Specific features of quantum oscillations of magnetization in quasi-two-dimensional antiferromagnetic semimetals

The specific features of quantum oscillations of the magnetization in quasi-two-dimensional wide-band-gap antiferromagnetic semimetals with a low concentration of charge carriers have been considered theoretically. It has been shown that, in these systems, the Fermi energy determined from the analysis of the frequency of the de Haas–van Alphen oscillations according to the standard procedure can differ significantly from the true value. For the correct determination of the Fermi energy in the canted phase, it has been proposed to analyze quantum oscillations of the magnetization M not as a function of the inverse magnetic field 1/H, but as a function of 1/cosγ, where the angle γ characterizes the inclination angle of the magnetic field with respect to the plane of the quasi-two-dimensional semimetal.     

de Haas van Alphen (dHvA) effect in two-dimensional (2D) conductors: susceptibility oscillations

A new scheme for analyzing the de Haas van Alphen (dHvA) effect in nearly two dimensional (2D) metals (i.e. with nearly cylindrical Fermi surface) is presented. The envelope of the magnetic susceptibility oscillations is calculated in the entire range of magnetic fields and temperatures. The resulting envelope function is found to be proportional to a universal function of the dimensionless parameter Q=hω_c/k _B T. The upper (i.e. paramagnetic) branch of the susceptibility envelope has a maximum at a certain Q = 5.45. This universal value may be useful for determining the effective cyclotron mass and the Fermi energy of nearly 2D metals. A simple relation between magnetization oscillations amplitude and calculated susceptibility amplitudes is derived. The corresponding limiting formulae for the magnetization oscillations envelope are found to match smoothly around the value X = 2π²/Q?2 of the Lifshitz-Kosevich (LK) smearing parameter. The influence of Fermi surface sheets with open orbits on magneto-quantum oscillations is considered. Triangle-like rather than saw-tooth-like oscillations at ultralow temperatures are obtained and substantially diminished magnetization and susceptibility amplitudes are calculated. This suggests the possibility of estimating the band structure parameters of Fermi surface sheets from magneto-quantum oscillations measurements.     

Quantum oscillations in a novel organic quasi-two-dimensional (BEDO-TTF)5[RbHg(SCN)4]2 metal

Quantum magnetization and magnetoresistance oscillations are detected in the quasi-two-dimensional organic metal (BEDO-TTF)₅[RbHg(SCN)₄]₂ for the first time. The magnetization oscillation spectrum corresponds to a calculated Fermi surface provided that a magnetic breakdown is realized. The magnetoresistance oscillation spectrum contains additional frequencies, one of which can unambiguously be related to quantum interference. An analysis of the angular dependence of the magnetoresistance oscillation amplitude indicates that the many-body interactions in this metal are weak.     

Effect of the Fermi surface shape on the de Haas-van Alphen oscillations in layered conductors

In this paper, we theoretically analyze the effects of the Fermi surface (FS) shape on the magnetic oscillations in quantizing magnetic fields in quasi-two-dimensional layered conductors. The theory is developed based on a phenomenological model for the electron energy spectra introduced in the paper. The model enables various Fermi surface profiles to be taken into consideration; this gives it an advantage over the commonly used tight-binding approximation. It is shown that, when the FS curvature becomes zero at an effective cross-section with maximum/minimum cross-sectional area, this can significantly affect the amplitude, shape, and phase of the oscillations, as well as the field dependence of the amplitude. The text was submitted by the author in English.     

The influence of the chemical potential oscillations on the de Haas-van Alphen effect in quasi-two-dimensional compounds

The de Haas-van Alphen effect in quasi-two-dimensional metals is studied at arbitrary parameters. Oscillations of the chemical potential can substantially change the temperature dependence of harmonic amplitudes that is commonly used to determine the effective electron mass. The processing of the experimental data using the standard Lifshitz-Kosevich formula can therefore lead to substantial errors even in the strong harmonic damping limit. This can explain the difference between the effective electron masses determined from the de Haas-van Alphen effect and the cyclotron resonance measurements. The oscillations of the chemical potential and the deviations from the Lifshitz-Kosevich formula depend on the reservoir density of states that exists in organic metals due to open sheets of the Fermi surface. This dependence can be used to determine the density of electron states on open sheets of the Fermi surface. We present analytical results of the calculations of harmonic amplitudes in some limiting cases that show the importance of the chemical potential oscillations. We also describe a simple algorithm for a numerical calculation of the harmonic amplitudes for arbitrary reservoir density of states, arbitrary warping, spin-splitting, temperature, and Dingle temperature.     

