Special issue on complex fluids at structured surfaces.

Journal of physics. Condensed matter : an Institute of Physics journal 29 (2017) 180301-

P Teixeira, JM Yeomans

Spinon decay in the spin-1/2 Heisenberg chain with weak next nearest neighbour exchange


S Groha, FHL Essler

Long coherence times for edge spins

Journal of Statistical Mechanics: Theory and Experiment 2017 (2017) 063105-063105

J Kemp, NY Yao, CR Laumann, P Fendley

Variation of the Contact Time of Droplets Bouncing on Cylindrical Ridges with Ridge Size.

Langmuir : the ACS journal of surfaces and colloids 33 (2017) 7583-7587

M Andrew, Y Liu, JM Yeomans

Reducing the contact time between bouncing droplets and an underlying solid surface is relevant to a broad range of industrial applications, such as anti-icing and self-cleaning. Previous work has found that placing cylindrical obstacles on the substrate leads to a reduction in contact time. For obstacles large compared to the drop, this is a result of hydrodynamic coupling between the azimuthal and axial spreading directions. For obstacles small compared to the drop, the reduction in contact time is interpreted as being due to fast retraction along the cylindrical ridge, followed by drop breakup. Here we use simulations to discuss in greater detail the effect of varying the obstacle size on the dynamics of the drop bouncing. We investigate the crossover between the two regimes and explain why the contact time is minimized when the radii of the drop and the cylindrical obstacle are comparable.

The macroscopic pancake bounce


JA Bro, KSB Jensen, AN Larsen, JM Yeomans, T Hecksher

The hydrodynamics of active systems(*)


JM Yeomans

On truncated generalized Gibbs ensembles in the Ising field theory


FHL Essler, G Mussardo, M Panfil

Onset of meso-scale turbulence in active nematics.

Nature communications 8 (2017) 15326-

A Doostmohammadi, TN Shendruk, K Thijssen, JM Yeomans

Meso-scale turbulence is an innate phenomenon, distinct from inertial turbulence, that spontaneously occurs at low Reynolds number in fluidized biological systems. This spatiotemporal disordered flow radically changes nutrient and molecular transport in living fluids and can strongly affect the collective behaviour in prominent biological processes, including biofilm formation, morphogenesis and cancer invasion. Despite its crucial role in such physiological processes, understanding meso-scale turbulence and any relation to classical inertial turbulence remains obscure. Here we show how the motion of active matter along a micro-channel transitions to meso-scale turbulence through the evolution of locally disordered patches (active puffs) from an ordered vortex-lattice flow state. We demonstrate that the stationary critical exponents of this transition to meso-scale turbulence in a channel coincide with the directed percolation universality class. This finding bridges our understanding of the onset of low-Reynolds-number meso-scale turbulence and traditional scale-invariant turbulence in confinement.

Multi-scale statistics of turbulence motorized by active matter


J Urzay, A Doostmohammadi, JM Yeomans

Efficient Representation of Fully Many-Body Localized Systems Using Tensor Networks

PHYSICAL REVIEW X 7 (2017) ARTN 021018

TB Wahl, A Pal, SH Simon

Strong peak in Tc of Sr2RuO4 under uniaxial pressure.

Science (New York, N.Y.) 355 (2017)

A Steppke, L Zhao, ME Barber, T Scaffidi, F Jerzembeck, H Rosner, AS Gibbs, Y Maeno, SH Simon, AP Mackenzie, CW Hicks

Sr2RuO4 is an unconventional superconductor that has attracted widespread study because of its high purity and the possibility that its superconducting order parameter has odd parity. We study the dependence of its superconductivity on anisotropic strain. Applying uniaxial pressures of up to ~1 gigapascals along a 〈100〉 direction (a axis) of the crystal lattice results in the transition temperature (Tc) increasing from 1.5 kelvin in the unstrained material to 3.4 kelvin at compression by ≈0.6%, and then falling steeply. Calculations give evidence that the observed maximum Tc occurs at or near a Lifshitz transition when the Fermi level passes through a Van Hove singularity, and open the possibility that the highly strained, Tc = 3.4 K Sr2RuO4 has an even-parity, rather than an odd-parity, order parameter.

