Contributed Talks

Model of antibiotic action on bacterial population growth
M. Evans

Monday, February 15 9:40-10:00

In this talk I will describe a simple model for the growth of a bacterial population under the challenge of ribosome-targetting antibiotics. The model is statistical physics-like in that it makes a coarse-grained description of the growth process, reduced to three variables within the bacterial cell - the antibiotic concentration, the concentration of ribosomes bound to antibiotics and the concentration of unbound ribosomes. Furthermore, there is biological input from empirically established physiological constraints which relate the three variables. Remarkably the model can explain several observations concerning antibiotic action and bacterial growth rate. In particular the growth-dependent bacterial susceptibility is controlled by a single, `universal' parameter and the extreme behaviours correspond to the phenomenological classification into bactericidal and bacteriostatic antibiotics. If time allows I will describe how the predictions of the model are backed up by experimental studies.

Anomalous transport of circular swimmers in disordered structures: classical edge-state percolation
T. Franosch

Monday, February 15 10:00-10:20

Recently micron-sized self-propelled particles have been realized in the lab as model systems [1] for complex living organisms such as swimming bacteria. If the agent is asymmetric a natural circular motion [2] emerges which yields characteristic skipping orbits when interacting with confining boundaries. 

Here, we investigate by event-driven molecular dynamics simulations the dynamics of circular swimmers in a two-dimensional model with
randomly distributed scatterers [3]. For small radii of the swimming motion 
the agents orbit only around isolated clusters of scatterers, while at large radii diffusive behavior emerges. 
For a certain critical radius a de-localization transition occurs which is generated by percolating skipping orbits along the edges of obstacle clusters. Directly at the transition the mean-square displacements displays subdiffusive transport. 
The dynamic exponents associated with the transition differ significantly from those of the conventional transport problem on percolating systems, thus establishing a new dynamic universality class. This difference is tentatively attributed to a weak-link scenario, which emerges naturally due to barely overlapping edge trajectories.
We draw an analogy to the field-induced localization transition in magnetotransport in two-dimensional electron gases in a disordered array of antidots.

If time permits we also elucidate the conventional localization transition occcuring at high scatterer density and revisit universality as the microdynamics is changed from ballistic to Brownian dynamics [4]. There the splitting of the universality class is rationalized within renormalization group arguments for the probing of the narrow channels.

[1] F. Kümmel, B. ten Hagen, R. Wittkowski, I. Buttinoni, R. Eichhorn, G. Volpe, H. Löwen, and C. Bechinger, Phys. Rev. Lett. 110, 198302 (2013)

[2] S. van Teeffelen and H. Löwen, Phys. Rev. E 78, 020101(R) (2008).

[3] W. Schirmacher, B. Fuchs, F. Höfling, and T. Franosch, submitted

[4] M. Spanner, F. Höfling, S.C. Kapfer, K.R. Mecke, G.E. Schröder-Turk, and T. Franosch, accepted for publication on PRL

Orientational Brownian motion in a viscoelastic ferrofluid. Magnetic and birefringence responses to ac field
Yu. Raikher

Monday, February 15 10:20-10:40

A kinetic theory of rotary Brownian motion of dipolar particles suspended in a viscoelastic fluid is developed. The dipolar (magnetic) moment is assumed to be “frozen” in the particle body, and, for simplicity, the particle motion is taken to be one-dimensional (plane rotator model). The viscoelastic fluid is approximated by the Jeffreys model, which includes two friction mechanisms acting in parallel. One of them is Newtonian friction with the viscosity coefficient ηN, while another is Maxwellian friction with the viscosity ηM and retardation time τM = ηM /GM with GM being the elasticity modulus. The specifics of Maxwellian mechanism – it is modelled by a spring-damper sequence – is that at short times it mostly produces an elastic restoring torque (t < τM), while at long times (t > τM) its main effect is viscous friction. For an ensemble of such particles affected by thermal fluctuations (torques), the set of Langevin equations is obtained and then transformed to a rotary diffusion (Fokker-Planck) equation. Provided the interparticle interactions are negligible, this equation provides a well-defined description for the orientation kinetics of a viscoelastic ferrofluid. The kinetic equation is considered for the case of ac probing field and, to construct the solution, the orientational distribution function is expanded in powers of the field amplitude. In each order of such a perturbation theory, the statistical moments are expanded over a special set of orthogonal functions [1]. This approach enables one to consistently find, first, the linear response of the ensemble that renders the dynamic magnetic susceptibility, and then the quadratic response that renders the dynamic birefringence. Those functions are obtained in the form of chain fractions and, thus, are well fit for evaluation. The dynamic susceptibility, as expected, coincides with the one following from the linear response theory [2]. A compact analytical form for the dynamic birefringence (one may call it optical anisotropy susceptibility) of a viscoelastic ferrofluid is derived for the first time. Support of Project 15-12-10003 from Russian Science Foundation is gratefully acknowledged.

