Modification of the neutral triptycene TPC-X molecule by increasing the size of the X substituent at the 9th position from X = H to Br and I affects the magnetic and conducting properties of the (MDABCO+)(C60 center dot- )(TPC-X) salts (1-3, respectively), where MDABCO+ is N-methyldiazabicyclooctanium. These salts feature hexagonal fullerene layers composed of C60 center dot- , which remain undimerized owing to spatial separation by TPC-X. Two distinct fullerene layers, A and B, differ in their surroundings. The calculated bandwidth of 0.133-0.141 eV for 3 indicates a narrow band nature and as a result both fullerene layers are close to a Mott insulating state. Only one type of fullerene layers demonstrate metallic conductivity in 2 and 3, while another type of fullerene layers have localized electronic state and shows strong antiferromagnetic spin coupling down to low temperatures. Magnetic coupling in 2 and 3 follows the Heisenberg model for 2D hexagonal layers, with J =-28 and-34 cm-1, respectively. The 2D metallic conductivity enables the observation of a Dysonian-type electron paramagnetic resonance signal from oriented single crystals of 2 and 3, similar to 1. Heating 2 and 3 above 300 K decreases their A/B ratio, reaches unity above 350 K, signaling a crossover to a nonmetallic state. The interfullerene center-to-center (ctc) distances increase upon heating, essentially reducing overlap and transfer integrals in (MDABCO+) (C60 center dot- )(TPC-X). Because these salts are narrow-band metals, such an increase in ctc distances may drive a transition of these metals to a Mott-insulating state.

We perform transport and noise measurements for device consisting of a thin NbSe2 flake laid onto the predefined gold electrodes and covered with a thin hBN flake. In the shot noise of a NbSe2/Au tunnel junction, we identify Andreev reflection regime by demonstrating the effective charge doubling. Further, by creating temperature gradient across the tunnel junction and measuring its delta- T noise in the normal state, we extract electron–phonon scattering length in NbSe2 and its T-dependence. The results of delta- T noise measurements in the absence of a magnetic field when the flake is superconducting are in qualitative agreement with expectations. The introduced approach is promising for the study of nonequilibrium configurations in superconductors.

A step-by-step description of the technique for manufacturing various van der Waals heterostructures is provided. First, the procedure to obtain monolayer and few-layer flakes from layered materials, in particular graphite and hexagonal boron nitride, is discussed. Next, different approaches to their assembly depending on the required final structure are considered. Finally, the procedure for making ohmic contacts is described in detail and the parameters for plasma chemistry and metal deposition are given. The field effect is discovered in transport measurements carried out at various temperatures, but a number of features, such as a strong shift of the charge neutral point from the zero-gate voltage, a large resistance away from the charge neutral point, and a low mobility, indicate a poor quality of the resulting devices. Nevertheless, one of the fabricated devices demonstrates good quality: the maximum mobility is estimated as 15 000 cm2/(V s), and the magnetic field dependences demonstrate the quantum Hall effect that is standard for high-quality graphene. Unexpectedly, scanning electron microscope images of the resulting devices reveal a large amount of contamination on the surface of the flakes, which may explain the corresponding quality of our devices. Preliminary results of flakes cleaning with chemical compounds and thermal treatment are presented.

Numerous diverse grown-in point defects in hexagonal boron nitride exhibit properties of single photon emitters stimulating the development of controllable methods for their local formation. In this work the defects created in hexagonal boron nitride by helium ion irradiation were investigated by means of cathodoluminescence and Raman spectroscopy. The irradiation with ion fluence above 10(15) cm(-2) resulted in a new Raman spectral band at about 1295 cm(-1), which can be attributed to the formation of vacancies or divacancies. The intensity of the defect-related luminescence was found to vary non-monotonically with ion fluence and possessed a maximum at about 10(14) cm(-2). On the basis of this result a new procedure to fabricate light emitting discs by means of the focused helium ion beam was suggested and demonstrated.

