) of 3-dimensional objects. To provide an efficient representation for this type of data, we introduce a new multiscale directional system of functions based on cylindrical shearlets and prove that this new approach achieves superior approximation properties with respect to conventional multiscale representations. We illustrate the advantages of our approach by applying a discrete implementation of the new representation to a challenging problem from dynamic tomography. Numerical results confirm the potential of our novel approach with respect to conventional multiscale methods. △ Less

Submitted 30 March, 2023; v1 submitted 7 October, 2021; originally announced October 2021.

Comments: 41 pages, 7 figures, 1 table

MSC Class: 42C10 (primary); 42C40; 92C55; 47A52 (secondary)

Journal ref: Journal of Computational and Applied Mathematics 429 (2023) 115206

Current rectification in junctions with spin-split superconductors

Authors: Stefan Ilić, P. Virtanen, T. T. Heikkilä, F. Sebastián Bergeret

Abstract: Spin-split superconductors exhibit an electron-hole asymmetric spin-resolved density of states, but the symmetry is restored upon averaging over spin. On the other hand, asymmetry appears again in tunneling junctions of spin-split superconductors with a spin-polarized barrier. As demonstrated recently in both theory and experiment, this fact leads to a particularly strong thermoelectric effect in… ▽ More Spin-split superconductors exhibit an electron-hole asymmetric spin-resolved density of states, but the symmetry is restored upon averaging over spin. On the other hand, asymmetry appears again in tunneling junctions of spin-split superconductors with a spin-polarized barrier. As demonstrated recently in both theory and experiment, this fact leads to a particularly strong thermoelectric effect in superconductor-ferromagnet structures. In this work we show another important effect stemming from the electron-hole asymmetry - current rectification. We calculate the charge current in spin-polarized tunnel junctions of normal metal and a spin-split superconductor with AC and DC voltage bias. In the DC case, the I-V curve is not fully antisymmetric and has a voltage-symmetric component due to spin polarization. This translates to the existence of a rectified current in the AC case, which is proportional to the spin polarization of the junction and strongly depends on the frequency of the applied bias. We discuss possible applications of the rectification effect, including a diode for superconducting electronics and radiation detectors. The analysis of the rectified charge current is supplemented by the discussion of heat current and relevant noise correlators, where electron-hole asymmetry also plays an important role, and which are useful for applications in detectors. △ Less

Submitted 21 September, 2021; originally announced September 2021.

Comments: 13 pages, 11 figures

Journal ref: Phys. Rev. Applied 17, 034049 (2022)

Controlling magnetism through Ising superconductivity in magnetic van der Waals heterostructures

Authors: Faluke Aikebaier, Tero T. Heikkilä, J. L. Lado

Abstract: 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 and superconductors as NbSe2 provide a natural platform to engineer novel phenomena at ferromagnet-superconductor interfaces. In particular, NbSe2 is… ▽ More 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 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 allows controlling magnetic states in van der Waals materials. Our results show how hybrid van der Waals ferromagnet/superconductor heterostructure can be used as a tunable materials platform for superconducting spin-orbitronics. △ Less

Submitted 27 January, 2022; v1 submitted 10 September, 2021; originally announced September 2021.

Comments: 10 pages, 4 figures

Journal ref: Phys. Rev. B 105, 054506 (2022)

Superconducting spintronic tunnel diode

Authors: E. Strambini, M. Spies, N. Ligato, S. Ilic, M. Rouco, C. G. Orellana, M. Ilyn, C. Rogero, F. S. Bergeret, J. S. Moodera, P. Virtanen, T. T. Heikkilä, F. Giazotto

Abstract: Diodes are key elements for electronics, optics, and detection. The search for a material combination providing the best performances for the required application is continuously ongoing. Here, we present a superconducting spintronic tunnel diode based on the strong spin filtering and splitting generated by an EuS thin film between a superconducting Al and a normal metal Cu layer. The Cu/EuS/Al tu… ▽ More Diodes are key elements for electronics, optics, and detection. The search for a material combination providing the best performances for the required application is continuously ongoing. Here, we present a superconducting spintronic tunnel diode based on the strong spin filtering and splitting generated by an EuS thin film between a superconducting Al and a normal metal Cu layer. The Cu/EuS/Al tunnel junction achieves a large rectification (up to $\sim40$\%) already for a small voltage bias ($\sim 200$ $μ$V) thanks to the small energy scale of the system: the Al superconducting gap. With the help of an analytical theoretical model we can link the maximum rectification to the spin polarization of the barrier and describe the quasi-ideal Schottky-diode behavior of the junction. This cryogenic spintronic rectifier is promising for the application in highly-sensitive radiation detection for which two different configurations are evaluated. In addition, the superconducting diode may pave the way for future low-dissipation and fast superconducting electronics. △ Less

Submitted 9 November, 2023; v1 submitted 2 September, 2021; originally announced September 2021.

