V. I. Marchenko

Frictional shear cracks

We discuss crack propagation along the interface between two dissimilar materials. The crack edge separates two states of the interface, “stick” and “slip.” In the slip region, we assume that the shear stress is proportional to the sliding velocity; i.e., the linear viscous friction law is valid. In this picture, the static friction appears as the tile Griffith threshold for crack propagation. We calculate the crack velocity as a function of the applied shear stress and find that the main dissipation comes from the macroscopic region and is mainly due to the friction at the interface. The relevance of our results to recent experiments, Baumberger et al., Phys. Rev. Lett. 88, 075509 (2002), is discussed.

Preparation of facilities for fundamental research with ultracold neutrons at PNPI

The WWR-M reactor of PNPI offers a unique opportunity to prepare a source for ultracold neutrons (UCN) in an environment of high neutron flux (about 3 � 10 12 n/cm 2 /s) but still acceptable radiation heat release (about 4 � 10 � 3 W/g). It can be realized within the thermal column situated close to the reactor core. With its large diameter of 1 m, this channel allows to install a 15-cm-thick bismuth shielding, a graphite premoderator (300 dm 3 at 20 K), and a superfluid helium converter (35 dm 3 ). At a temperature of 1.2 K it is possible to remove the heat release power of about 20 W. Using 4p flux of cold neutrons within the reactor column can bring more than a factor 100 of cold neutron flux incident on the superfluid helium with respect to the present cold neutron beam conditions at the ILL reactor. The storage lifetime for UCN in superfluid He at 1.2 K is about 30 s, which is sufficient when feeding experiments requiring a similar filling time. The calculated density of UCN with energy between 50 and 230 neV in an experimental volume of 40 l is about 10 4 n/cm 3 . Technical solutions for realization of the project are discussed.

Fracture and friction: Stick-slip motion

Abstract.We discuss the stick-slip motion of an elastic block sliding along a rigid substrate. We argue that for a given external shear stress this system shows a discontinuous nonequilibrium transition from a uniform stick state to uniform sliding at some critical stress which is nothing but the Griffith threshold for crack propagation. An inhomogeneous mode of sliding occurs when the driving velocity is prescribed instead of the external stress. A transition to homogeneous sliding occurs at a critical velocity, which is related to the critical stress. We solve the elastic problem for a steady-state motion of a periodic stick-slip pattern and derive equations of motion for the tip and resticking end of the slip pulses. In the slip regions we use the linear friction law and do not assume any intrinsic instabilities even at small sliding velocities. We find that, as in many other pattern forming system, the steady-state analysis itself does not select uniquely all the internal parameters of the pattern, especially the primary wavelength. Using some plausible analogy to first-order phase transitions we discuss a “soft” selection mechanism. This allows to estimate internal parameters such as crack velocities, primary wavelength and relative fraction of the slip phase as functions of the driving velocity. The relevance of our results to recent experiments is discussed.

Influence of strain on the kinetics of phase transitions in solids.

We consider a sharp interface kinetic model of phase transitions accompanied by elastic strain, together with its phase-field realization. Quantitative results for the steady-state growth of a new phase in a strip geometry are obtained, and different pattern formation processes in this system are investigated.

Project of the ultracold and cold neutron source at the WWR-M reactor with superfluid helium as a moderator

The WWR-M reactor of the Petersburg Nuclear Physics Institute provides a unique opportunity for creating conditions of low radiative heat release (∼4 × 10−3 W/g) at a sufficiently high neutron flux (∼3 × 1012 neutrons/(cm2 s)). This opportunity can be implemented in the reactor thermal column, which represents a 1-m-diameter channel adjacent to the reactor core. This diameter of the channel allows the arrangement of the core gamma shielding made of bismuth (15 cm thick), a graphite premoderator (300 dm3) at a temperature of 20 K, and a converter with superfluid helium (35 dm3) at a temperature of 1.2 K. Calculations show that the heat release in the source (20 W) can be removed by pumping helium vapor, and the density of ultracold neutrons in an experimental trap will be ∼104 neutrons/cm3, which is higher than that of existing sources of ultracold neutrons by two to three orders of magnitude.

On the phases of compressed matter

It is shown that matter during the process of compression under the conditions of complete thermodynamic equilibrium at low temperatures should sequentially pass through 14 monoisotopic phase states. The nuclear subsystem is a Wigner crystal in almost all phases and is a quantum liquid at the maximum pressure.

Phase diagram of surface superconductivity

The Ginzburg-Landau theory is used to numerically analyze the stability of surface superconductivity. The singularities in the behavior of the solution on approaching the stability limit are described within the Landau theory of second-order phase transitions applied to a metastable state. It is found that the wetting must be observed upon transition from type I to type II superconductors: the thickness of surface superconductivity goes to infinity when the magnetic field tends to critical.

On antiferromagnetic transition in CuCrO2

It has been established that peculiar properties of the antiferromagnetic state of CuCrO2 can be explained naturally using the Dzyaloshinskii-Landau theory of magnetic phase transitions.

On the NMR spectrum in antiferromagnetic CsMnI3

An explanation is proposed for the spin-reduction anisotropy observed in an investigation of NMR in the noncollinear six-sublattice antiferromagnet CsMnI3.

New possibilities in crystal morphology

Two new morphological phenomena are predicted in crystals: meniscus disappearance and meniscus fixation. Helium crystals are the most convenient objects for their observation.