arXiv subjects

G. E. Volovik

Publications and source records attributed to G. E. Volovik.

At least 73 records · Page 4Linked to original sources

arXiv2020

Dimensionless physics

We discuss two scenarios of emergent gravity. In one of them the quantum vacuum is considered as superplastic crystal, and the effective gravity describes the dynamical elastic deformations of this crystal. In the other one the gravitational tetrads emerge as the bilinear form of the fermionic fields. In spite of the essentially different mechanisms of emergent gravity, these two scenarios have one important common property: the metric field has dimension of the inverse square of length $[g_{μν}]=1/[l]^2$, as distinct from the conventional dimensionless metric, $[g_{μν}]=1$, in general relativity. As a result the physical quantities, which obey diffeomorphism invariance, become dimensionless. This takes place for such quantities as Newton constant, the scalar curvature, the cosmological constant, particle masses, fermionic and scalar bosonic fields, etc. This may suggest that the dimensionless physics can be the natural consequence of the diffeomorphism invariance, and thus can be the general property of any gravity, which emerges in the quantum vacuum. One of the nontrivial consequences of the shift of dimensions is related to topology. Due to the shift of dimensions some operators become topological, and contain the integer or fractional prefactors in the action. This in particular concerns the intrinsic 3+1 quantum Hall effect and the Nieh-Yan quantum anomaly in terms of torsion.

gr-qc↗

arXiv2020

Varying Newton constant and black hole to white hole quantum tunneling

The thermodynamics of black holes is discussed for the case, when the Newton constant $G$ is not a constant, but is the thermodynamic variable. This gives for the first law of the Schwarzschild black hole thermodynamics: $dS_\text{BH}= -AdK + \frac{dM}{T_\text{BH}}$, where the gravitational coupling $K=1/4G$, $M$ is the black hole mass, $A$ is the area of horizon, and $T_\text{BH}$ is Hawking temperature. From this first law it follows that the dimensionless quantity $M^2/K$ is the adiabatic invariant, which in principle can be quantized if to follow the Bekenstein conjecture. From the Euclidean action for the black hole it follows that $K$ and $A$ serve as dynamically conjugate variables. This allows us to calculate the quantum tunneling from the black hole to the white hole, and determine the temperature and entropy of the white hole.

gr-qc↗

arXiv2020

Exceeding the Landau superflow speed limit with topological Bogoliubov Fermi surfaces

A common property of topological systems is the appearance of topologically protected zero-energy excitations. In a superconductor or superfluid such states set the critical velocity of dissipationless flow $v_{\mathrm{cL}}$, proposed by Landau, to zero. We check experimentally whether stable superflow is nevertheless possible in the polar phase of p-wave superfluid $^3$He, which features a Dirac node line in the energy spectrum of Bogoliubov quasiparticles. The fluid is driven by rotation of the whole cryostat, and superflow breakdown is seen as the appearance of single- or half-quantum vortices. Vortices are detected using the relaxation rate of a Bose-Einstein condensate of magnons, created within the fluid. The superflow in the polar phase is found to be stable up to a finite critical velocity $v_{\rm c}\approx 0.2\,$cm/s, despite the zero value of the Landau critical velocity. We suggest that the stability of the superflow above $v_{\mathrm{cL}}$ but below $v_{\rm c}$ is provided by the accumulation of the flow-induced quasiparticles into pockets in the momentum space, bounded by Bogoliubov Fermi surfaces. In the polar phase this surface has non-trivial topology which includes two pseudo-Weyl points. Vortices forming above the critical velocity are strongly pinned in the confining matrix, used to stabilize the polar phase, and hence stable macroscopic superflow can be maintained even when the external drive is brought to zero.