Transition metal dichalcogenides are studied due to the possibility of creating nanoscale semiconductor devices, as well as fundamental issues of magnetic ordering. We researched the crystal structure and magnetic properties of niobium dichalcogenide Mn0.30NbS2. The results of the X-ray study showed the possible existence of an intermediate 23a0·23a0 structure between the “basic” superstructures. Also, two local maximums were found in the temperature dependence of the dynamic magnetic susceptibility. These features can indirectly confirm the presence of a transition superstructure and reflect the two-step nature of the magnetic ordering.

Photoluminescence spectra of excitons and trions in MoSe2 monolayers encapsulated with hexagonal boron nitride under nonresonant laser excitation were studied. When the size of the laser excitation spot decreases from 8 to 3 nm, individual peaks with a line width of ~2 meV emerge in the photoluminescence spectra, which were unresolved with a larger spot. Studies of the sample surface using a scanning electron microscope revealed the existence of a large number of features at the interfaces of structures with characteristic sizes ranging from submicron to micron and more. It was expected that the lines appearing in the spectrum at small excitation spot sizes were associated with similar submicron inhomogeneities. A study of a specially made heterostructure covered by a metal mask with holes of 1.6 microns in diameter confirmed this assumption.

A study is performed of the photoluminescence spectra of excitons and trions in MoSe2 monolayers encapsulated with hexagonal boron nitride upon nonresonant laser excitation. When the size of the laser spot of excitation is reduced from 8 to 3 nm, individual peaks with line widths of ~2 meV start to resolve in the photoluminescence spectra. Such peaks are unresolved when there is a larger spot. Studies of the surface of the sample using a scanning electron microscope reveal a great many features at the interfaces of structures that have characteristic sizes ranging from submicrometer to micrometer and larger. It is expected the lines that appear in the spectrum with small spots of excitation are associated with similar submicrometer inhomogeneities. Studies of a specially fabricated heterostructure that has a metal mask with holes 1.6 μm in diameter sputtered onto its surface confirm this assumption.

The reliability of fundamental studies of superconductivity depends on the quality of the materials under study. Optical zone melting yields high-quality single crystals without impurities, which can be difficult for other technologies. The paper describes the growth procedure for single crystals of several families of superconductors: bismuth high-temperature superconductors Bi2Sr2CaCu2O8 + δ and Bi2Sr2–xLaxCuO6 + δ and a superconductor with an assumed p symmetry of the superconducting order parameter Sr2RuO4. We discuss the search criteria for synthesizing high-temperature yttrium superconductors YBa2Cu3O7 + δ by optical zone melting, which do not lead to the formation of single crystals. The procedure for obtaining single crystals includes several stages. The first is to anneal a mixture of powders of the required oxides and carbonates, taken in specific proportions, at temperatures up to 850°C. A solid-phase reaction takes place, resulting in the desired polycrystalline complex oxide; rods with a length of ~5–10 cm are obtained from this oxide using a hydraulic press. The second stage is annealing of the rods in air at temperatures up to 940°C and, if necessary, melting in an optical-zone-melting unit using lamps with a rated power of 500 W at an adjustable power from 20 to 95% with a drawing speed of 20–30 mm/h. The third stage is the growth of a single crystal at 20–95% power at a rate of 0.1–20 mm/h. The result is a mixture that disintegrates upon cracking into single crystals up to several millimeters in size. Measurements of the temperature dependence of the dynamic magnetic susceptibility of the synthesized single crystals at a frequency of 100 kHz are carried out, which makes it possible to determine the temperature of the superconducting transition and its width.

The superconducting properties of hierarchical nanostructured samples of Pb–In alloys have been studied by the measurement of dynamic susceptibility χ(T) temperature dependence. Symmetric samples with different shapes and sizes were formed on a brass metallic net by cathode-metal electrodeposition with a programmed pulsing current. Two different kinds of χ(T) dependence were observed in synthesized structures. The first kind was a broad superconductive transition without energy dissipation with a very weak response to the external magnetic field. The second kind was, conversely, an abrupt transition signifying an energy dissipation with a significant field response. This behavior depends on the ratio between a superconducting domain size (defined by the London penetration depth λ) and a crystallite size. In these cases, one or several superconducting domains are present in a sample. This result paves the way to controlling a superconducting domain size in materials with the parameters of a pulsed current.

