arXiv subjects

P. Törmä

Publications and source records attributed to P. Törmä.

At least 19 recordsLinked to original sources

arXiv2026

Incoherent Spectral Weight Emerging from a Van Hove Singularity in a Kagome Metal

Waterfall-like spectral features observed by angle-resolved photoemission spectroscopy (ARPES) are commonly associated with strong electronic correlations or electron-boson coupling. Here, we investigate this connection in the kagome metal LuFe6Ge6, a structurally simple member of the FeGe-derived family, using polarization-dependent ARPES, density functional theory and dynamical mean-field theory. We observe a pronounced vertical spectral feature at the L point extending over several hundred meV in binding energy. Its orbital character is identified as predominantly Fe dyz, and its energy onset coincides with a van Hove singularity located approximately 0.3 eV below EF . Despite the waterfall-like spectral response, dynamical mean-field theory (DMFT) reveals only weak electronic correlations, while the calculated electronic structure closely reproduces the experimental bulk bands. We propose that the anomalous spectral weight may arise from inelastic scattering of the outgoing photoelectrons, potentially associated with the large density of states near the van Hove singularity. Our results suggest that waterfall-like features can emerge without strong electronic correlations and highlight the possible role of the underlying band structure and photoelectron energy-loss processes in their interpretation.

cond-mat.str-el↗

arXiv2025

Superconducting junctions with flat bands

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.

cond-mat.supr-con↗

arXiv2025

Cascade of pressure induced competing CDWs in the kagome metal FeGe

Electronic ordering is prevalent in correlated systems, which commonly exhibit competing interactions. Here, we use x-ray diffraction to demonstrate a cascade of pressure induced order-disorder transformations, with propagation vectors $\mathbf{q}_\mathrm{CDW}$= $\left(\frac{1}{2}\ 0\ \frac{1}{2}\right)$, $\mathbf{q}^*$=$\left(\frac{1}{3}\ \frac{1}{3}\ \frac{1}{2}\right)$ and $\mathbf{q}^\dagger$=$\left(\frac{1}{3}\ \frac{1}{3}\ \frac{1}{3}\right)$, in the kagome metal FeGe. In the pressure interval between 4$<p<$10 GPa, $\mathbf{q}_\mathrm{CDW}$ and $\mathbf{q}^*$ coexist and the spatial extent of the $\sqrt{3}\times\sqrt{3}$ order is nearly long-range at $\sim$15 GPa, $\sim$30 unit cells. Above $\sim$25 GPa, the periodic lattice distortion has a propagation wavevector of $\mathbf{q}^\dagger$=$\left(\frac{1}{3}\ \frac{1}{3}\ \frac{1}{3}\right)$ at room temperature. The cascade of phase transitions are captured by the Ising model of frustrated triangular lattices and modeled by Monte Carlo simulations based on the dimerization of trigonal Ge$_1$. The pressure dependence of the integrated intensities and correlation lengths of $\mathbf{q}_\mathrm{CDW}$, $\mathbf{q}^*$ and $\mathbf{q}^\dagger$ demonstrate a competition between the 2$\times$2 and $\sqrt{3}\times\sqrt{3}$ phases prove the tunability of order-disorder phase transitions under pressure and the rich landscape of metastable/fragile phases of FeGe.

cond-mat.str-el↗

arXiv2023

Superfluidity and Quantum Geometry in Twisted Multilayer Systems

Designer 2D materials where the constituent layers are not aligned may result in band structures with dispersionless, "flat" bands. Twisted bilayer graphene has been found to show correlated phases as well as superconductivity related to such flat bands. In parallel, theory work has discovered that superconductivity and superfluidity is determined by the quantum geometry and topology of the band structure. These recent key developments are merging to a flourishing research topic: understanding the possible connection and ramifications of quantum geometry on the induced superconductivity and superfluidity in moiré multilayer and other flat band systems. This article presents an introduction to how quantum geometry governs superfluidity in platforms including, and beyond, graphene. We explain how a new type of topology discovered in TBG could affect superconductivity, pinpoint the geometric contribution in its Beretzinskii-Kosterlitz-Thouless (BKT) critical temperature, and mention moiré materials beyond TBG. Ultracold gases are introduced as a complementary platform for quantum geometric effects and a comparison is made to moiré materials. An outlook sketches the prospects of twisted multilayer systems in providing the route to room temperature superconductivity.