Comments: 17 pages, 12 figures

Journal ref: Nature Communications (2022)

Searching For Fermi GRB Optical Counterparts With The Prototype Gravitational-Wave Optical Transient Observer (GOTO)

Authors: Y. -L. Mong, K. Ackley, D. K. Galloway, M. Dyer, R. Cutter, M. J. I. Brown, J. Lyman, K. Ulaczyk, D. Steeghs, V. Dhillon, P. OBrien, G. Ramsay, K. Noysena, R. Kotak, R. Breton, L. Nuttall, E. Palle, D. Pollacco, E. Thrane, S. Awiphan, U. Burhanudin, P. Chote, A. Chrimes, E. Daw, C. Duffy , et al. (23 additional authors not shown)

Abstract: The typical detection rate of $\sim1$ gamma-ray burst (GRB) per day by the \emph{Fermi} Gamma-ray Burst Monitor (GBM) provides a valuable opportunity to further our understanding of GRB physics. However, the large uncertainty of the \emph{Fermi} localization typically prevents rapid identification of multi-wavelength counterparts. We report the follow-up of 93 \emph{Fermi} GRBs with the Gravitatio… ▽ More The typical detection rate of $\sim1$ gamma-ray burst (GRB) per day by the \emph{Fermi} Gamma-ray Burst Monitor (GBM) provides a valuable opportunity to further our understanding of GRB physics. However, the large uncertainty of the \emph{Fermi} localization typically prevents rapid identification of multi-wavelength counterparts. We report the follow-up of 93 \emph{Fermi} GRBs with the Gravitational-wave Optical Transient Observer (GOTO) prototype on La Palma. We selected 53 events (based on favourable observing conditions) for detailed analysis, and to demonstrate our strategy of searching for optical counterparts. We apply a filtering process consisting of both automated and manual steps to 60\,085 candidates initially, rejecting all but 29, arising from 15 events. With $\approx3$ GRB afterglows expected to be detectable with GOTO from our sample, most of the candidates are unlikely to be related to the GRBs. Since we did not have multiple observations for those candidates, we cannot confidently confirm the association between the transients and the GRBs. Our results show that GOTO can effectively search for GRB optical counterparts thanks to its large field of view of $\approx40$ square degrees and its depth of $\approx20$ mag. We also detail several methods to improve our overall performance for future follow-up programs of \emph{Fermi} GRBs. △ Less

Submitted 26 August, 2021; originally announced August 2021.

Coupling the Higgs mode and ferromagnetic resonance in spin-split superconductors with Rashba spin-orbit coupling

Authors: Yao Lu, Risto Ojajärvi, P. Virtanen, M. A. Silaev, Tero T. Heikkilä

Abstract: We consider the Higgs mode at nonzero momentum in superconductors and demonstrate that in the presence of Rashba spin-orbit coupling, it couples linearly with an external exchange field. The Higgs-spin coupling dramatically modifies the spin susceptibility near the superconducting critical temperature and consequently enhances the spin pumping effect in a ferromagnetic insulator/superconductor bil… ▽ More We consider the Higgs mode at nonzero momentum in superconductors and demonstrate that in the presence of Rashba spin-orbit coupling, it couples linearly with an external exchange field. The Higgs-spin coupling dramatically modifies the spin susceptibility near the superconducting critical temperature and consequently enhances the spin pumping effect in a ferromagnetic insulator/superconductor bilayer system. We show that this effect can be detected by measuring the magnon-induced voltage generated by the inverse spin Hall effect. △ Less

Submitted 21 February, 2022; v1 submitted 13 August, 2021; originally announced August 2021.