Quantum Hall edges with hard confinement: Exact solution beyond Luttinger liquid

PHYSICAL REVIEW B 95 (2017) ARTN 201108

R Fern, SH Simon

A solvable model of axisymmetric and non-axisymmetric droplet bouncing.

Soft matter 13 (2017) 985-994

M Andrew, JM Yeomans, DO Pushkin

We introduce a solvable Lagrangian model for droplet bouncing. The model predicts that, for an axisymmetric drop, the contact time decreases to a constant value with increasing Weber number, in qualitative agreement with experiments, because the system is well approximated as a simple harmonic oscillator. We introduce asymmetries in the velocity, initial droplet shape, and contact line drag acting on the droplet and show that asymmetry can often lead to a reduced contact time and lift-off in an elongated shape. The model allows us to explain the mechanisms behind non-axisymmetric bouncing in terms of surface tension forces. Once the drop has an elliptical footprint the surface tension force acting on the longer sides is greater. Therefore the shorter axis retracts faster and, due to the incompressibility constraints, pumps fluid along the more extended droplet axis. This leads to a positive feedback, allowing the drop to jump in an elongated configuration, and more quickly.

Electric-field-induced shape transition of nematic tactoids

PHYSICAL REVIEW E 96 (2017) ARTN 022706

L Metselaar, I Dozov, K Antonova, E Belamie, P Davidson, JM Yeomans, A Doostmohammadi

Valley-selective Landau-Zener oscillations in semi-Dirac p-n junctions

Physical Review B 96 (2017)

K Saha, R Nandkishore, SA Parameswaran

© 2017 American Physical Society. We study transport across p-n junctions of gapped two-dimensional semi-Dirac materials: nodal semimetals whose energy bands disperse quadratically and linearly along distinct crystal axes. The resulting electronic properties - relevant to materials such as TiO2/VO2 multilayers and α-(BEDT-TTF)2I3 salts - continuously interpolate between those of mono- and bilayer graphene as a function of propagation angle. We demonstrate that tunneling across the junction depends on the orientation of the tunnel barrier relative to the crystalline axes, leading to strongly nonmonotonic current-voltage characteristics, including negative differential conductance in some regimes. In multivalley systems, these features provide a natural route to engineering valley-selective transport.

Disorder-driven destruction of a non-Fermi liquid semimetal studied by renormalization group analysis

Physical Review B 95 (2017)

RM Nandkishore, SA Parameswaran

© 2017 American Physical Society. We investigate the interplay of Coulomb interactions and short-range-correlated disorder in three-dimensional systems where absent disorder the noninteracting band structure hosts a quadratic band crossing. Though the clean Coulomb problem is believed to host a non-Fermi liquid phase, disorder and Coulomb interactions have the same scaling dimension in a renormalization group (RG) sense, and thus should be treated on an equal footing. We therefore implement a controlled expansion and apply it at leading order to derive RG flow equations valid when disorder and interactions are both weak. We find that the non-Fermi liquid fixed point is unstable to disorder, and demonstrate that the problem inevitably flows to strong coupling, outside the regime of applicability of the perturbative RG. An examination of the flow to strong coupling suggests that disorder is asymptotically more important than interactions, so that the low-energy behavior of the system can be described by a noninteracting sigma model in the appropriate symmetry class (which depends on whether exact particle-hole symmetry is imposed on the problem). We close with a discussion of general principles unveiled by our analysis that dictate the interplay of disorder and Coulomb interactions in gapless semiconductors, and of connections to many-body localized systems with long-range interactions.