cond-mat.supr-con↗

arXiv2021

Light-matter coupling and quantum geometry in moiré materials

Quantum geometry has been identified as an important ingredient for the physics of quantum materials and especially of flat-band systems, such as moiré materials. On the other hand, the coupling between light and matter is of key importance across disciplines and especially for Floquet and cavity engineering of solids. Here we present fundamental relations between light-matter coupling and quantum geometry of Bloch wave functions, with a particular focus on flat-band and moiré materials, in which the quenching of the electronic kinetic energy could allow one to reach the limit of strong light-matter coupling more easily than in highly dispersive systems. We show that, despite the fact that flat bands have vanishing band velocities and curvatures, light couples to them via geometric contributions. Specifically, the intra-band quantum metric allows diamagnetic coupling inside a flat band; the inter-band Berry connection governs dipole matrix elements between flat and dispersive bands. We illustrate these effects in two representative model systems: (i) a sawtooth quantum chain with a single flat band, and (ii) a tight-binding model for twisted bilayer graphene. For (i) we highlight the importance of quantum geometry by demonstrating a nonvanishing diamagnetic light-matter coupling inside the flat band. For (ii) we explore the twist-angle dependence of various light-matter coupling matrix elements. Furthermore, at the magic angle corresponding to almost flat bands, we show a Floquet-topological gap opening under irradiation with circularly polarized light despite the nearly vanishing Fermi velocity. We discuss how these findings provide fundamental design principles and tools for light-matter-coupling-based control of emergent electronic properties in flat-band and moiré materials.

physics.optics↗

arXiv2017

Fast trimers in one-dimensional extended Fermi-Hubbard model

We consider a one-dimensional two component extended Fermi-Hubbard model with nearest neighbor interactions and mass imbalance between the two species. We study the stability of trimers, various observables for detecting them, and expansion dynamics. We generalize the definition of the trimer gap to include the formation of different types of clusters originating from nearest neighbor interactions. Expansion dynamics reveal rapidly propagating trimers, with speeds exceeding doublon propagation in strongly interacting regime. We present a simple model for understanding this unique feature of the movement of the trimers, and we discuss the potential for experimental realization.

cond-mat.quant-gas↗

arXiv2017

Impurity and soliton dynamics in a Fermi gas with nearest-neighbor interactions

We study spinless fermions with repulsive nearest-neighbor interactions perturbed by an impurity particle or a local potential quench. Using the numerical time-evolving block decimation method and a simplified analytic model, we show that the pertubations create a soliton-antisoliton pair. If solitons are already present in the bath, the two excitations have a drastically different dynamics: The antisoliton does not annihilate with the solitons and is therefore confined close to its origin while the soliton excitation propagates. We discuss the consequences for experiments with ultracold gases.

cond-mat.quant-gas↗

arXiv2017

Fermi surface deformations and pairing in mixtures of dipolar and non-dipolar fermions

We study mass-imbalanced two-component Fermi mixtures, where one of the components consists of dipolar fermions. We specifically study the mass imbalances corresponding to the atomic ${}^{163}$Dy-${}^{40}$K and ${}^{53}$Cr-${}^{6}$Li mixtures. We study the onset of the $s$-wave superfluid phase, as a function of population imbalance and the dipolar interaction strength. We find the critical temperature and the Fermi surface deformations at the transition to depend on the dipolar interaction strength, where the critical temperature increases (decreases) with dipolar interaction strength for a majority (minority) of dipolar atoms. We present momentum distributions of both components where the Fermi surface deformations are visible.