Journal ref: Phys. Rev. B 106, 024514 (2022)

Dynamics of two ferromagnetic insulators coupled by superconducting spin current

Authors: Risto Ojajärvi, F. S. Bergeret, M. A. Silaev, Tero T. Heikkilä

Abstract: A conventional superconductor sandwiched between two ferromagnets can maintain coherent equilibrium spin current. This spin supercurrent results from the rotation of odd-frequency spin correlations induced in the superconductor by the magnetic proximity effect. In the absence of intrinsic magnetization, the superconductor cannot maintain multiple rotations of the triplet component but instead prov… ▽ More A conventional superconductor sandwiched between two ferromagnets can maintain coherent equilibrium spin current. This spin supercurrent results from the rotation of odd-frequency spin correlations induced in the superconductor by the magnetic proximity effect. In the absence of intrinsic magnetization, the superconductor cannot maintain multiple rotations of the triplet component but instead provides a Josephson type weak link for the spin supercurrent. We determine the analogue of the current-phase relation in various circumstances and show how it can be accessed in experiments on dynamic magnetization. In particular, concentrating on the magnetic hysteresis and the ferromagnetic resonance response, we show how the spin supercurrent affects the nonequilibrium dynamics of magnetization which depends on a competition between spin supercurrent mediated static exchange contribution and a dynamic spin pumping contribution. Depending on the outcome of this competition, a mode crossing in the system can either be an avoided crossing or mode locking. △ Less

Submitted 19 January, 2022; v1 submitted 21 July, 2021; originally announced July 2021.

Comments: 6 pages, 5 figures; Supplementary material 5 pages, 3 figures

Journal ref: Phys. Rev. Lett. 128, 167701 (2022)

Magnomechanics in suspended magnetic beams

Authors: Kalle S. U. Kansanen, Camillo Tassi, Harshad Mishra, Mika A. Sillanpää, Tero T. Heikkilä

Abstract: Cavity optomechanical systems have become a popular playground for studies of controllable nonlinear interactions between light and motion. Owing to the large speed of light, realizing cavity optomechanics in the microwave frequency range requires cavities up to several mm in size, hence making it hard to embed several of them on the same chip. An alternative scheme with much smaller footprint is… ▽ More Cavity optomechanical systems have become a popular playground for studies of controllable nonlinear interactions between light and motion. Owing to the large speed of light, realizing cavity optomechanics in the microwave frequency range requires cavities up to several mm in size, hence making it hard to embed several of them on the same chip. An alternative scheme with much smaller footprint is provided by magnomechanics, where the electromagnetic cavity is replaced by a magnet undergoing ferromagnetic resonance, and the optomechanical coupling originates from magnetic shape anisotropy. Here, we consider the magnomechanical interaction occurring in a suspended magnetic beam -- a scheme in which both magnetic and mechanical modes physically overlap and can also be driven individually. We show that a sizable interaction can be produced if the beam has some initial static deformation, as is often the case due to unequal strains in the constituent materials. We also show how the magnetism affects the magnetomotive detection of the vibrations, and how the magnomechanics interaction can be used in microwave signal amplification. Finally, we discuss experimental progress towards realizing the scheme. △ Less

Submitted 7 December, 2021; v1 submitted 2 July, 2021; originally announced July 2021.

Comments: 17 pages, 10 figures; a few added paragraphs and several typographical errors corrected

Journal ref: Physical Review B 104, 214416 (2021)

Light curve classification with recurrent neural networks for GOTO: dealing with imbalanced data

Authors: U. F. Burhanudin, J. R. Maund, T. Killestein, K. Ackley, M. J. Dyer, J. Lyman, K. Ulaczyk, R. Cutter, Y. -L. Mong, D. Steeghs, D. K. Galloway, V. Dhillon, P. O'Brien, G. Ramsay, K. Noysena, R. Kotak, R. P. Breton, L. Nuttall, E. Pallé, D. Pollacco, E. Thrane, S. Awiphan, P. Chote, A. Chrimes, E. Daw , et al. (20 additional authors not shown)

Abstract: The advent of wide-field sky surveys has led to the growth of transient and variable source discoveries. The data deluge produced by these surveys has necessitated the use of machine learning (ML) and deep learning (DL) algorithms to sift through the vast incoming data stream. A problem that arises in real-world applications of learning algorithms for classification is imbalanced data, where a cla… ▽ More The advent of wide-field sky surveys has led to the growth of transient and variable source discoveries. The data deluge produced by these surveys has necessitated the use of machine learning (ML) and deep learning (DL) algorithms to sift through the vast incoming data stream. A problem that arises in real-world applications of learning algorithms for classification is imbalanced data, where a class of objects within the data is underrepresented, leading to a bias for over-represented classes in the ML and DL classifiers. We present a recurrent neural network (RNN) classifier that takes in photometric time-series data and additional contextual information (such as distance to nearby galaxies and on-sky position) to produce real-time classification of objects observed by the Gravitational-wave Optical Transient Observer (GOTO), and use an algorithm-level approach for handling imbalance with a focal loss function. The classifier is able to achieve an Area Under the Curve (AUC) score of 0.972 when using all available photometric observations to classify variable stars, supernovae, and active galactic nuclei. The RNN architecture allows us to classify incomplete light curves, and measure how performance improves as more observations are included. We also investigate the role that contextual information plays in producing reliable object classification. △ Less

Submitted 25 May, 2021; v1 submitted 24 May, 2021; originally announced May 2021.