Master Equation for Random Network Dynamics
T. Biro

Monday, February 15 11:50-12:10

We suggest a classification of random networks based on a simple, diffusion-like master equation. We view a network by its entries in the connectivity matrix, considering oriented and multiple connections in the general case. Generalizing further for real values in this matrix, flows on the network can be modeled. Based on a dynamics, changing the connection strength entries by one unit in a time-step, we classify the node-connectedness distributions via the stationary solutions of this master equation. From the simplest preference assumptions the Poisson, the binomial and the negative binomial (including the exponential) distributions emerge. Viewing subnetworks of huge network-environments thermodynamical principles can be easily recognized

Percolation transitions in the survival of interdependent agents on multiplex networks, catastrophic cascades, and SOS
P. Grassberger

Monday, February 15 12:10-12:30

The "SOS" in the title does not refer to the international distress signal, but to "solid-on-solid" (SOS)
surface growth. The catastrophic cascades are those observed by Buldyrev et al. in interdependent
networks, which we re-interpret as multiplex networks with agents that can only survive if
they mutually support each other, and whose survival struggle
we map onto an SOS type growth model. This mapping not only reveals non-trivial structures in the phase space
of the model, but also leads to a new and extremely efficient simulation algorithm. We use
this algorithm to study interdependent agents on duplex Erdös-Rényi (ER) networks and on lattices with
dimensions 2, 3, 4, and 5. We obtain new and surprising results in all these cases, and we correct statements
in the literature for ER networks and for 2-d lattices. In particular, we find that $d=4$ is the upper critical
dimension, that the percolation transition is continuous for $d\leq 4$ but -- at least for $d\neq 3$ -- not
in the universality class of ordinary percolation. For ER networks  we verify that the cluster statistics is
exactly described by mean field
theory, but find evidence that the cascade process is not. For $d=5$ we find a first order transition as for 
ER networks, but we find also that small clusters have a nontrivial mass distribution that scales at the
transition point. Finally, for $d=2$ with intermediate range dependency links we propose a scenario different
from that proposed in W. Li et al., PRL 108, 228702 (2012).

Can we predict the difficulty of optimization problems without solving them?
H. Katzgraber

Monday, February 15 12:30-12:50

Surprisingly often. Based on previous results of a large-scale numerical study of the equilibrium three-dimensional Edwards-Anderson Ising spin glass where it was demonstrated that autocorrelation times are directly correlated with the roughness of the free-energy landscape [Phys. Rev. E 87, 012104 (2013)], we show that a generalized spin-glass order parameter can be used as a proxy to the computational difficulty of various paradigmatic optimization problems. Our results are illustrated with different optimization algorithms and on a quantum annealer, as well as optimization problems. Furthermore, we show numerical evidence that the order-parameter distribution does mirror salient features in the free-energy landscape of complex systems for moderate system sizes. Finally, we corroborate and improve on these results using the family entropy of population annealing Monte Carlo [arXiv:1508.05647]. 
Work done in collaboration with Chao Fang, Richard Lawrence, Oliver Melchert, Humberto Munoz-Bauza, Andrew J. Ochoa, Wenlong Wang, and Zheng Zhu.

Localisation dynamics of ballistic tracers in the two-dimensional Lorentz model interpreted as a renormalisation group flow
F. Höfling

Monday, February 15 16:00-16:20

The Lorentz model serves as a minimal model to explain many facets of the rich phenomenology of anomalous transport, as frequently observed in porous materials and cellular transport [1]. Here, I will discuss the localisation transition of “ballistic” tracers (subject to Newton's equations of motion) in the two-dimensional, overlapping Lorentz model. Extensive simulations provide evidence for the universality of the dynamic critical exponent, which has been crucial in the interpretation of recent studies [2,3]. The long-time asymptotes, however, are obscured by non-universal corrections to scaling, explaining the contradicting values for the diffusivity exponent in the literature. A spectral analysis of the obtained correlation functions allows for an interpretation of the dynamics as an renormalisation flow of the transport at long times and gives insight into the fixed point structure of the RG flow.