cond-mat.quant-gas↗

arXiv2017

Spin-asymmetric Josephson plasma oscillations

The spin-asymmetric Josephson effect is a proposed quantum-coherent tunnelling phenomenon where Cooper-paired fermionic spin-$\frac{1}{2}$ particles, which are subjected to spin-dependent potentials across a Josephson junction, undergo frequency-synchronized alternating-current Josephson oscillations with spin-dependent amplitudes. Here, in line with present-day techniques in ultracold Fermi gas setups, we consider the regime of small Josephson oscillations and show that the Josephson plasma oscillation amplitude becomes spin-dependent in the presence of spin-dependent potentials while the Josephson plasma frequency is the same for both spin-components. Detecting these spin-dependent Josephson plasma oscillations provides a possible means to establish the yet-unobserved spin-asymmetric Josephson effect with ultracold Fermi gases using existing experimental tools.

cond-mat.quant-gas↗

arXiv2016

Geometry dependence of surface lattice resonances in plasmonic nanoparticle arrays

Plasmonic nanoarrays which support collective surface lattice resonances (SLRs) have become an exciting frontier in plasmonics. Compared with the localized surface plasmon resonance (LSPR) in individual particles, these collective modes have appealing advantages such as angle-dependent dispersions and much narrower linewidths. Here, we investigate systematically how the geometry of the lattice affects the SLRs supported by metallic nanoparticles. We present a general theoretical framework from which the various SLR modes of a given geometry can be straightforwardly obtained by a simple comparison of the diffractive order (DO) vectors and orientation of the nanoparticle dipole given by the polarization of the incident field. Our experimental measurements show that while square, hexagonal, rectangular, honeycomb and Lieb lattice arrays have similar spectra near the $Γ$-point ($k=0$), they have remarkably different SLR dispersions. Furthermore, their dispersions are highly dependent on the polarization. Numerical simulations are performed to elucidate the field profiles of the different modes. Our findings extend the diversity of SLRs in plasmonic nanoparticle arrays, and the theoretical framework provides a simple model for interpreting the SLRs features, and vice versa, for designing the geometrical patterns.

physics.optics↗

arXiv2016

Lasing in dark and bright modes of a finite-sized plasmonic lattice

Lasing at the nanometre scale promises strong light-matter interactions and ultrafast operation. Plasmonic resonances supported by metallic nanoparticles have extremely small mode volumes and high field enhancements, making them an ideal platform for studying nanoscale lasing. At visible frequencies, however, the applicability of plasmon resonances is limited due to strong ohmic and radiative losses. Intriguingly, plasmonic nanoparticle arrays support non-radiative dark modes that offer longer life-times but are inaccessible to far-field radiation. Here, we show lasing both in dark and bright modes of an array of silver nanoparticles combined with optically pumped dye molecules. Linewidths of 0.2 nanometers at visible wavelengths and room temperature are observed. Access to the dark modes is provided by a coherent out-coupling mechanism based on the finite size of the array. The results open a route to utilize all modes of plasmonic lattices, also the high-Q ones, for studies of strong light-matter interactions, condensation and photon fluids.

cond-mat.mes-hall↗

arXiv2016

Decoherence of an impurity in a one-dimensional fermionic bath with mass imbalance

We study the transport, decoherence and dissipation of an impurity interacting with a bath of free fermions in a one-dimensional lattice. Numerical simulations are made with the time-evolving block decimation method. We introduce a mass imbalance between the impurity and bath particles and find that the fastest decoherence occurs for a light impurity in a bath of heavy particles. By contrast, the fastest dissipation of energy occurs when the masses are equal. We present a simple model for decoherence in the heavy bath limit, and a linear density response description of the interaction which predicts maximum dissipation for equal masses.

cond-mat.quant-gas↗

arXiv2016

Excitations and impurity dynamics in a fermionic Mott insulator with nearest-neighbor interactions

