arXiv subjects

Malte Henkel

Publications and source records attributed to Malte Henkel.

At least 19 recordsLinked to original sources

arXiv2026

Harnessing finite-size effects to gauge aging in the $2D$ Ising model

The relaxation behavior towards equilibrium of the $2D$ Ising model with nearest-neighbor interactions has been studied with focus on the two-time autocorrelator $C(t,s)$. Finite-size effects affecting the growing magnetic domains lead to the saturation of $C(t,s)$ with a distinct plateau of height $C_{\infty}^{(2)}(s,L)$ scaling algebraically with waiting time $s$ and lattice size $L$. These scaling relations are used to produce precise estimates for the autocorrelation exponent $\lambda$ and dynamical exponent $z$ with deliberately small lattices. Treating smooth domain walls in a similar manner to the lattice boundaries, their effect on $C(t,s)$ can be understood as premature finite-size phenomenon, extending our ansatz to systems not yet in equilibrium.

cond-mat.stat-mech↗

arXiv2026

Exact solution of the Glauber-Ising model on the finite-length semi-open chain

The exact time-space correlation function of the $1D$ Glauber-Ising model, quenched to temperature $T=0$ and on a semi-open lattice of finite size $N$, is obtained. This also allows to deduce the exact empty-interval probability of the dual $1D$ coagulation-diffusion process on a periodic finite ring and to reproduce the long-time decay of the particle concentration. These results are consistent with the generic expectations of dynamical finite-size scaling theory.

cond-mat.stat-mech↗

arXiv2026

Fractal and Spectral Dimensions as Determinants of Thermal Ablation Outcomes in Cancer Tissues

Clinical thermal ablation outcomes display significant variability that classical bio-heat models cannot fully explain. One reason may lie in the fractal architecture of biological tissues, which has been identified as a robust biomarker directly correlated with cancer grades. This structural heterogeneity, together with memory effects (e.g., thermotolerance), causes heat transfer in living tissues to differ from Fourier diffusion, resulting in anomalous biological transport. In this work, we implemented a realistic fractal-fractional bio-heat model, with non-linear perfusion and PI-controlled power delivery, to quantify the role of tissue fractality in ablation outcomes. Our results reveal that the expansion of coagulation zones is jointly controlled by fractal geometry and its associated topological connectivity. These findings highlight spectral dimension as a key driver of clinical variability, successfully reproducing the reduced ablative efficacy in liver metastases compared to primary carcinomas, and provide evidence for topologically informed treatment strategies for the thermal ablation of malignant neoplasms.

cond-mat.stat-mech↗

arXiv2025

Schr\"odinger-invariance in phase-ordering kinetics

The generic shape of the single-time and two-time correlators in non-equilibrium phase-ordering kinetics with ${z}=2$ is obtained from the co-variance of the four-point response functions. Their non-equilibrium scaling forms follow from a new non-equilibrium representation of the Schr\"odinger algebra.

cond-mat.stat-mech↗

arXiv2025

Short-time dynamics in phase-ordering kinetics

Short-time dynamics in the $2D$ Blume-Capel model, with a non-conserved order-parameter and short-ranged interactions, is analysed. For non-equilibrium dynamics, both at a critical point in the $2D$ Ising universality class and at the tricritical point, we reproduce the values $\Theta=0.190({5})$ and $\Theta=-0.542({5})$, respectively, of the critical initial slip exponent. These agree with more early estimates and with the Janssen-Schaub-Schmittmann scaling relation. In phase-ordering kinetics, after a quench into the ordered phase, we establish the validity of short-time dynamics. In the $2D$ Ising universality class, we find $\Theta=0.39({1})$ in agreement with the scaling relation $\lambda=d-2\Theta$.

cond-mat.stat-mech↗

arXiv2025

Schr\"odinger-invariance in non-equilibrium critical dynamics

The scaling functions of single-time and two-time correlators in systems undergoing non-equilibrium critical dynamics with dynamical exponent ${z}=2$ are predicted from a new time-dependent non-equilibrium representation of the Schr\"odinger algebra. These explicit predictions are tested and confirmed in the ageing of several exactly solvable models.

cond-mat.stat-mech↗

arXiv2025

Generalised fractional Rabi problem

Fractional quantum dynamics provides a natural framework to capture nonlocal temporal behavior and memory effects in quantum systems. In this work, we analyze the physical consequences of fractional-order quantum evolution using a Green's function formulation based on the Caputo fractional derivative. Explicit iterative expressions for the evolved state are derived and applied to an extended two-level Rabi model, a paradigmatic setting for coherent quantum control. We find that even in the absence of external driving, the static Hamiltonian term induces non-trivial spin dynamics with damping features directly linked to the fractional temporal nonlocality. When a periodically varying driving field is introduced, the competition between energy injection and memory effects gives rise to a richer dynamical behavior, manifest in the evolution of spin polarization, autocorrelation function, and fidelity. Unlike the standard Rabi oscillations characterized by a fixed frequency, the fractional regime introduces controllable damping and dephasing governed by the degree of fractionality. These distinctive signatures could be observable through the Loschmidt echo and autocorrelation function, and would offer potential routes to probe fractional quantum dynamics experimentally. Our findings open pathways toward exploring memory-induced dynamical phenomena in other systems effectively described by a two-level approximation, such as graphene-like materials and topological SSH chains, where non-integer order evolution may reveal novel topological or relaxation effects.