De Haas-van Alphen effect in unconventional superconductors

A theory of the de Haas-van Alphen effect in type-II p-wave and D-wave superconductors (the latter corresponds to the B _1g one-dimensional representation of group D _4h ) has been developed. Solutions for the order parameter and density of quasiparticle states near the upper critical field have been calculated. If the curve enclosing the extremal cross section of the Fermi surface in the plane perpendicular to the external magnetic field coincides with the line of nodes of the superconducting order parameter, the effect of the transition to the superconducting state on the amplitude of magnetization oscillations is negligible. If the line of nodes is oriented differently with respect to the applied magnetic field, the de Haas-van Alphen oscillations are suppressed in a manner qualitatively similar to the case of conventional superconductors. Zh. éksp. Teor. Fiz. 113, 2174–2192 (June 1998)     

De Haas-van Alphen oscillations in the quasi-two-dimensional organic metal (BEDO-TTF)5[CsHg(SCN)4]2

The magnetization oscillations in the quasi-two-dimensional organic metal (BEDO-TTF)₅[CsHg(SCN)₄]₂ are thoroughly investigated over a wide range of magnetic field directions at different temperatures down to 0.4 K. The results obtained are in good agreement with the shape and sizes of the Fermi surface calculated from the x-ray diffraction data. Apart from the fundamental frequencies, the combination frequencies are found in the magnetization oscillation spectrum. It is demonstrated that these combination frequencies are governed by the motion of charge carriers along the real closed orbits inside the network of magnetic breakdown orbits formed under the action of the magnetic field. It is uniquely established that the combination frequencies previously revealed in the magnetoresistance oscillation spectrum of the same metal are associated with the quantum interference effect. The angular dependences of the oscillation amplitude exhibit minima, which are explained by the spin splitting of the Landau levels.     

Thermoelectric effects in layered conductors in a strong magnetic field

Thermoelectric effects are investigated theoretically in layered conductors with a quasi-two-dimensional electron energy spectrum of arbitrary type in a strong magnetic field. It is shown that, at temperatures sufficiently low for quantization of the orbital motion of charge carriers in a magnetic field to be required, there exist giant quantum oscillations of the thermoelectric field. Thermoelectric emf is studied as a f unction of the orientation of the magnetic field with respect to the layers; experimental investigation of this function allows one to determine the velocity distribution of conduction electrons on the Fermi surface.     

Pulsed magnetization reversal of single-crystal iron borate in the presence of a transverse magnetic field

This paper describes the first investigations of the process of pulsed 180° magnetization reversal in iron borate in the presence of a transverse magnetic field. How the intensity of magnetoelastic oscillations depends on the amplitude of the magnetization reversal field and the duration of the primary period of the transient process is studied, and also the analysis of hodographs of the magnetization vector, which show that the primary reason why the pulsed magnetization reversal curve exhibits a kink is a decrease in the energy lost to excitation of magnetoelastic oscillations, caused by lagging of the elastic subsystem of the crystal behind the magnetic subsystem for magnetization reversal times less than 13–16 ns. Fiz. Tverd. Tela (St. Petersburg) 40, 305–309 (February 1998)     

Angular studies of the magnetoresistance in the density wave state of the quasi-two-dimensional purple bronze KMo<Subscript>6</Subscript>O<Subscript>17</Subscript>

The purple molybdenum bronze KMo₆O₁₇ is a quasi-two-dimensional compound which shows a Peierls transition towards a commensurate metallic charge density wave (CDW) state. High magnetic field measurements have revealed several transitions at low temperature and have provided an unusual phase diagram “temperature-magnetic field”. Angular studies of the interlayer magnetoresistance are now reported. The results suggest that the orbital coupling of the magnetic field to the CDW is the most likely mechanism for the field induced transitions. The angular dependence of the magnetoresistance is discussed on the basis of a warped quasi-cylindrical Fermi surface and provides information on the geometry of the Fermi surface in the low temperature density wave state.     