[1] F. Höfling and T. Franosch, Rep. Prog. Phys. 76, 046602 (2013).

[2] S. K. Schnyder, M. Spanner, F. Höfling, T. Franosch, and J. Horbach, Soft Matter 11, 701 (2015).

[3] W. Schirmacher, B. Fuchs, F. Höfling, and T. Franosch, arXiv:1511.05218, Phys. Rev. Lett. in print (2015).

Interfaces near criticality
G. Delfino

Monday, February 15 17:30-17:50

We present the exact theory of interfaces in near-critical planar systems at phase coexistence. The results include order parameter profiles, interface structure, passage probabilities and wetting properties, for the different universality classes and different geometries (strip, half-plane, wedge). 


[1] G. Delfino, J. Viti, Phase separation and interface structure in two dimensions from field theory, J. Stat. Mech. (2012) P10009. 

[2] G. Delfino, A. Squarcini, Exact theory of intermediate phases in two dimensions, Annals of Physics 342 (2014) 171. 

[3] G. Delfino, A. Squarcini, Phase separation in a wedge. Exact results, Phys. Rev. Lett. 113 (2014) 066101. 

[4] G. Delfino, A. Squarcini, Multiple phases and vicious walkers in a wedge, Nucl. Phys. B 901 (2015) 430.

Finite-size scaling of free-energy barrier in droplet formation and nucleation-like processes
J. Zierenberg

Monday, February 15 17:50-18:10

We study the nucleation free-energy barrier of the droplet formation process upon a temperature change. Employing generalized-ensemble methods allows us to directly access estimates of the free-energy barrier from energy probability distributions. Phenomenological arguments reveal that in this scenario the free-energy barrier scales with $N^{1/2}$, confirmed by an extensive finite-size scaling analysis. The same scaling is supposed to remain true for other nucleation-like processes such as polymer aggregation. 

Driving rate dependence of avalanche statistics and shapes at the yielding transition
E. Ferrero

Monday, February 15 18:10-18:30

n this talk I will first briefly introduce a coarse-grained approach for the study of amorphous solids under deformation, the so-called "elasto-plastic" models. Then, I will present some results from our study on avalanches statistics at the yielding transition. 

We study the stress time series caused by plastic avalanches in athermally sheared disordered materials. Using extensive simulations of a mesoscopic elasto-plastic model, we find that critical exponents differ from mean-field predictions, that we approach only further away from the yielding point, at larger driving rates. We analyze the avalanche duration and size distributions introducing a scaling to account for the rate dependency of the dynamics. A probability distribution for local yielding is also discussed in the marginal stability picture and its driving rate dependence displayed. The average temporal shape of the stress drops also depends strongly on the imposed shear rate and system size. When individual avalanches are considered, they show a clear asymmetry. 

Ref. arXiv:1506.08161

'Civilized' Model for Protein-Salt Mixtures in Water
V. Vlachy

Tuesday, February 16 9:40-10:00

The effect of the nature of salt on properties of a model protein solution in water is studied theoretically and compared with experimental data. In our approach all the interacting species, proteins, ions of low-molecular-mass salt, and water molecules are accounted for explicitly (1). This is in contrast with the majority of other theoretical studies (see, for example,  (2) and the Refs. therein), which treat the composed solvent as a structure-less continuum. The model proteins have simultaneously present positive and/or negative charges on the surface, mimicking this way the realistic situation occurring in such solutions. To solve numerically this complex model we utilize the associative mean spherical approximation, developed earlier for simple symmetric electrolytes and solutions of macroions (1,3). From measurable properties we choose to calculate the second virial coefficient, the quantity, which reflects stability of protein solutions and is closely related with the tendency of proteins to aggregate and crystallize. We show that osmotic second virial coefficient does not depend only on the magnitude of the net charge of the protein but also on its sign, as also on the nature of the present low-molecular-mass electrolyte. We find the specific ion effects to be correlated with differences in hydration free energy between the ions in solution and charged groups on the protein. The calculations capture experimental trends for lysozyme solutions reasonably well.