quant-ph↗

arXiv2025

Schr\"odinger-invariance in the voter model

Exact single-time and two-time correlations and the two-time response function are found for the order-parameter in the voter model with nearest-neighbour interactions. Their explicit dynamical scaling functions are shown to be continuous functions of the space dimension $d>0$. Their form reproduces the predictions of non-equilibrium representations of the Schr\"odinger algebra for models with dynamical exponent ${z}=2$ and with the dominant noise-source coming from the heat bath. Hence the ageing in the voter model is a paradigm for relaxations in non-equilibrium critical dynamics, without detailed balance, and with the upper critical dimension $d^*=2$.

cond-mat.stat-mech↗

arXiv2025

Correlators in phase-ordering from Schr\"odinger-invariance

Systems undergoing phase-ordering kinetics after a quench into the ordered phase with $0<T<T_c$ from a fully disordered initial state and with a non-conserved order-parameter have the dynamical exponent ${z}=2$. The long-time behaviour of their single-time and two-time correlators, determined by the noisy initial conditions, is derived from Schr\"odinger-invariance and we show that the generic ageing scaling forms of the correlators follow from the Schr\"odinger covariance of the four-point response functions. The autocorrelation exponent $\lambda$ is related to the passage exponent $\zeta_p$ which describes the time-scale for the cross-over into the ageing regime. Both Porod's law and the bounds $d/2 \leq \lambda \leq d$ are reproduced in a simple way. The dynamical scaling in fully finite systems and of global correlators is found and the low-temperature generalisation $\lambda= d-2\Theta$ of the Janssen-Schaub-Schmittmann scaling relation is derived.

cond-mat.stat-mech↗

arXiv2025

Critical ageing correlators from Schr\"odinger-invariance

For ageing systems, quenched onto a critical temperature $T=T_c$ such that the dominant noise comes from the thermal bath, with a non-conserved order-parameter and in addition with dynamical exponent ${z}=2$, the form of the two-time auto-correlator as well as the time-space form of the single-time correlator are derived from Schr\"odinger-invariance, generalised to non-equilibrium ageing. These findings reproduce the exact results in the $1D$ Glauber-Ising model at $T=0$ and the critical spherical model in $d>2$ dimensions.

cond-mat.stat-mech↗

arXiv2025

Physical ageing from generalised time-translation-invariance

A generalised form of time-translation-invariance permits to re-derive the known generic phenomenology of ageing, which arises in classical many-body systems after a quench from an initially disordered system to a temperature $T\leq T_c$, at or below the critical temperature $T_c$. Generalised time-translation-invariance is obtained, out of equilibrium, from a change of representation of the Lie algebra generators of the dynamical symmetries of scale-invariance and time-translation-invariance. Observable consequences include the algebraic form of the scaling functions for large arguments of the two-time auto-correlators and auto-responses, the equality of the auto-correlation and the auto-response exponents $\lambda_C=\lambda_R$, the cross-over scaling form for an initially magnetised critical system and the explanation of a novel finite-size scaling if the auto-correlator or auto-response converge for large arguments $y=t/s\gg 1$ to a plateau. For global two-time correlators, the time-dependence involving the initial critical slip exponent $\Theta$ is confirmed and is generalised to all temperatures below criticality and to the global two-time response function, and their finite-size scaling is derived as well. This also includes the time-dependence of the squared global order-parameter. The celebrate Janssen-Schaub-Schmittmann scaling relation with the auto-correlation exponent is thereby extended to all temperatures below the critical temperature. A simple criterion on the relevance of non-linear terms in the stochastic equation of motion is derived, taking the dimensionality of couplings into account. Its applicability in a wide class of models is confirmed, for temperatures $T\leq T_c$. Relevance to experiments is also discussed.

cond-mat.stat-mech↗

arXiv2025

Bio-heat regimes in fractal-based models of tumors

Anomalous heat diffusion is investigated for biological tissues displaying a fractal structure and long-term thermal memory, which is modeled via a fractional derivative. For increasing values of the fractional derivation order, the tissue temperature displays three kinds of bio-heat regimes: damped (or sub-diffusive), critical damping and under-damped oscillations. The temperature profiles depend on the fractal dimension of the tissue but notably also on a parameter related to its topology: the spectral dimension. The parametric analysis reveals that these two parameters have antagonistic effects on the pseudo period of the temperature oscillations and their amplitudes. We discuss how our results might impact some treatment protocols.

