6 research outputs found

### Excitonic instability towards a Potts-nematic quantum paramagnet

Magnetic frustration can lead to peculiar magnetic orderings that break a
discrete symmetry of the lattice in addition to the fundamental magnetic
symmetries (i.e., spin rotation invariance and time-reversal symmetry). In this
work, we focus on frustrated quantum magnets and study the nature of the
quantum phase transition between a paramagnet and a magnetically ordered state
with broken threefold ($Z_3$) crystal rotation symmetry. We predict the
transition to happen in two stages, giving rise to an intermediate nematic
phase in which rotation symmetry is broken but the system remains magnetically
disordered. The nematic transition is described by the three-state Potts model.
This prediction is based on an analysis of bound states formed from two-magnon
excitations in the paramagnet, which become gapless while single-magnon
excitations remain gapped. By considering three different lattice models, we
demonstrate a generic instability towards two-magnon bound state formation in
the Potts-nematic nematic channel. We present both numerical results and a
general analytical perturbative formula for the bound state binding energy
similar to BCS theory. We further discuss a number of different materials that
realize key features of the model considered, and thus provide promising venues
for possible experimental observation.Comment: 17 pages, 8 figure

### Mesoscopic conductance fluctuations and noise in disordered Majorana wires

Superconducting wires with broken time-reversal and spin-rotational
symmetries can exhibit two distinct topological gapped phases and host bound
Majorana states at the phase boundaries. When the wire is tuned to the
transition between these two phases and the gap is closed, Majorana states
become delocalized leading to a peculiar critical state of the system. We study
transport properties of this critical state as a function of the length $L$ of
a disordered multichannel wire. Applying a non-linear supersymmetric sigma
model of symmetry class D with two replicas, we identify the average
conductance, its variance and the third cumulant in the whole range of $L$ from
the Ohmic limit of short wires to the regime of a broad conductance
distribution when $L$ exceeds the correlation length of the system. In
addition, we calculate the average shot noise power and variance of the
topological index for arbitrary $L$. The general approach developed in the
paper can also be applied to study combined effects of disorder and topology in
wires of other symmetries.Comment: 21 pages, 7 figure

### Signatures of Z$_3$ Vestigial Potts-nematic order in van der Waals antiferromagnets

Layered van der Waals magnets have attracted much recent attention as a
promising and versatile platform for exploring intrinsic two-dimensional
magnetism. Within this broader class, the transition metal phosphorous
trichalcogenides $M$P$X_3$ stand out as particularly interesting, as they
provide a realization of honeycomb lattice magnetism and are known to display a
variety of magnetic ordering phenomena as well as superconductivity under
pressure. One example, found in a number of different materials, is
commensurate single-$Q$ zigzag antiferromagnetic order, which spontaneously
breaks the spatial threefold $(C_3)$ rotation symmetry of the honeycomb
lattice. The breaking of multiple distinct symmetries in the magnetic phase
suggests the possibility of a sequence of distinct transitions as a function of
temperature, and a resulting intermediate $\mathbb{Z}_3$-nematic phase which
exists as a paramagnetic vestige of zigzag magnetic order -- a scenario known
as vestigial ordering. Here, we report the observation of key signatures of
vestigial Potts-nematic order in rhombohedral FePSe$_3$. By performing linear
dichroism imaging measurements -- an ideal probe of rotational symmetry
breaking -- we find that the $C_3$ symmetry is already broken above the N\'eel
temperature. We show that these observations are explained by a general
Ginzburg-Landau model of vestigial nematic order driven by magnetic
fluctuations and coupled to residual strain. An analysis of the domain
structure as temperature is lowered and a comparison with zigzag-ordered
monoclinic FePS$_3$ reveals a broader applicability of the Ginzburg-Landau
model in the presence of external strain, and firmly establishes the $M$P$X_3$
magnets as a new experimental venue for studying the interplay between
Potts-nematicity, magnetism and superconductivity.Comment: 6 pages, 4 figures + supplementary materia

### Mirror proteorhodopsins

Abstract Proteorhodopsins (PRs), bacterial light-driven outward proton pumps comprise the first discovered and largest family of rhodopsins, they play a significant role in life on the Earth. A big remaining mystery was that up-to-date there was no described bacterial rhodopsins pumping protons at acidic pH despite the fact that bacteria live in different pH environment. Here we describe conceptually new bacterial rhodopsins which are operating as outward proton pumps at acidic pH. A comprehensive function-structure study of a representative of a new clade of proton pumping rhodopsins which we name â€śmirror proteorhodopsinsâ€ť, from Sphingomonas paucimobilis (SpaR) shows cavity/gate architecture of the proton translocation pathway rather resembling channelrhodopsins than the known rhodopsin proton pumps. Another unique property of mirror proteorhodopsins is that proton pumping is inhibited by a millimolar concentration of zinc. We also show that mirror proteorhodopsins are extensively represented in opportunistic multidrug resistant human pathogens, plant growth-promoting and zinc solubilizing bacteria. They may be of optogenetic interest