We present the results of measuring the dynamic magnetic susceptibility and surface impedance of a unique layered organic superconductor κ-(BEDT-TTF) 4 Hg 2.89 Br 8 (κ-HgBr). In this material, strong electronic correlations coexist with weak doping associated with lattice incommensurability. The superconducting properties of this material are studied by several methods: the temperature dependences of the resistance across the conducting layers, the dynamic magnetic susceptibility, and the surface impedance in the conducting planes are measured. The results of measuring the resistance and dynamic magnetic susceptibility at a frequency of 100 kHz demonstrated the presence of a superconducting state at temperatures below T c = 3.1 K. The results of measuring the temperature dependence of the surface impedance Z ( T ) = R ( T ) + iX ( T ) of several samples at a frequency of 28 GHz in the temperature range from 0.5 K to 50 K turned out to be unusual. In the studied samples at T < 3 K, a sharp change in Z ( T ) is not observed, and some samples even demonstrate weak “dielectricization” at low temperatures.

We develop a self-consistent approach for calculating the local impedance at a rough surface of a chiral p-wave superconductor. Using the quasiclassical Eilenberger-Larkin-Ovchinnikov formalism, we numerically find the pair potential, pairing functions, and the surface density of states taking into account diffusive electronic scattering at the surface. The obtained solutions are then employed for studying the local complex conductivity and surface impedance in the broad range of microwave frequencies (ranging from subgap to above-gap values). We identify anomalous features of the surface impedance caused by generation of odd-frequency superconductivity at the surface. The results are compared with experimental data for Sr2RuO4 and provide a microscopic explanation of the phenomenological two-fluid model suggested earlier to explain anomalous features of the microwave response in this material.

The composition and the structure of ceramic EuBa2Cu3O6 + δ (Eu-123) oxide samples annealed in steps with varying processing conditions (in air or oxygen and argon atmosphere at a temperature of 940–960°С for 1–70 h with or without homogenization) were studied by the X-ray phase and chemical analysis, electron diffraction pattern analysis, elemental analysis, and high-resolution transmission electron microscopy. Regardless of the processing conditions, Eu-123 nanostructured oxide with a tetragonal or orthorhombic structure and domains 1–20 nm in size was obtained as a result of annealing. Nanostructuring of the samples, which was revealed by high-resolution electron microscopy, is attributed to their chemical nature: the presence of identical structural elements in members of the homologous Eu n Ba m Cum + nO y series of oxides allows them to intergrow coherently and create an illusion of a single crystal. Just like any other member of the Eu n Ba m Cum + nO y series, oxide Eu-123 is disproportionate depending on the annealing conditions to form other members of this series located on either side of the dominant oxide. Temperature Tc of the superconducting transition of each member of the series depends on the average oxidation state of copper \(\overline {Cu} \). At \(\overline {Cu} \) < 2, all members of the series have a tetragonal structure and do not exhibit superconducting properties. At \(\overline {Cu} \) = 2.28, five members of the Eu n Ba m Cum + nO y series with matrices (Ba : Cu) 5 : 8, 3 : 5, 2 : 3, 5 : 7, and 3 : 4 exhibit superconducting properties with Tc = 82–90 K.

Oxidation of {cryptand(Na+)}(C-60*(-)) and {cryptand(Na+)}(2)(C-60(2-)), which display integer -1 and -2 charges on the fullerene unit, by iron(II) octaethyltetrapyrazinoporphyrazine allowed the crystallization of complexes with noninteger average charges on C-60. The complex {cryptand(Na+)}(C-60)(3)center dot 2C(6)H(4)Cl(2) (1), with a partial charge of -0.33 on C-60, exhibits a closely-packed 3D structure formed by fragments of hexagonal fullerene layers. Complex 1 shows high conductivity in the 295-120 K range: conductivities of 50-60 and 100-250 S/cm(-1) were measured using four-probe and microwave techniques, respectively. Metallic behavior of 1 is observed down to 150-120 K. Metallic conductivity is still observed in 1 after one month of exposure to air. The complexes {cryptand(Na+)}(C-60)(2) - (2) and {cryptand (Na+)}(8)(C-60)(6)(8-) (3), which display average charges of -0.5 and -1.33, respectively, on C-60, do not show metallic conductivity. This can be explained by charge disproportionation and the formation of neutral and -1-charged C-60 in 2, and -1- and -2-charged C-60 in 3. Despite the close packing of the fullerene molecules in crystal structures, no dimerization of C-60 is observed in 1 and 2. On the contrary, diamagnetic, singly-bonded (C60(-))(2) dimers are formed in {cryptand(Li+)} 2(C-60(-))(2)center dot C6H4Cl2 center dot C6H14 (4), which contains fullerene molecules bearing a full -1 charge.