1. Yu. V. Kalyuzhnyi and V. Vlachy, Model for a mixture of macroions, counterions, and co-ions in a water-like fluid, Phys. Rev. E., 90, 012308 (2014).

2. M. Kastelic, Yu.V. Kalyuzhnyi, B. Hribar Lee, K. A. Dill, V. Vlachy, Protein aggregation in salt solutions, Proc. Natl. Acad. Sci. US.

3. Yu. V. Kalyuzhnyi, V. Vlachy, and K. A. Dill, Aqueous alkali halide solutions: can osmotic coefficients be explained on the basis of the ionic sizes alone? Phys. Chem. Chem. Phys., 12, 6260-6266 (2010).

Mixing random organization and jamming
M. Schmiedeberg

Tuesday, February 16 9:00-09:20

By choosing appropriate protocols, both the random organization and the athermal jamming transition can be studied within a unifying model system [1]. We explore the model system for a mixture of the protocols and argue that the result is comparable to a glassy system of soft spheres at small but non-zero temperatures.
In our model system, we start with a random configuration of spheres. In order to obtain random organization, we displace overlapping particles randomly in each step. On the other side, athermal jamming is realized by deterministically shifting overlapping particles and heading for the local minimum of total overlaps without crossing energy barriers. 
Finally, we mix these protocols. In case of mainly deterministic but also a few random displacements we obtain a system that corresponds to a system of soft spheres at small but non-zero temperatures. Therefore, we employ the model system to obtain insights into the differences between the glass transition at small but non-zero temperatures and the purely athermal jamming transition. For example, we determine the critical exponents of the transitions. Interestingly, the exponents of the random organization transition as well as the transitions in case of the mixed protocol correspond to the exponents from the universality class of directed percolation or conserved directed percolation, while athermal jamming seems to be a completely different transition. 

Finally, we are interested in the relation to other transitions that occur in similar packing models, e.g., the contact percolation transition.

[1] L. Milz and M. Schmiedeberg, Connecting the random organization transition and jamming within a unifying model system, Phys. Rev. E. 88, 062308 (2013).

Mercury Droplet Spreading on Thin Metal Film on Glass: Statistical Physics of Advancing Interfaces
H. Taitelbaum

Tuesday, February 16 9:20-09:40

When a droplet of mercury is transferred to a thin metal (silver / gold) film deposited on a glass substrate, it starts to dissolve and spread in a very non-trivial manner. This is the only known reactive-wetting system in room temperature. It exhibits two main regimes, the bulk propagation regime and the interface kinetic roughening regime. The bulk propagating dynamics is very different from classical wetting characteristics. In the kinetic roughening regime, rich spatio-temporal patterns are observed. The latter are studied and characterized using statistical physics tools, such as the growth, roughness and persistence exponents. We also discuss the decohesion and structural instability of the thin film due to the mercury penetration towards the metal-glass interface through the granular film structure. Various aspects of this complex sequence of phenomena will be addressed.

Towards flexible and dynamic self-assembly from colloids with anisotropic interactions
G. Steinbach

Tuesday, February 16 10:00-10:20

Colloidal particles are a suitable model system for the study of self-assembly and dynamic processes on the microscale. Here, we demonstrate the potential of anisotropic interactions between the particles for the design of complex networks with structural variability. The special feature of the presented approach lies in the presence of different types of connections in a homogeneous particle system. It enables the spontaneous formation of flexible architectures, which resemble the modular design of many biological systems.

As an example, we present a system of colloidal microspheres that have an off-centered net magnetic moment pointing perpendicular to the particle surface. They are an experimental realization of the theoretical model of spheres with radially shifted point dipole (sd-particles). Experimentally we observed the formation of branched structures as result of two coexisting self-assembly patterns, which is untypical for homogeneous systems. We show that the structural bistability can be explained by an extended model of sd-particles. This framework takes the broad and anisotropic magnetization distribution in the experimental particles into account. 

We will further show that the interacting particles exhibit interesting non-equilibrium dynamics when exposed to time-dependent fields. Reversible structural reconfigurations emerge from the anisotropic interactions between the shifted net magnetic moments. The wealth of observable deformations and transformations particularly benefits from the complex self-assembly behavior.