Giant Nernst-Ettingshausen oscillations in semiclassically strong magnetic fields

We consider the Nernst-Ettingshausen (NE) effect in the presence of semiclassically strong magnetic fields for a quasi-two-dimensional system with a parabolic or linear dispersion of carriers. We show that the occurring giant oscillations of the NE coefficient are coherent with the recent experimental observation in graphene, graphite, and bismuth. In the 2D case we find the exact shape of these oscillations and show that their magnitude decreases (increases) with enhancement of the Fermi energy for Dirac fermions (normal carriers). With a crossover to the 3D spectrum the phase of the oscillations shifts, their amplitude decreases, and the peaks become asymmetric.     

The distribution of equilibrium magnetization currents in systems with dimensional quantization in a finite magnetic field

The distribution of equilibrium magnetization currents in two-dimensional bounded systems placed in an external magnetic field is studied. A half-plane, a quantum disk, and a wide quantum ring are considered. The passage from classical to quantizing magnetic fields is investigated. The edge currents near the boundary of the half-plane are shown to experience damped (far from the boundaries) spatial oscillations related to the Fermi electron wavelength. The region occupied by currents was found to narrow with increasing field. Apart from these oscillations, the current contains a component that smoothly changes with distance but oscillationally depends on the position of the Fermi level relative to the Landau levels. The suppression of the oscillations by temperature is studied. The spatial distribution of the current in a circular disk and a ring is shown to significantly depend on the position of the Fermi level.     

On the propagation of electromagnetic waves in the tetrathiafulvalene family of organic conducting salts

The propagation of electromagnetic waves is investigated theoretically in organic layered conductors with metallic conductivity in magnetic fields strong enough that the characteristic radius of curvature of a conduction-electron orbit is much smaller than its mean-free path l. It is shown that when groups of charge carriers with quasi-two-dimensional and quasi-one-dimensional energy spectra coexist in such a material, the penetration depth of the waves into it, which is a strong function of the polarization of the waves and the orientation of the magnetic field, also has an interesting dependence on the magnitude of the magnetic field and the low-dimensionality parameters of the charge-carrier spectra. This property makes it possible to recover details of the Fermi surface from the experimental data. Fiz. Tverd. Tela (St. Petersburg) 40, 1401–1404 (August 1998)     

Cyclotron resonance in quasi-two-dimensional conductors

A theoretical analysis is made of the resonant absorption of RF electromagnetic wave energy in layered conductors having a quasi-two-dimensional electron energy spectrum. An analysis is made of the influence of possible drift of carriers into the conductor and their Fermi-liquid interaction on the position of the cyclotron resonance lines. A reason is given for the substantial difference between the cyclotron effective masses determined from the temperature dependence of the amplitude of the Shubnikov-de Haas oscillations and from measurements of the cyclotron resonance frequencies in [5–7]. It is shown that by means of an experimental investigation of cyclotron resonance in a magnetic field parallel to the layers, it is possible to find the carrier velocity distribution at the Fermi surface in layered organic conductors.     

Magnetoquantum de Haas-van Alphen oscillations in spin-split two-dimensional Fermi liquid

Theory of magnetoquantum oscillations with spin-split structure in strongly anisotropic (two-dimensional (2D)) metal is developed in the formalism of level approach. Parametric method for exact calculation of oscillations wave forms and amplitudes, developed earlier for spin degenerate levels is generalized on a 2D electron system with spin-split levels. General results are proved: 1) proportionality relation between magnetization and chemical potential oscillations accounting for spin-split energy levels and magnetic field unperturbed levels (states of reservoir), 2) basic equation for chemical potential oscillations invariant to various models of 2D and 1D energy bands (intersecting or overlapping) and localized states. Equilibrium transfer of carriers between overlapping 2D and 1D bands, characterizing the band structure of organic quasi 2D metals, is considered. Transfer parameter, calculated in this model to be of the order of unity, confirms the fact that the wave form of oscillations in organic metals should be quasisymmetric up to ultralow temperature. Presented theory accounts for spin-split magnetization oscillations at magnetic field directions tilted relative to the anisotropic axis of a metal. Theoretical results are compared with available experimental data on organic quasi-2D metal α-(BEDT-TTF)₂KHg(SNC)₄ explaining the appearance of clear split structure under the kink magnetic field and absence above by the corresponding change in the electron g-factor rather than cyclotron mass. Received 20 December 2000 and Received in final form 13 July 2001     