cond-mat.other↗

arXiv2020

On thermal Nieh-Yan anomaly in Weyl superfluids

We discuss the possibility of the torsional Nieh-Yan anomaly of the type $μ(ej^μ_5) =γT^2({\cal T}^a \wedge {\cal T}_a)$ in Weyl superfluids, where $T$ is temperature and ${\cal T}^a$ is the effective or emergent torsion from the superfluid order parameter. As distinct from the dimensionful ultraviolet (UV) parameter $Λ^2$ in the conventional torsional Nieh-Yan anomaly, the parameter $γ$ is dimensionless in canonical units. This suggests that such dimensionless parameter may be fundamental, being determined by the geometry, topology and number of chiral quantum fields in the system. By comparing this to a Weyl superfluid with low-temperature corrections, $T\llΔ_0$, we show that such a term does exist in the hydrodynamics of a chiral $p$-wave superfluid, such as $^3$He-A, or a chiral superconductor. We also discuss and show how other $T^2$ terms of similar form and of the same order in gradients, coming from e.g. Fermi-liquid corrections and the chiral chemical potential, can also be expressed in terms of dimensionless fundamental parameters with emergent low-energy relativistic fields. Lastly, we discuss our results in comparison to relativistic Weyl fermions and the connection of the torsional gravitational anomalies to thermal transport in Weyl systems.

cond-mat.str-el↗

arXiv2020

String monopoles, string walls, vortex-skyrmions and nexus objects in polar distorted B-phase of $^3$He

The composite cosmological objects -- Kibble-Lazarides-Shafi (KLS) walls bounded by strings and cosmic strings terminated by Nambu monopoles -- could be produced during the phase transitions in the early Universe. Recent experiments in superfluid $^3$He reproduced the formation of the KLS domain walls, which opened the new arena for the detailed study of those objects in human controlled system with different characteristic lengths. These composite defects are formed by two successive symmetry breaking phase transitions. In the first transition the strings are formed, then in the second transition the string becomes the termination line of the KLS wall. In the same manner, in the first transition monopoles are formed, and then in the second transition these monopoles become the termination points of strings. Here we show that in the vicinity of the second transition the composite defects can be described by relative homotopy groups. This is because there are two well separated length scales involved, which give rise to two different classes of the degenerate vacuum states, $R_1$ and $R_2$, and the composite objects correspond to the nontrivial elements of the group $π_n(R_1,R_2)$. We discuss this on example of the so-called polar distorted B phase, which is formed in the two-step phase transition in liquid $^3$He distorted by aerogel. In this system the string monopoles terminate spin vortices with even winding number, while KLS string walls terminate on half quantum vortices. In the presence of magnetic field, vortex-skyrmions are formed, and the string monopole transforms to the nexus. We also discuss the integer-valued topological invariants of those objects. Our consideration can be applied to the composite defects in other condensed matter and cosmological systems.

hep-ph↗

arXiv2020

Spin vortex lattice in the Landau vortex-free state of rotating superfluids

We show that the Landau vortex-free state in rotating container may give rise to the lattice of spin vortices. We consider this effect on example of spin vortices in magnon Bose-Einstein condensate (the phase coherent spin precession) in the B-phase of superfluid $^3$He, and on example of spin vortices in the polar phase of $^3$He.

cond-mat.other↗

arXiv2020

On dimension of tetrads in effective gravity

Two different sources of emergent gravity lead to the inverse square of length dimension of metric field, $[g_{μν}]=1/[l]^2$, as distinct from the conventional dimensionless metric, $[g_{μν}]=1$, for $c = 1$. In both scenarios all the physical quantities, which obey diffeomorphism invariance, such as the Newton constant, the scalar curvature, the cosmological constant, particle masses, fermionic and scalar bosonic fields, etc., are dimensionless.

gr-qc↗

arXiv2019

Composite topological objects in topological superfluids

Superfluid phases of $^3$He discovered in 1972 opened the new area of the application of topological methods to condensed matter systems. Due to the multi-component order parameter which characterizes the broken $SO(3)\times SO(3)\times U(1)$ symmetry in these phases, there are many inhomogeneous objects -- textures and defects in the order parameter field -- which are protected by topology and are characterized by topological quantum numbers. Among them there are quantized vortices, skyrmions and merons, solitons and vortex sheets, monopoles and boojums, Alice strings, Kibble-Lazarides-Shafi walls terminated by Alice strings, spin vortices with soliton tails, etc. Most of them have been experimentally identified and investigated using nuclear magnetic resonance (NMR) techniquie, and in particular the phase coherent spin precession discovered in 1984 in $^3$He-B by Borovik-Romanov, Bunkov, Dmitriev and Mukharskiy in collaboration with Fomin.

