arXiv subjects

K. Knakkergaard Nielsen

Publications and source records attributed to K. Knakkergaard Nielsen.

12 recordsLinked to original sources

arXiv2025

Dual Spectroscopy of Quantum Simulated Fermi-Hubbard Systems

Quantum gas microscopy with atoms in optical lattices provides remarkable insights into the real space properties of many-body systems, but does not directly reveal the nature of their fundamental excitation spectrum. Here, we demonstrate that radio-frequency spectroscopy can reveal the quasi-particle nature of doped quantum many-body systems, crucial for our understanding of, e.g., high-temperature superconductors. In particular, we showcase how the existence and energy of magnetic polaron quasi-particles in doped Fermi-Hubbard systems may be probed, revealed by hallmark peaks in the spectroscopic spectrum. In combination with fundamental dualities of the Fermi-Hubbard model, we describe how these findings may be tested using several experimental platforms.

cond-mat.quant-gas↗

arXiv2025

Dopant pairing in a disordered magnetic spin ladder

I demonstrate a pairing mechanism of dopants in a magnetic lattice, emerging from the underlying high-temperature disorder of the spins. The effect is demonstrated in a mixed-dimensional model, where dopants travel along a two-leg ladder, while the spins feature Ising interactions along the rungs and the legs of the ladder. The dynamics following the sudden immersion of two nearest neighbor dopants in an infinite temperature spin lattice shows that thermal spin disorder frustrates the relative motion of the dopants and enforces them to co-propagate. The predictions are shown to be realistically testable in quantum simulation experiments.

cond-mat.str-el↗

arXiv2025

Probing a quantum spin liquid with equilibrium and nonequilibrium hole dynamics

The properties and experimental identification of quantum spin liquids (QSLs) remain an important topic with many fundamental questions. Here, we explore the dynamics of a single charge dopant (hole) in a $t$-$J_{1}$-$J_{2}$ model on a square lattice, which realizes a gapless $\mathbb{Z}_2$ QSL at half filling. Using a field theory approach based on the parton construction, which includes an infinite number of scatterings between the low-energy quasiparticle excitations of the QSL via a self-consistent Born approximation, we calculate both the equilibrium and nonequilibrium properties of the hole for weak and strong interactions. Quasiparticle branches as well as stringlike excitations of the holon are identified, and we furthermore explore the time-dependent spreading of a hole throughout the QSL after it has been injected at a given lattice site. The final ballistic expansion speed is shown to exhibit a nonmonotonic behavior as a quantum phase transition between an antiferromagnetic and the QSL phase is crossed, which is caused by a qualitative change in the fundamental kinematics of the interactions between the hole and the surrounding spins. Our results demonstrate how charge dopants can be used as a quantum probe for QSLs and are directly relevant to optical lattice experiments with single site resolution.

cond-mat.str-el↗

arXiv2024

Localized dopant motion across the 2D Ising phase transition

I investigate the motion of a single hole in 2D spin lattices with square and triangular geometries. While the spins have nearest neighbor Ising spin couplings $J$, the hole is allowed to move only in 1D along a single line in the 2D lattice with nearest neighbor hopping amplitude $t$. The non-equilibrium hole dynamics is initialized by suddenly removing a single spin from the thermal Ising spin lattice. I find that for any nonzero spin coupling and temperature, the hole is localized. This is an extension of the thermally induced localization phenomenon [Phys. Rev. Research 6, 023325 (2024)] to the case, where there is a phase transition to a long-range ordered ferromagnetic phase. The dynamics depends only on the ratio of the temperature to the spin coupling, $k_BT / |J|$, and on the ratio of the spin coupling to the hopping $J/t$. I characterize these dependencies in great detail. In particular, I find universal behavior at high temperatures, common features for the square and triangular lattices across the Curie temperatures for ferromagnetic interactions, and highly distinct behaviors for the two geometries in the presence of antiferromagnetic interactions due geometric frustration in the triangular lattice.