Marangoni flow and anomalous diffusion at interfaces
M. Oettel

Tuesday, February 16 10:20-10:40

Collective diffusion for particles trapped at
fluid interfaces is characterized by an anomalous speedup
due to hydrodynamic interactions mediated through the
bulk phases forming the interface [1]. Profiles at later times
of an initial density fluctuation exhibit
an intermediate range power-law decay in space which leads to
diverging collective mean square displacements, experimentally
signalled in a strong enhancement of the collective diffusion
coefficient in the limit of zero wavenumber [2,3].
Confinement for particles and essentially unconfined hydrodynamic flow defines the notion of partial confinement.
In a wider interpretation, the diffusing particles at an interface
are sources of a Marangoni flow which in turn acts back on
its sources. Thus, the well-known spreading of soap films at interfaces
is actually anomalous diffusion in the above sense.
Furthermore, this anomaly always occurs when the lengthscale on
which lateral spreading is observed is larger than the confinement width 
at the interface of the spreading particles or molecules. Therefore, anomalous diffusion is a rather general phenomenon, based on the
presence of partial confinement. 

[1] J. Bleibel, A. Dominguez, F. Günther, J. Harting, and M. Oettel,
Hydrodynamic interactions induce anomalous diffusion under partial confinement, Soft Matter 10, 2945 (2014).

[2] B. Lin, B. Cui, X. Xu, R. Zangi, H. Diamant, and S. A. Rice,
Divergence of the long-wavelength collective diffusion coefficient in quasi-one- and quasi-two-dimensional colloidal suspensions,
 Phys. Rev. E 89, 022303 (2014).

[3] J. Bleibel, A. Dominguez, and M. Oettel,
3D hydrodynamic interactions lead to divergences in 2D diffusion,   
  J. Phys.: Condens. Matter 27, 194113 (2015).

Phase Transitions in Driven Diffusion and Brownian Motor Systems
P. Maass

Tuesday, February 16 11:50-12:10

In connection with directed transport on the molecular level, two research areas have attracted much attention in the past: Brownian motors and driven diffusion systems under a static bias. Brownian motors are operated by a periodic process in time, where, in contrast to classical engines, fluctuations caused by thermal noise and thermally assisted overcoming of energy barriers are important. Driven diffusion under a static bias has received particular interest in connection with transport through open tube-like compartments and for general studies of non-equilibrium steady states (NESS). I shall first address the problem of variables controlling NESS in the presence of particle interactions beyond hard-core repulsions and present a theoretical approach based on time-dependent density functional theory to predict boundary-induced bulk phases in generic situations [2,3]. Then I will show that boundary-induced phase transitions also appear in collective Brownian motors and argue that their occurrence is generic [3].

[1] M. Dierl, P. Maass, M. Einax: Classical Driven Transport in Open Systems with Particle Interactions and General Couplings to Reservoirs. Phys. Rev. Lett. 108, 060603 (2012).
[2] M. Dierl, M. Einax, P. Maass: One-dimensional Transport of Interacting Particles: Currents, Density Profiles, Phase Diagrams, and Symmetries. Phys. Rev. E 87, 062126 (2013).
[3] M. Dierl, W. Dieterich, M. Einax, P. Maass: Phase Transitions in Brownian Pumps. Phys. Rev. Lett. 112, 150601 (2014).

Fluctuation relations for a hot Brownian swimmer
K. Kroy

Tuesday, February 16 12:10-12:30

Laser-heating of Janus particles provides a versatile nano-scale swimming mechanism with a great potential for technological applications. To characterize and control the dynamics of such hot nano-swimmers, one needs to understand both their enhanced Brownian fluctuations and their active self-thermophoresis. We performed non-equilibrium molecular dynamics simulations of a heated Janus bead in a Lennard--Jones fluid. They reveal some non-trivial spatio-temporal symmetries in the current statistics of the swimmer. In the Markovian limit, these can be cast in the form of analytically computable stationary fluctuation relations, in spite of the non-deterministic character of the driving and the strongly non-isothermal conditions.

Markov State Modeling of Sliding Friction
F. Pellegrini

Tuesday, February 16 12:30-12:50

Markov State Modeling has recently emerged as one of the key techniques for analyzing rare events in molecular simulations. In particular, in biochemistry this approach is successfully exploited to find the metastable states of complex systems in thermal equilibrium, such as a protein undergoing a folding event. We show that this technique can be exported to the study of friction, where strongly non-equilibrium events are induced by an external force. The approach is benchmarked on a Frenkel-Kontorova model, whose properties are well established. We demonstrate that the approach allows identifying the minimal basis of natural microscopic states necessary for describing the dynamics of sliding, including frictional dissipation and stick-slip events. We anticipate that the same technique can be applied to the analysis of realistic frictional systems. 
This work is supported by ERC Advanced Research Grant No. 320796 MODPHYSFRICT.