physics.bio-ph↗

arXiv2025

Asymptotics of the Humbert functions $\Psi_1$ and $\Psi_2$

A compilation of new results on the asymptotic behaviour of the Humbert functions $\Psi_1$ and $\Psi_2$, and also on the Appell function $F_2$, is presented. As a by-product, we confirm a conjectured limit which appeared recently in the study of the $1D$ Glauber-Ising model. We also propose two elementary asymptotic methods and confirm through some illustrative examples that both methods have great potential and can be applied to a large class of problems of asymptotic analysis. Finally, some directions of future research are pointed out in order to suggest ideas for further study.

math.CA↗

arXiv2025

Finite-size scaling in the ageing dynamics of the $1D$ Glauber-Ising model

Single-time and two-time correlators are computed exactly in the $1D$ Glauber-Ising model after a quench to zero temperature and on a periodic chain of finite length $N$, using a simple analytical continuation technique. Besides the general confirmation of finite-size scaling in non-equilibrium dynamics, this allows to test the scaling behaviour of the plateau height $C_{\infty}^{(2)}$ to which the two-time auto-correlator converges, when deep into the finite-size regime.

cond-mat.stat-mech↗

arXiv2025

Finite-Size Effects in Aging can be Interpreted as Sub-Aging

Systems brought out of equilibrium through a rapid quench from a disordered initial state into an ordered phase undergo physical aging in the form of phase-ordering kinetics, with characteristic dynamical scaling. In many systems, notably glasses, dynamical scaling is often described through sub-aging, where a phenomenological sub-aging exponent $0<\mu< 1$ is empirically chosen to achieve the best possible data collapse. Here it is shown that finite-size effects modify the dynamical scaling behavior, away from simple aging with $\mu=1$ towards $\mu<1$, such that phenomenologically it would appear as sub-aging. This is exemplified for the exactly solved dynamical spherical model in dimensions $2<d<4$ and numerical simulations of the two-dimensional Ising model, with short-ranged and long-ranged interactions.

cond-mat.stat-mech↗

arXiv2024

Schr\"odinger symmetry: a historical review

This paper reviews the history of the conformal extension of Galilean symmetry, now called Schr\"odinger symmetry. In the physics literature, its discovery is commonly attributed to Jackiw, Niederer and Hagen (1972). However, Schr\"odinger symmetry has a much older ancestry: the associated conserved quantities were known to Jacobi in 1842/43 and its euclidean counterpart was discovered by Sophus Lie in 1881 in his studies of the heat equation. A convenient way to study Schr\"odinger symmetry is provided by a non-relativistic Kaluza-Klein-type "Bargmann" framework, first proposed by Eisenhart (1929), but then forgotten and re-discovered by Duval {\it et al.} only in 1984. Representations of Schr\"odinger symmetry differ by the value $z=2$ of the dynamical exponent from the value $z=1$ found in representations of relativistic conformal invariance. For generic values of $z$, whole families of new algebras exist, which for $z=2/\ell$ include the $\ell$-conformal galilean algebras. We also review the non-relativistic limit of conformal algebras and that this limit leads to the $1$-conformal galilean algebra and not to the Schr\"odinger algebra. The latter can be recovered in the Bargmann framework through reduction. A distinctive feature of Galilean and Schr\"odinger symmetries are the Bargmann super-selection rules, algebraically related to a central extension. An empirical consequence of this was known as "mass conservation" already to Lavoisier. As an illustration of these concepts, some applications to physical ageing in simple model systems are reviewed.

hep-th↗

arXiv2024

Fractional diffusion equations interpolate between damping and waves

The behaviour of the solutions of the time-fractional diffusion equation, based on the Caputo derivative, is studied and its dependence on the fractional exponent is analysed. The time-fractional convection-diffusion equation is also solved and an application to Pennes bioheat model is presented. Generically, a wave-like transport at short times passes over to a diffusion-like behaviour at later times.

math-ph↗

arXiv2023

Dynamical symmetries in the non-equilibrium dynamics of the directed spherical model

The dynamical scaling and ageing in the relaxational dynamics of the quenched directed spherical model is analysed. The exact two-time correlation and response functions display new regimes of ballistic or anisotropic ballistic scaling, at larger distances than probed in the usual regime of diffusive scaling. The rôle of long-ranged initial correlations on the existence of these scaling regimes is clarified. Their dynamical symmetries are described in terms of extensions of the Schrödinger algebra appropriate to non-equilibrium dynamics in that the anisotropic ballistic scaling regime can be interpreted in terms of meta-Schrödinger invariance while the regime of isotropic ballistic scaling is meta-conformally invariant.

cond-mat.stat-mech↗