cond-mat.str-el↗

arXiv2024

Thermally induced localization of dopants in a magnetic spin ladder

I unveil a novel variant of Anderson localization. This emergent phenomenon pertains to the motion of a dopant in a thermal spin lattice, rendered localized by thermal fluctuations. This is in stark contrast to the intrinsic origin of localization for quenched disorder. The system of interest consists of spin-$1/2$ particles organized in a two-leg ladder with nearest neighbor Ising interactions $J$. The motion of a hole -- the dopant -- is initialized by suddenly removing a spin from the thermal spin ensemble, which then moves along the ladder via nearest neighbor hopping $t$. I find that the hole remains \emph{localized} for all values of $J/t$ and for \emph{all} nonzero temperatures. The origin is an effective disorder potential seen by the hole and induced by thermal spin fluctuations. Its length scale is found to match with the underlying spin-spin correlation length at low temperatures. For ferromagnetic couplings ($J<0$), the associated localization length of the hole increases with decreasing temperature and becomes proportional to the correlation length at low temperatures, asymptotically delocalizing at low temperatures. For antiferromagnetic couplings ($J>0$), there is a smooth crossover between thermal localization at high temperatures to localization driven by the antiferromagnetic order at low temperatures. At infinite temperatures, the dynamics becomes independent of the sign of the spin coupling, whereby the localization length is a universal function of $|J|/t$, diverging as $(t/J)^{2}$ for $|J| \ll t$. Finally, I analyze a setup with Rydberg-dressed atoms, which naturally realizes finite range Ising interactions, accessible in current experimental setups. I show that the discovered localization phenomenon can be probed on experimentally accessible length- and timescales, providing a strong testing ground for my predictions.

cond-mat.str-el↗

arXiv2023

Quantum beat spectroscopy of repulsive Bose polarons

The physics of impurities in a bosonic quantum environment is a paradigmatic and challenging many-body problem that remains to be understood in its full complexity. Here, this problem is investigated for impurities with strong repulsive interactions based on Ramsey interferometry in a quantum degenerate gas of 39K atoms. We observe an oscillatory signal that is consistent with a quantum beat between two co-existing coherent quasiparticle states: the attractive and repulsive polarons. The interferometric signal allows us to extract the polaron energies for a wide range of interaction strengths, complimenting earlier spectroscopic measurements. We furthermore identify several dynamical regimes towards the formation of the Bose polaron in good agreement with theory. Our results improve the understanding of quantum impurities interacting strongly with a bosonic environment, and demonstrate how quasiparticles as well as short-lived non-equilibrium many-body states can be probed using Ramsey interferometry.

cond-mat.quant-gas↗

arXiv2023

Exact dynamics of two holes in two-leg antiferromagnetic ladders

We study the motion of holes in a mixed-dimensional setup of an antiferromagnetic ladder, featuring nearest neighbor hopping $t$ along the ladders and Ising-type spin interactions along, $J_\parallel$, and across, $J_\perp$, the ladder. We determine exact solutions for the low-energy one- and two-hole eigenstates. The presence of the trans-leg spin coupling, $J_\perp$, leads to a linear confining potential between the holes. As a result, holes on separate legs feature a super-linear binding energy scaling as $(J_\perp / t)^{2/3}$ in the strongly correlated regime of $J_\perp,J_\parallel \leq t$. This behavior is linked to an emergent length scale $λ\propto (t/J_\perp)^{1/3}$, stemming from the linear confining potential, and which describes how the size of the two-hole molecular state diverges for $J_\perp,J_\parallel \ll t$. On the contrary, holes on the same leg unbind at sufficiently low spin couplings. This is a consequence of the altered short-range boundary condition for holes on the same leg, yielding an effective Pauli repulsion between them, limiting their kinetic energy and making binding unfavorable. Finally, we determine the exact nonequilibrium quench dynamics following the sudden immersion of initially localized nearest neigbhor holes. The dynamics is characterized by a crossover from an initial ballistic quantum walk to an aperiodic oscillatory motion around a finite average distance between the holes due to the confining potential between them. In the strongly correlated regime of low spin couplings, $J_\perp, J_\parallel \leq t$, we find this asymptotic distance to diverge as $t / J_\perp$, showing a much stronger scaling than the eigenstates. The predicted results should be amenable to state-of-the-art quantum simulation experiments using currently implemented experimental techniques.

