arXiv subjects

Emir Sezik

Publications and source records attributed to Emir Sezik.

7 recordsLinked to original sources

arXiv2026

Large Spin-Wave Fluctuations Suppress Activity in Malthusian Flocks

Novel phases, beyond long-range order in two dimensions, have continued to be discovered within flocking models, establishing flocking as one of the pivotal paradigms in active matter. However, much of the discussion around ``Malthusian'' (constant density) flocks, an analytically more tractable alternative to the Vicsek model, has centred around the scaling exponents governing the intermediate regime prior to the proliferation of asters, leaving open the question of what other phases the model might display. Here, we study the two-dimensional dynamics of Malthusian flocks and identify a previously unnoticed phase, where the dynamics is that of the equilibrium XY Model. By identifying the symmetries of the model, we derive the effective equations of motion for the Goldstone modes and analyse the spin-wave fluctuations. We identify a novel critical point separating two distinct phases and, using a perturbative RG procedure, determine the RG flows in its vicinity. This allows us to calculate the universal scaling behaviour at the critical point, along with its logarithmic corrections. The novel phase transition here is due to the interaction of activity and spin-waves, unlike the equilibrium counterpart, which undergoes a phase transition in effective degrees of freedom, namely vortices. Nevertheless, the RG flows are similar to those of the Berezinskii-Kosterlitz-Thouless transition, and we show that for sufficiently strong noise, the activity becomes irrelevant and the system crosses over to the equilibrium XY universality class.

cond-mat.soft↗

arXiv2026

Non-Reciprocal yet Equilibrium Critical Dynamics

Non-reciprocal interactions find broad applicability in non-equilibrium and living systems. Their canonical implementation involves asymmetric couplings between two entities, which generally induce spatio-temporal patterns and time-dependent steady states that break time-translational invariance, representing a clear deviation from equilibrium physics. Although their phenomenology is well understood, whether non-reciprocal interactions induce new universality classes, and if so, under what conditions, remains an open question. In the present work, we perform a field-theoretic renormalization group (RG) analysis of the dynamics of two non-reciprocally coupled $n$-vector order parameters possessing a $U(n)$ symmetry, generalizing previous results to order parameters with multiple components, a feature that has been shown to generate novel non-equilibrium critical behavior in certain non-reciprocal systems. To lowest order in $\epsilon = 4-d$, we find that the non-reciprocal coupling is RG-irrelevant, and the critical behavior is governed by the equilibrium fixed point of the Model A universality class of Hohenberg and Halperin with $2n$ vector components, even though the transition is into a non-equilibrium state. Our results demonstrate that non-reciprocity alone may not be sufficient to induce novel universality classes.

cond-mat.stat-mech↗

arXiv2026

Critical Dynamics of Non-Reciprocally Coupled Conserved Systems

Non-reciprocal systems have been shown to sustain time-dependent patterns, most prominently travelling waves. The transition into these time-dependent states generally breaks time-translational invariance, representing a clear deviation from equilibrium dynamics. Though common implementations of non-reciprocity lead to such phenomenology, these spatio-temporal patterns are absent in other models. In the same vein, the ensuing scaling behaviour also depends on the precise way non-reciprocity is implemented. To better understand the effects of different non-reciprocal interactions, we study the critical conserved dynamics of non-reciprocally coupled spin systems. Specifically, we consider the dynamics of two $n$-component order parameter fields $\boldsymbol{\phi}_i$ with $i \in\{1,2\}$. Unlike the common implementations of non-reciprocal interactions, we introduce the non-reciprocity solely through the non-linear interaction between the distinct species. Using the field-theoretic renormalisation group (RG) procedure, we perform a one-loop analysis and show that at one-loop level, the critical behaviour depends on the microscopic value of certain quantities. Using the flow functions, we elucidate the behaviour of the fixed points for different bare microscopic values. We also show that for $n \geq 4$, there is a fixed point where the ensuing critical dynamics asymptotically obey detailed-balance, implying the emergent dynamics are agnostic to the microscopic non-reciprocity on large scales. Finally, we show that the conserved dynamics reduces the number of independent scaling exponents, mimicking the effect of a standard fluctuation-dissipation relation.