cond-mat.other↗

arXiv2019

On thermal Nieh-Yan anomaly in topological Weyl materials

We discuss the possibility of a gravitional Nieh-Yan anomaly of the type $\partial_μj^μ_5 =γT^2{\cal T}^a\wedge {\cal T}_a$ in topological Weyl materials, where $T$ is temperature and ${\cal T}^a$ is the effective or emergent torsion. As distinct from the non-universal parameter $Λ$ in the conventional (zero temperature) Nieh-Yan anomaly -- with canonical dimensions of momentum -- the parameter $γ$ is dimensionless. This suggests that the dimensionless parameter is fundamental, being determined by the geometry, topology and number of the number of chiral quantum fields without any explicit non-universal UV scales. This conforms with previous results in the literature, as well as spectral flow calculations using torsional magnetic field at finite temperature.

cond-mat.str-el↗

arXiv2019

Vacuum energy decay from a q-bubble

We consider a finite-size spherical bubble with a nonequilibrium value of the $q$-field, where the bubble is immersed in an infinite vacuum with the constant equilibrium value $q_{0}$ for the $q$-field (this $q_{0}$ has already cancelled an initial cosmological constant). Numerical results are presented for the time evolution of such a $q$-bubble with gravity turned off and with gravity turned on. For small enough bubbles and a $q$-field energy scale sufficiently below the gravitational energy scale $E_\text{Planck}$, the vacuum energy of the $q$-bubble is found to disperse completely. For large enough bubbles and a finite value of $E_\text{Planck}$, the vacuum energy of the $q$-bubble disperses only partially and there occurs gravitational collapse near the bubble center.

gr-qc↗

arXiv2019

Elasticity tetrads, mixed axial-gravitational anomalies, and 3+1d quantum Hall effect

For two-dimensional topological insulators, the integer and intrinsic (without external magnetic field) quantum Hall effect is described by the gauge anomalous (2+1)-dimensional [2+1d] Chern-Simons (CS) response for the background gauge potential of the electromagnetic U(1) field. The Hall conductance is given by the quantized prefactor of the CS term, which is a momentum-space topological invariant. Here, we show that three-dimensional crystalline topological insulators with no other symmetries are described by a topological (3+1)-dimensional [3+1d] mixed CS term. In addition to the electromagnetic U(1) gauge field, this term contains elasticity tetrad fields $E^{\ a}_μ({\bf r},t) = \partial_μX^a(\mathbf{r},t)$ which are gradients of crystalline U(1) phase fields $X^a(\mathbf{r},t)$ and describe the deformations of the crystal. For a crystal in three spatial dimensions $a=1,2,3$ and the mixed axial-gravitational response contains three parameters protected by crystalline symmetries: the weak momentum-space topological invariants. The response of the Hall conductance to the deformations of the crystal is quantized in terms of these invariants. In the presence of dislocations, the anomalous 3+1d CS term describes the Callan-Harvey anomaly inflow mechanism. The response can be extended to all odd spatial dimensions. The elasticity tetrads, being the gradients of the lattice U(1) fields, have canonical dimension of inverse length. Similarly, if such tetrad fields enter general relativity, the metric becomes dimensionful, but the physical parameters, such as Newton's constant, the cosmological constant, and masses of particles, become dimensionless.