Directed assembly of soft colloids through rapid solvent exchange
A. Nikoubashman

Tuesday, February 16 14:20-14:40

We studied the directed assembly of soft nanoparticles through rapid micromixing of polymers in solution with a non-solvent. Both experiments and computer simulations were performed to elucidate the underlying physics and to investigate the role of various process parameters. In particular, we discovered that no external stabilizing agents or charged end-groups are required to keep the colloids separated from each other, when water is used as the non-solvent. The size of the nanoparticles can be reliably tuned through the mixing rate and the ratio between polymer solution and non-solvent. Furthermore, we were able to fabricate a wide variety of patchy colloids, such as Janus particles, when polymer blends were used in the feed stream. Our results demonstrate that this mechanism is highly promising for the mass fabrication of uniformly-sized colloidal particles, using a wide variety of polymeric feed materials.

Thermodynamic and dynamic properties at the intrinsic liquid surface
P. Jedlovszky

Tuesday, February 16 14:40-15:00

Interfaces are ubiquitous objects, whose thermodynamic behavior we only recently started to understand at the microscopic detail. In computer simulations, when such interfaces are seen in atomistic resolution even the definition of the interface itself, in other words, the determination of the full and exact list of the molecules that are located right at the interface (i.e., at the boundary of the two coexisting phases) is not an obvious task at all. However, such a rigorous distinction between interfacial and non-interfacial molecules is a pre-requisite of any meaningful analysis of the interfacial properties. The development of various intrinsic surface analyzing methods in the last decade now enables us to discuss the thermodynamic properties associated with fluid (”soft”) interfaces, such as the density, stress, energy, and free energy distribution across them by analyzing the respective contributions coming from successive layers. Also, having the successive subsurface molecular layers defined the variation of the dynamical properties of the particles along these layers can be discussed. In this presentation the basic concept of the identification of the olecules (ITIM) is explained, it is demonstrated how the distribution of the aforementioned thermodynamic quantities across the interface can be discussed both in terms of continuous profiles and in a layer-wise manner, several applications of this method is presented, and finally the diffusion of the interfacial molecules at the liquid water surface is discussed in detail.

Absence of the ideal glass transition in a binary Lennard-Jones glass former
M. Ozawa

Wednesday, February 17 9:40-10:00

The dynamics of glass forming liquids shows a tremendous slowing down if temperature is decreased: Relaxation times, transport coefficients such as the diffusion constant or the shear viscosity grow by more than 15 orders of magnitude within narrow temperature range. One of the central goals in the field of glass physics is to understand the origin of this dramatic slowing down of the dynamics. There are several scenarios that seem to be able to explain the slowing down of the dynamics and the low temperature state of the glass former. A very popular one is to rationalize the slow dynamics by invoking the existence of a thermodynamic transition, the so-called ideal glass transition from the liquid to an ideal glass states. This ideal glass transition occurs when the configurational entropy, defined as the logarithm of the number of available states, becomes zero. Due to this singular behavior of the configurational entropy, the relaxation time and transport coefficients diverge if the transition point is approached [1]. However, confirming the existence of the ideal glass transition is a very difficult task since in practice the system falls out of equilibrium before reaching this putative ideal glass transition temperature, thus making the test of this theory most difficult. Furthermore, other scenarios are also able to explain the slow dynamics without resorting any thermodynamic transition [2]. In the present study, we investigate the existence of the ideal glass transition by using computer simulation of a canonical binary Lennard-Jones glass former in three dimensions. Massive computational effort and efficient sampling algorithm (parallel tempering) allow us to reach equilibrium states that are very deeply supercooled. By measuring the configurational entropy directly (obtained via thermodynamic integration), we find that the configuration entropy does not go to zero at a finite temperature. This implies that the thermodynamic singularity is indeed avoided before reaching a putative ideal glass transition temperature predicted by previous studies. Our results indicates that contrary to the previous results the system remains in a liquid state down to zero temperature without showing an ideal glass transition. Furthermore, we analyze the microscopic structure and the potential energy landscape of the system to clarify the mechanism that leads to the avoidance of the ideal glass transition. We find that locally favored structures present in the liquid state hardly change in the temperature range we consider, which rules out another possibility for the slow dynamics, e.g. the transformation between two distinct liquids, so-called liquid-liquid transition [3]. Instead, we find the potential energy landscape contains extensive number of minima even very low temperature, which rationalize the avoided singularity. Our results support a theoretical argument that the ideal glass transition can not exist at bulk system in finite dimensions [5].
[1] Adam G and Gibbs JH, J. Chem. Phys., 43, 139 (1965)
[2] Chandler D and Garrahan JP, Annu. Rev. Phys. Chem., 61, 191 (2010)
[3] Speck T, Royall CP, and Williams SR, arXiv:1409.0751, (2014)
[4] Saksaengwijit A and Heuer A, Phys. Rev. Lett., 93, 235701 (2004)
[5] Stillinger FH, J. Chem. Phys., 88, 7818 (1988)