Spin-catalyzed hopping conductivity in disordered strongly interacting quantum wires

Physical Review B 95 (2017)

SA Parameswaran, S Gopalakrishnan

© 2017 American Physical Society. In one-dimensional electronic systems with strong repulsive interactions, charge excitations propagate much faster than spin excitations. Such systems therefore have an intermediate temperature range [termed the "spin-incoherent Luttinger liquid" (SILL) regime] where charge excitations are "cold" (i.e., have low entropy) whereas spin excitations are "hot." We explore the effects of charge-sector disorder in the SILL regime in the absence of external sources of equilibration. We argue that the disorder localizes all charge-sector excitations; however, spin excitations are protected against full localization, and act as a heat bath facilitating charge and energy transport on asymptotically long time scales. The charge, spin, and energy conductivities are widely separated from one another. The dominant carriers of energy in much of the SILL regime are neither charge nor spin excitations, but neutral "phonon" modes, which undergo an unconventional form of hopping transport that we discuss. We comment on the applicability of these ideas to experiments and numerical simulations.

Eigenstate phase transitions and the emergence of universal dynamics in highly excited states

Annalen der Physik 529 (2017)

SA Parameswaran, AC Potter, R Vasseur

© 2017 by WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim We review recent advances in understanding the universal scaling properties of non-equilibrium phase transitions in non-ergodic disordered systems. We discuss dynamical critical points (also known as eigenstate phase transitions) between different many-body localized (MBL) phases, and between MBL and thermal phases. (Figure presented.).

Dancing disclinations in confined active nematics.

Soft matter 13 (2017) 3853-3862

TN Shendruk, A Doostmohammadi, K Thijssen, JM Yeomans

The spontaneous emergence of collective flows is a generic property of active fluids and often leads to chaotic flow patterns characterised by swirls, jets, and topological disclinations in their orientation field. However, the ability to achieve structured flows and ordered disclinations is of particular importance in the design and control of active systems. By confining an active nematic fluid within a channel, we find a regular motion of disclinations, in conjunction with a well defined and dynamic vortex lattice. As pairs of moving disclinations travel through the channel, they continually exchange partners producing a dynamic ordered state, reminiscent of Ceilidh dancing. We anticipate that this biomimetic ability to self-assemble organised topological disclinations and dynamically structured flow fields in engineered geometries will pave the road towards establishing new active topological microfluidic devices.

Topological defects in epithelia govern cell death and extrusion.

Nature 544 (2017) 212-216

TB Saw, A Doostmohammadi, V Nier, L Kocgozlu, S Thampi, Y Toyama, P Marcq, CT Lim, JM Yeomans, B Ladoux

Epithelial tissues (epithelia) remove excess cells through extrusion, preventing the accumulation of unnecessary or pathological cells. The extrusion process can be triggered by apoptotic signalling, oncogenic transformation and overcrowding of cells. Despite the important linkage of cell extrusion to developmental, homeostatic and pathological processes such as cancer metastasis, its underlying mechanism and connections to the intrinsic mechanics of the epithelium are largely unexplored. We approach this problem by modelling the epithelium as an active nematic liquid crystal (that has a long range directional order), and comparing numerical simulations to strain rate and stress measurements within monolayers of MDCK (Madin Darby canine kidney) cells. Here we show that apoptotic cell extrusion is provoked by singularities in cell alignments in the form of comet-shaped topological defects. We find a universal correlation between extrusion sites and positions of nematic defects in the cell orientation field in different epithelium types. The results confirm the active nematic nature of epithelia, and demonstrate that defect-induced isotropic stresses are the primary precursors of mechanotransductive responses in cells, including YAP (Yes-associated protein) transcription factor activity, caspase-3-mediated cell death, and extrusions. Importantly, the defect-driven extrusion mechanism depends on intercellular junctions, because the weakening of cell-cell interactions in an α-catenin knockdown monolayer reduces the defect size and increases both the number of defects and extrusion rates, as is also predicted by our model. We further demonstrate the ability to control extrusion hotspots by geometrically inducing defects through microcontact printing of patterned monolayers. On the basis of these results, we propose a mechanism for apoptotic cell extrusion: spontaneously formed topological defects in epithelia govern cell fate. This will be important in predicting extrusion hotspots and dynamics in vivo, with potential applications to tissue regeneration and the suppression of metastasis. Moreover, we anticipate that the analogy between the epithelium and active nematic liquid crystals will trigger further investigations of the link between cellular processes and the material properties of epithelia.