cond-mat.mes-hall↗

arXiv2019

Flat band and Planckian metal

We discuss the recent extension of the Sachdev-Ye-Kitaev (SYK) microscopic model by Patel and Sachdev in arXiv:1906.03265, which demonstrates the characteristic features of the Khodel-Shaginyan fermion condensate -- the existence of the finite region of momenta, where the energy of electrons is exactly zero (the flat band). The microscopic derivation of the flat band in this interacting model supports the original idea of Khodel and Shaginyan based on the phenomenological approach. It also suggests that it is the flat band, which is responsible for the linear dependence of resistivity on temperature in "strange metals".

cond-mat.str-el↗

arXiv2019

Tetrads and q-theory

As the microscopic structure of the deep relativistic quantum vacuum is unknown, a phenomenological approach ($q$-theory) has been proposed to describe the vacuum degrees of freedom and the dynamics of the vacuum energy after the Big Bang. The original $q$-theory was based on a four-form field strength from a three-form gauge potential. However, this realization of $q$-theory, just as others suggested so far, is rather artificial and does not take into account the fermionic nature of the vacuum. We now propose a more physical realization of the $q$-variable. In this approach, we assume that the vacuum has the properties of a plastic (malleable) fermionic crystalline medium. The new approach unites general relativity and fermionic microscopic (trans-Planckian) degrees of freedom, as the approach involves both the tetrad of standard gravity and the elasticity tetrad of the hypothetical vacuum crystal. This approach also allows for the description of possible topological phases of the quantum vacuum.

gr-qc↗

arXiv2019

50 years in the Landau Institute environment

It is impossible to review all the fundamental works of Landau Institute, which made important contribution to physics in general. Here I will discuss only a part of these works, only those which directly influence my own work during 50 years (1968-2018). Khalatnikov created the unique Institute, where practically all important areas of theoretical physics have been represented, opening broad possibilities for collaboration. The multidisciplinary environment of the Landau Institute was the main element of inspiration.

physics.hist-ph↗

arXiv2019

Comment to the CPT-symmetric Universe: Two possible extensions

In Ref.1 (L. Boyle, K. Finn and N. Turok, CPT-Symmetric Universe, Phys. Rev. Lett. {\bf 121}, 251301 (2018)) the antispacetime Universe was suggested as the analytic continuation of our Universe across the Big Bang singularity in conformal time. We consider two different scenarios of analytic continuation. In one of them the analytic continuation is extended to the temperature of the system. This extension suggests that if such analytic continuation is valid, then it is possible that the initial stage of the evolution of the Universe on our side of the Big Bang was characterized by the negative temperature. In the second scenario, the analytic continuation is considered in the proper time. In this scenario the Big Bang represents the bifurcation point at which the $Z_2$ symmetry between the spacetime and antispacetime is spontaneously broken.

gr-qc↗

arXiv2019

Topological Superfluids

There are many topological faces of the superfluid phases of $^3$He. These superfluids contain various topological defects and textures. The momentum space topology of these superfluids is also nontrivial, as well as the topology in the combined (${\bf p},{\bf r})$ phase space, giving rise to topologically protected Dirac, Weyl and Majorana fermions living in bulk, on the surface and within the topological objects. The nontrivial topology lead to different types of anomalies, which extended in many different directions the Landau-Khalatninkov theory of superfluidity.

cond-mat.other↗

arXiv2019

Two roads to antispacetime in polar distorted B phase: Kibble wall and half-quantum vortex

We consider the emergent tetrad gravity and the analog of antispacetime realized in the recent experiments ( J.T. Makinen, et al., Nat. Comm. 10, 237 (2019)) on the composite defects in superfluid $^3$He: the Kibble walls bounded by strings (the half quantum vortices). The antispacetime can be reached in two different ways: by the "safe" route around the Alice string or by dangerous route across the Kibble wall. This consideration also suggests the scenario of the formation of the discrete symmetry -- the parity $P$ in Dirac equations -- from the continuous symmetry existing on the more fundamental level.

cond-mat.other↗

arXiv2018

More on cold dark matter from q-theory

We consider the rapidly-oscillating part of a $q$-field in a cosmological context and find that its energy density behaves in the same way as a cold-dark-matter component, namely proportional to the inverse cube of the cosmic scale factor.

gr-qc↗