Computer simulation of glasses under shear: From inhomogeneous flow patterns to mechanical properties
J. Horbach

Wednesday, February 17 10:00-10:20

The response of glasses to mechanical loading often leads to the formation of inhomogeneous flow patterns that may strongly affect the material properties. Among them, shear bands, associated with strain localization in form of band-like structures, are ubiquitous in a wide variety of materials, ranging from soft matter systems to metallic alloys. Molecular dynamics simulations of a model of a glass-forming binary Lennard-Jones mixture are performed to investigate the formation of shear bands, using different flow protocols. Under an externally applied constant stress, persistent creep in the form of shear-banded structures is observed around the yield stress, whereas under the application of a constant strain rate, shear bands occur at sufficiently low strain rates. For both cases, we give evidence that flow is initiated by a directed percolation transition. We analyze the nucleation of the shear-banded structures as well as the mechanical properties of the deformed glasses..

Fabricating large two-dimensional single colloidal crystals by doping with active particles
L. Filion

Wednesday, February 17 10:20-10:40

Self-propelled particles, also known as active particles, incessantly convert energy into self-propulsion, and as such are intrinsically out-of-equilibrium. While traditionally such particles occured solely within the purview of natural systems (e.g. bacteria), recent experimental breakthroughs have led to many novel types of synthetic colloidal swimmers. These systems exhibit a wealth of new phase behaviour, including motility-induced phase separation into dense and dilute phases, giant density fluctuations, and swarming. Moreover, experimental and simulation studies have shown that the dynamics of a passive system can be altered dramatically by incorporating as little as 1% of active particles into the system.  At these concentrations, the self-propelled particles can be viewed as active ``dopants", which like passive dopants can strongly alter the properties (e.g. dynamics) of the underlying passive system. 

In this talk I use simulations to explore the behaviour of two-dimensional colloidal (poly)crystals doped with active particles. We show that these active dopants can provide an elegant new route to removing grain boundaries in polycrystals. Specifically, we show that active dopants both generate and are attracted to defects, such as vacancies and interstitials, which leads to clustering of dopants at grain boundaries. The active particles both broaden and enhance the mobility of the grain boundaries, causing rapid coarsening of the crystal domains. The remaining defects recrystallize upon turning off the activity of the dopants, resulting in a large-scale single-domain crystal.

A projection operator approach for the quantum canonical ensemble
W. Magnus

Wednesday, February 17 11:50-12:10

Constraining the particle number N in the canonical ensemble hampers the systematic calculation of the partition function Z_N for non-interacting fermions and bosons, unlike in the case of the grand-canonical ensemble, the reason being that the multiple summation over the single-particle states that involves products of Gibbs factors cannot be rewritten as a multiple product of sums over single-particle modes.
Recently, however, we have bypassed this difficulty and shown that the summation can be performed anyway by invoking a projection operator approach that automatically imposes the particle number constraint in the many-particle Hilbert space. Until now, only a recursion relation expressing Z_N in all
partition functions of lower particle numbers is known to be available. The projection operator approach, however, provides a direct integral representation for Z_N as well as for the two-point and four-point correlation functions, thus reducing the old summation problem to a numerical integration task. As an
illustration, the Helmholtz free energy and the chemical potential are computed for a two-dimensional electron gas typically residing in the inversion layer of a field-effect transistor and the results are compared with those conventionally obtained from the grand-canonical ensemble. One of the conclusions is that the chemical potential not only emerges as a by-product of the calculation of the canonical partition function but may also take incorrect, unphysical values for low particle numbers when calculated within the grand-canonical picture. This new insight may be considered useful as such, but has also relevant consequences for the evaluation of the so-called sub-threshold slope of nano-scale transistors with few electrons, which is a hot topic today and which heavily relies on reliable values of the chemical potential and the gate metal work functions.