We study analytically and with the numerical time-evolving block decimation method the dynamics of an impurity in a bath of spinless fermions with nearest-neighbor interactions in a one-dimensional lattice. The bath is in a Mott insulator state with alternating sites occupied and the impurity interacts with the bath by repulsive on-site interactions. We find that when the magnitudes of the on-site and nearest-neighbor interactions are close to each other, the system shows excitations of two qualitatively distinct types. For the first type, a domain wall and an anti-domain wall of density propagate in opposite directions, while the impurity stays at the initial position. For the second one, the impurity is bound to the anti-domain wall while the domain wall propagates, an excitation where the impurity and bath are closely coupled.

cond-mat.quant-gas↗

arXiv2015

Population imbalance in the extended Fermi-Hubbard model

We study the interplay between population imbalance in a two-component fermionic system and nearest-neighbor interaction using matrix product states method. Our analysis reveals the existence of a new type of Fulde-Ferrell-Larkin-Ovchinnikov phase in the presence of competing interactions. Furthermore, we find distinct evidence for the presence of hidden order in the system. We present an effective model to understand the emergent oscillations in the string correlations due to the imbalance, and show how they can become an efficient tool to investigate systems with imbalance.

cond-mat.quant-gas↗

arXiv2015

Superfluid phases of fermions with hybridized $s$ and $p$ orbitals

We explore the superfluid phases of a two-component Fermi mixture with hybridized orbitals in optical lattices. We show that there exists a general mapping of this system to the Lieb lattice. By using simple multiband models with hopping between $s$ and $p$-orbital states, we show that superfluid order parameters can have a $π$-phase difference between lattice sites, which is distinct from the case with hopping between $s$-orbitals. If the population imbalance between the two spin species is tuned, the superfluid phase may evolve through various phases due to the interplay between hopping, interactions and imbalance. We show that the rich behavior is observable in experimentally realizable systems.

cond-mat.quant-gas↗

arXiv2015

Larkin-Ovchinnikov phases in two-dimensional square lattices

We consider a two-component gas of fermions in optical lattices in the presence of a population imbalance within a mean-field theory. We study phase transitions from a normal gas of unpaired fermions to a superfluid phase of Bose-condensed Cooper pairs. The possibility of Cooper pairs with a nonzero center-of-mass momentum is included, which corresponds to a so-called Fulde-Ferrel-Larkin-Ovchinnikov (FFLO) state. We find that for population imbalanced systems such states can form the ground state. The FF and LO state are compared and it is shown that actually the LO state is energetically more favorable. We complete the mean-field phase diagram for the LO phase and show that it is qualitatively in excellent agreement with recent diagrammatic Monte Carlo calculations. Subsequently, we calculate the atomic density modulations in the LO phase.

cond-mat.quant-gas↗

arXiv2015

Topological States with Broken Translational and Time-Reversal Symmetries in a Honeycomb-Triangular Lattice

We study fermions in a lattice, with on-site and nearest neighbor attractive interactions between two spin species. We consider two geometries: both spins in a triangular lattice, and a mixed geometry with up-spins in honeycomb and down-spins in triangular lattices. We focus on the interplay between spin-population imbalance, on-site and valence bond pairing, and order parameter symmetry. The mixed geometry leads to a rich phase diagram of topologically non-trivial phases. In both geometries, we predict order parameters with simultaneous time-reversal and translational symmetry breaking.

cond-mat.quant-gas↗

arXiv2014

Moving perturbation in a one-dimensional Fermi gas

We simulate a balanced attractively interacting two-component Fermi gas in a one-dimensional lattice perturbed with a moving potential well or barrier. Using the time-evolving block decimation method, we study different velocities of the perturbation and distinguish two velocity regimes based on clear differences in the time evolution of particle densities and the pair correlation function. We show that, in the slow regime, the densities deform as particles are either attracted by the potential well or repelled by the barrier, and a wave front of hole or particle excitations propagates at the maximum group velocity. Simultaneously, the initial pair correlations are broken and coherence over different sites is lost. In contrast, in the fast regime, the densities are not considerably deformed and the pair correlations are preserved.

cond-mat.quant-gas↗