(MDABCO(+))(C-60(center dot-))(TPC) (1), in which MDABCO(+) is N-methyldiazabicyclooctanium, TPC is triptycene, and both have threefold symmetry, is a rare example of a fullerene-based quasi-2D metal and contains closely packed hexagonal fullerene layers with interfullerene center-to-center distances of 10.07 angstrom at 300 K. Evidence for the metallic nature of 1 was obtained by optical and microwave conductivity measurements on single crystals. The metal is characterized by a nontypical Drude response and relatively large optical mass (m*/m(0) = 6.7). The latter indicates a narrow-band nature, which is consistent with the calculated bandwidth of 0.10-0.15 eV. The coexistence of metallic and antiferromagnetic nonmetallic 2D layers was observed in 1 above 200-230 K. It was assumed that the nonmetallic layers undergo a transition to the metallic state below 200 K due to ordering of the fullerene and cationic sublattices. New layered complex (MQ(+))(C-60(center dot-))(TPC) (2) with a hexagonal arrangement of C-60(center dot-) was obtained by increasing the interfullerene distance with the bulkier N-methylquinuclidinium cations (MQ(+)) having threefold symmetry. The structure of 2 is characterized by increased interfullerene center-to-center distances in the layers (10.124, 10.155, and 10.177 angstrom at 250 K). Unit-cell doubling parallel to the 2D layer (along the b axis) was observed at low temperatures. In contrast to metallic 1, 2 exhibits a nonmetallic spin-frustrated state with an antiferromagnetic interaction of spins (the Weiss temperature is -27 K) and no magnetic ordering down to 1.9 K. It was supposed that the expanded interfullerene distances in the triangular arrangement decrease the bandwidth and suppress metallic conductivity in 2, and thus a Mott-Hubbard insulating state with antiferromagnetically frustrated spins results.

Mass transfer during the melt electrolysis of Y0.02Ba0.30Cu0.70O y and Y0.02Ba0.25Cu0.75O y samples is investigated at a temperature of 950°C (0.5 h) and a current of 5–1050 A in a cell. Crystal deposits of YBa2CVu3O6 + δ tetragonal oxide (123) are grown, and their cation composition and structure are investigated by means of X-ray phase analysis, electron diffraction, elemental analysis, and high resolution on a transmission electron microscope. Deviation of the cation composition of oxide (123) from the stoichiometric ratio and its nanostructured state at nanocrystallite sizes of 2–5 nm are observed. The temperature dependence of magnetic susceptibility after oxygen annealing (450°C, 1 h) has four curve bends, indicating there are four superconducting phases with T s = 45, 52, 75, and 86 K.

Mass transfer during the electrolysis of melts of Y 0.02 Ba 0.30 Cu 0.70 O y and Y 0.02 Ba 0.25 Cu 0.75 O y samples was studied at 950°C (for 0.5 h) and currents of 5–1050 mA. YBa 2 Cu 3 O 6 + δ ( 123 ) tetragonal oxide crystal boules were grown, and their cationic composition and structure were studied by X-ray powder diffraction and by electron diffraction and elemental analysis in a transmission electron microscope (ED/TEM and EA/TEM). The 123 oxide was found to have cationic off-stoichiometry and to have a domain structure with domain sizes of 20–50 Å. Magnetic susceptibility versus temperature curves measured in the crystals after oxygen annealing (450°C, 1 h) feature four kinks, which indicate the occurrence of four superconducting phases with T c = 45, 52, 75, and 86 K. Electrolysis byproducts are platinum-containing oxides Ba 9 Pt 4 Cu 3 O y and Ba 50 Pt 15 Y 16 Al 13 Cu 7 O y unknown hitherto; we report structure data for them.