Exact non-adiabatic qubit manipulation on Rashba rings
A. Ramsak

Wednesday, February 17 12:10-12:30

First we will present exact solutions for an electron in a quantum wire with time dependent spin-orbit interaction and driven by external time-dependent potential [1,2]. By the virtue of the exact solution one can construct analytically the corresponding geometric Anandan phase or in the adiabatic limit the Wilczek-Zee phase, which enables holonomic qubit transformations. By breaking the time reversal symmetry the results lead to the Aharonov-Anandan phase and in the adiabatic limit reproduce the usual Berry phase. Next the result will be generalized and an exact solution will be presented for the time-dependent wavefunction of a Kramers doublet which propagates around a quantum ring with tuneable Rashba spin-orbit interaction [3]. By propagating in segments it will be shown that Kramers-doublet qubits may be defined for which transformations on the Bloch sphere may be performed for an integral number of revolutions around the ring. The conditions for full coverage of the Bloch sphere will be determined and explained in terms of sequential qubit rotations due to electron motion along the segments, with change of rotation axes between segments due to adiabatic changes in the Rashba spin-orbit interaction. Prospects and challenges for possible realizations will be discussed for which rings based on InAs quantum wires are promising candidates [4].

[1] T. Cadez, J. H. Jefferson, and A. Ramsak, New J. Phys. 15, 013029 (2013).
[2] T. Cadež, J. H. Jefferson, and A. Ramsak, Phys. Rev. Lett. 112, 150402 (2014).
[3] A. Kregar, J. H. Jefferson, and A. Ramsak, arXiv:1511.06608.
[4] L. Ulcakar, A. Kregar, and A. Ramsak, to be submitted.

Random transverse-field Ising chain with long-range interactions
R. Juhasz

Wednesday, February 17 12:30-12:50

We study the low-energy properties of the long-range random transverse-field Ising chain with ferromagnetic interactions decaying as a power α of the distance. Using variants of the strong-disorder renormalization group method, the critical behavior is found to be controlled by a strong-disorder fixed point with a finite dynamical exponent zc=α. Approaching the critical point, the correlation length diverges exponentially. In the critical point, the magnetization shows an α-independent logarithmic finite-size scaling and the entanglement entropy follows the area law. These observations are argued to hold even in higher dimensions. The same fixed point is expected to describe the critical behavior of an epidemic model, the random contact process with long-range spreading.


1. R. Juhász, I. A. Kovács, F. Iglói, Europhys. Lett., 107, 47008 (2014). 

2. R. Juhász, I. A. Kovács, F. Iglói, Phys. Rev. E 91, 032815 (2015).

The associative nature of conference participation
J. Smiljanic

Wednesday, February 17 16:00-16:20

Patterns of scientific publications, collaborations and citations in scientific journals have been extensively studied in last decade while conference attendance patterns remain mostly unexplored from the perspective of statistical physics. In this work we study the conference participation at six national and international conferences of different sizes and from different fields of science. The gathered data contain detailed information about abstracts presented at the conferences for the period of 30 years, which enables us to analyse the total and successive number of participations, as well as the distribution of time lags between two successive participations for each author. All these properties exhibit truncated power-law behavior, regardless of the conference size and degree of specialization. This indicates that the probability for a scientist to re-attend the specific conference is not constant, but rather it depends on the balance between the number of previous participations and non-participations. In order to further investigate the mechanism behind conference attendance patterns, we propose a microscopic stochastic model based on two key ingredients, 2-bin generalized Polya process and random termination time of a career. Using Monte Carlo simulations, we estimate the model parameters and demonstrate that this model, with positive feedback, can successively reproduce the empirically observed results. It is shown that an active participation in a conference series at the beggining strengthens the scientists association with that particular conference community and thus increases the probability of future participations. We expect to see similar participation patterns and dynamics in other types of social groups. In general, understanding mechanisms that underlie the repeated participation in the same conference series could have implications in social networks creation and optimization.

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