cond-mat.stat-mech↗

arXiv2026

Run, Tumble and Paint

The visit probability, quantifying whether a particle has reached a given point for the first time by a specified time, provides access to various extreme value statistics and serves as a fundamental tool for characterising active matter models. However, previous studies have largely neglected how the visit probability depends on the internal degree of freedom driving the active particle. To address this, we calculate the "state-dependent'' visit probability for a Run-and-Tumble particle, that is the probability that the particle first passes through $x$ before time $t$, keeping track of its internal state during first passage. This process may be thought of as the particle "painting'' the positions it passes through for the time in the colour of its self-propulsion state. We perform this calculation in one dimension using Doi-Peliti field theory, by extending the tracer mechanism from previous works to incorporate such "polar deposition'' and demonstrate that state-dependent visit probabilities can be elegantly captured within this field-theoretic framework. We further derive the total volume covered by a right- (or left-) moving Run-and-Tumble particle and compare our results with known expressions for Brownian motion.

cond-mat.stat-mech↗

arXiv2025

Controlling the Glass Transition through Active Fluctuating Interactions

Fluctuating pairwise interactions are understood to drive fluid-like states in dense biological systems. These states find a broad range of functionalities, such as directing growth during morphogenesis and forming aggregates with heightened mechanical response. However, a tractable model capturing the role of microscopic fluctuating interactions in these structural transitions is crucially lacking. Here, we study a $p$-spin model with fluctuating pairwise couplings (of strength $D_a$ and persistence time $t_a$) as a schematic model for interaction-mediated fluidization. We find that while stronger fluctuations suppress the glass transition, more persistent fluctuations have the opposite effect. We identify the presence of an emergent fluctuation-dissipation relation at long times. We numerically extract the critical temperature $T_c(D_a, t_a)$ from a scaling relation near the transition, illustrating how microscopic fluctuations control the glass transition.

cond-mat.stat-mech↗

arXiv2025

Conditional splitting probabilities for hidden-state inference in drift-diffusive processes

Splitting probabilities quantify the likelihood of particular outcomes out of a set of mutually-exclusive possibilities for stochastic processes and play a central role in first-passage problems. For two-dimensional Markov processes $\{X(t),Y(t)\}_{t\in T}$, a joint analogue of the splitting probabilities can be defined, which captures the likelihood that the variable $X(t)$, having been initialised at $x_0 \in \mathbb{L}$, exits $\mathbb{L}$ for the first time via either of the interval boundaries \emph{and} that the variable $Y(t)$, initialised at $y_0$, is given by $y_{\rm exit}$ at the time of exit. We compute such joint splitting probabilities for two classes of processes: processes where $X(t)$ is Brownian motion and $Y(t)$ is a decoupled internal state, and unidirectionally coupled processes where $X(t)$ is drift-diffusive and depends on $Y(t)$, while $Y(t)$ evolves independently. For the first class we obtain generic expressions in terms of the eigensystem of the Fokker-Planck operator for the $Y$ dynamics, while for the second we carry out explicit derivations for three paradigmatic cases (run-and-tumble motion, diffusion in an intermittent piecewise-linear potential and diffusion with stochastic resetting). Drawing on Bayes' theorem, we subsequently introduce the related notion of conditional splitting probabilities, defined as the posterior likelihoods of the internal state $Y$ \emph{given} that the observable degree of freedom $X$ has undergone a specific exit event. After computing these conditional splitting probabilities, we propose a simple scheme that leverages them to partially infer the assumedly hidden state $Y(t)$ from point-wise detection events.

math-ph↗

arXiv2024

Ratchet-mediated resetting: Current, efficiency, and exact solution

We model an overdamped Brownian particle that is subject to resetting facilitated by a ratchet potential on a spatially periodic domain. This asymmetric potential switches on with a constant rate, but switches off again only upon the particle's first passage to a resetting point at the minimum of the potential. Repeating this cycle sustains a non-equilibrium steady-state, as well as a directed steady-state current which can be harnessed to perform useful work. We derive exact analytic expressions for the probability densities of the free-diffusion and resetting phases, the associated currents for each phase, and an efficiency parameter that quantifies the return in current for given power input. These expressions allow us to fully characterise the system and obtain experimentally relevant results such as the optimal current and efficiency. Our results are corroborated by simulations, and have implications for experimentally viable finite-time resetting protocols.

cond-mat.stat-mech↗