Comments: 16 pages, 12 figures, to be published in Monthly Notices of the Royal Astronomical Society

Processing GOTO data with the Rubin Observatory LSST Science Pipelines II: Forced Photometry and light curves

Authors: L. Makrygianni, J. Mullaney, V. Dhillon, S. Littlefair, K. Ackley, M. J. Dyer, J. Lyman, K. Ulaczyk, R. Cutter, Y. -L. Mong, D. Steeghs, D. K. Galloway, P. O'Brien, G. Ramsay, S. Poshyachinda, R. Kotak, L. Nuttall, E. Pallé, D. Pollacco, E. Thrane, S. Aukkaravittayapun, S. Awiphan, R. Breton, U. Burhanudin, P. Chote , et al. (23 additional authors not shown)

Abstract: We have adapted the Vera C. Rubin Observatory Legacy Survey of Space and Time (LSST) Science Pipelines to process data from the Gravitational-Wave Optical Transient Observer (GOTO) prototype. In this paper, we describe how we used the Rubin Observatory LSST Science Pipelines to conduct forced photometry measurements on nightly GOTO data. By comparing the photometry measurements of sources taken on… ▽ More We have adapted the Vera C. Rubin Observatory Legacy Survey of Space and Time (LSST) Science Pipelines to process data from the Gravitational-Wave Optical Transient Observer (GOTO) prototype. In this paper, we describe how we used the Rubin Observatory LSST Science Pipelines to conduct forced photometry measurements on nightly GOTO data. By comparing the photometry measurements of sources taken on multiple nights, we find that the precision of our photometry is typically better than 20~mmag for sources brighter than 16 mag. We also compare our photometry measurements against colour-corrected PanSTARRS photometry, and find that the two agree to within 10~mmag (1$σ$) for bright (i.e., $\sim14^{\rm th}$~mag) sources to 200~mmag for faint (i.e., $\sim18^{\rm th}$~mag) sources. Additionally, we compare our results to those obtained by GOTO's own in-house pipeline, {\sc gotophoto}, and obtain similar results. Based on repeatability measurements, we measure a $5σ$ L-band survey depth of between 19 and 20 magnitudes, depending on observing conditions. We assess, using repeated observations of non-varying standard SDSS stars, the accuracy of our uncertainties, which we find are typically overestimated by roughly a factor of two for bright sources (i.e., $<15^{\rm th}$~mag), but slightly underestimated (by roughly a factor of 1.25) for fainter sources ($>17^{\rm th}$~mag). Finally, we present lightcurves for a selection of variable sources, and compare them to those obtained with the Zwicky Transient Factory and GAIA. Despite the Rubin Observatory LSST Science Pipelines still undergoing active development, our results show that they are already delivering robust forced photometry measurements from GOTO data. △ Less

Submitted 11 May, 2021; originally announced May 2021.

Comments: Accepted for publication in PASA

4D Dual-Tree Complex Wavelets for Time-Dependent Data

Authors: Tatiana A. Bubba, Tommi Heikkilä, Samuli Siltanen

Abstract: The dual-tree complex wavelet transform (DT-$\mathbb{C}$WT) is extended to the 4D setting. Key properties of 4D DT-$\mathbb{C}$WT, such as directional sensitivity and shift-invariance, are discussed and illustrated in a tomographic application. The inverse problem of reconstructing a dynamic three-dimensional target from X-ray projection measurements can be formulated as 4D space-time tomography.… ▽ More The dual-tree complex wavelet transform (DT-$\mathbb{C}$WT) is extended to the 4D setting. Key properties of 4D DT-$\mathbb{C}$WT, such as directional sensitivity and shift-invariance, are discussed and illustrated in a tomographic application. The inverse problem of reconstructing a dynamic three-dimensional target from X-ray projection measurements can be formulated as 4D space-time tomography. The results suggest that 4D DT-$\mathbb{C}$WT offers simple implementations combined with useful theoretical properties for tomographic reconstruction. △ Less

Submitted 29 March, 2021; originally announced March 2021.

Journal ref: 21st International Conference on Computational Science and Its Applications (ICCSA), 2021, pp. 146-156