cond-mat.quant-gas↗

arXiv2023

Omnipresent bound state of two holes in antiferromagnetic Bethe lattices

For decades, it has remained an open question, whether two dopants in the $t$--$J$ model bind in the strongly correlated regime of small spin couplings versus hopping, $J \ll t$. Here, we investigate this problem in Bethe lattice structures with Ising coupling $J_z$, and mainly focus on the case of a coordination number equal to 4. The special geometry circumvents subtle effects in regular lattices, but importantly still contains the non-trivial dependency on the rotational symmetry and particle statistics. It, furthermore, allows us to reach numerical convergence, and we conclusively answer whether binding occurs or not. In particular, we find that the rotationally symmetric $s$ waves unbind below $J_z \simeq 0.3 t$, which we unveil is tied to Pauli blocking of symmetrical hole configurations. This is further substantiated by the fact that holes in a bosonic spin environment shows strong -- superlinear -- binding at low $J_z / t$. Additionally, higher rotational fermionic $p$ and $d$ waves partially overcome the blocking, are perfectly degenerate, and bind for all $J_z / t$. In the strongly correlated regime of $J_z \ll t$, this binding occurs sublinearly with scaling $\sim t (J_z / t)^{3/2}$.

cond-mat.str-el↗

arXiv2022

Nonequilibrium Hole Dynamics in Antiferromagnets: Damped Strings and Polarons

We develop a nonperturbative theory for hole dynamics in antiferromagnetic spin lattices, as described by the $t$-$J$ model. This is achieved by generalizing the selfconsistent Born approximation to nonequilibrium systems, making it possible to calculate the full time-dependent many-body wave function. Our approach reveals three distinct dynamical regimes, ultimately leading to the formation of magnetic polarons. Following the initial ballistic stage of the hole dynamics, coherent formation of string excitations gives rise to characteristic oscillations in the hole density. Their damping eventually leaves behind magnetic polarons that undergo ballistic motion with a greatly reduced velocity. The developed theory provides a rigorous framework for understanding nonequilibrium physics of defects in quantum magnets and quantitatively explains recent observations from cold-atom quantum simulations in the strong coupling regime.

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arXiv2022

Exact nonequilibrium hole dynamics, magnetic polarons and string excitations in antiferromagnetic Bethe lattices

We investigate a rare instance of an exactly solvable nonequilibrium many-body problem. In particular, we derive an exact solution for the nonequilibrium dynamics of an initially localized single hole in a fully anisotropic antiferromagnetic Bethe lattice, described by the $t$-$J_z$ model. The solvability of the model relies on the fractal self-similarity of Bethe lattices, making it possible to compute the full motion of the hole as it moves through the lattice, as well as exactly characterizing the resulting effect on spin-spin correlation functions. We find that the hole remains bound to its initial position with large aperiodic oscillations in the hole density distribution. We track this back to the irregular pattern of the eigenenergies of the magnetic polaron ground state and string excitations, which we also determine exactly.

cond-mat.str-el↗

arXiv2020

Superfluid flow of polaron polaritons above Landau's critical velocity

We develop a theory for the interaction of light with superfluid optical media, describing the motion of quantum impurities that are created and dragged through the liquid by propagating photons. It is well known that a mobile impurity suffers dissipation due to phonon emission as soon as it moves faster than the speed of sound in the superfluid - Landau's critical velocity. Surprisingly we find that in the present hybrid light-matter setting, polaritonic impurities can be protected against environmental decoherence and be allowed to propagate well above the Landau velocity without jeopardizing the superfluid response of the medium.

cond-mat.quant-gas↗

arXiv2020

Polariton dynamics in strongly interacting quantum many-body systems

We develop a theory for light propagating in an atomic Bose-Einstein condensate in the presence of strong interactions. The resulting many-body correlations are shown to have profound effects on the optical properties of this interacting medium. For weak atom-light coupling, there is a well-defined quasiparticle, the polaron-polariton, supporting light propagation with spectral features differing significantly from the noninteracting case. The damping of the polaron-polariton depends nonmonotonically on the light-matter coupling strength, initially increasing and then decreasing. This gives rise to an interesting crossover between two quasiparticles: a bare polariton and a polaron-polariton, separated by a complex and lossy mixture of light and matter.

cond-mat.quant-gas↗