A review of: “Classical electricity and magnetism”

The article reviews the considerable progress that has been made recently in the experimental determination of the electronic structure of metals and in particular the determination of Fermi surfaces and neighbouring surfaces of constant energy. In Part I the concept of electronic structure is briefly explained and this is followed by a simple analysis of the dynamics of electron motion in a magnetic field. The geometry of the orbit carried out by an electron in real space is related to the geometry of surfaces of constant energies in k-space and it is shown how the dimensions of orbits may be inferred from size-effect and ultrasonic experiments if perfect enough samples are available. Finally the frequency of rotation of the electrons in a magnetic field is related to a differential property of the constant energy surfaces and it is shown how this frequency can be measured in experiments on cyclotron resonance.

Part II starts by considering the effect of purification of the orbital motion and it is shown that this leads to an oscillatory field dependence of the magnetic properties known as the de Haas-van Alphen effect. The conditions for practical observation of this effect are discussed and it is shown that it can provide valuable information about the electronic structure. The frequency of the oscillations gives extreme areas of the Fermi surface, the temperature dependence of amplitude gives information similar to that from cyclotron resonance, while the field dependence of amplitude gives information about scattering time. As an illustration of the experimental methods a fairly detailed account is given of the determination of the Fermi surface of copper and of the variation of electron velocity over this Fermi surface. The article concludes with a brief mention of the more complicated Fermi surfaces of a few polyvalent metals.     

de Haas-van Alphen oscillations in the underdoped high-temperature superconductor YBa2Cu3O6.5

The de Haas-van Alphen effect was observed in the underdoped cuprate YBa2Cu3O6.5 via a torque technique in pulsed magnetic fields up to 59 T. Above a field of approximately 30 T the magnetization exhibits clear quantum oscillations with a single frequency of 540 T and a cyclotron mass of 1.76 times the free electron mass, in excellent agreement with previously observed Shubnikov-de Haas oscillations. The oscillations obey the standard Lifshitz-Kosevich formula of Fermi-liquid theory. This thermodynamic observation of quantum oscillations confirms the existence of a well-defined, closed, and coherent, Fermi surface in the pseudogap phase of cuprates.     

Frequency combinations in the magnetoresistance oscillations spectrum of a linear chain of coupled orbits with a high scattering rate

The oscillatory magnetoresistance spectrum of the organic metal (BEDO)₅Ni(CN)₄. 3C₂H₄(OH)₂ has been studied up to 50 T, in the temperature range from 1.5 K to 4.2 K. In high magnetic field, its Fermi surface corresponds to a linear chain of quasi-two-dimensional orbits coupled by magnetic breakdown (MB). The scattering rate consistently deduced from the data relevant to the basic α and the MB-induced β orbits is very large which points to a significant reduction of the chemical potential oscillation. Despite of this feature, the oscillations spectrum exhibits many frequency combinations. Their effective masses and (or) Dingle temperature are not in agreement with either the predictions of the quantum interference model or the semiclassical model of Falicov and Stachowiak.     

Quantum oscillations of resistance and magnetization in the degenerate semiconductor n-HgCr2Se4

In the magnetic field range ΔH=8–60 kOe we observed and studied the anomalous oscillations in the magnetic field dependence of the resistance and magnetization of single crystals of n-HgCr₂Se₄. The absence of periodicity in 1/H in the ΔH=8–20 kOe range can be explained by the non-Fermi-liquid behavior of the electron subsystem and agrees with the theory of the de Haas-van Alphen in systems with intermediate valence. In stronger fields, ΔH=20–60 kOe, the amplitude of the fundamental harmonic decreases, with the number and amplitude of the higher-order harmonics increasing. As a result, noise is superimposed on the signal as magnetic field strength grows. The temperature dependence of the magnetization is the sum of the monotonic spin-wave contribution and the oscillating part. Zh. éksp. Teor. Fiz. 113, 1877–1882 (May 1998)     

Galvanomagnetic phenomena in layered conductors

The dependence of the resistance and the Hall field in a layered conductor with a quasi-two-dimensional electron energy spectrum of arbitrary shape on the magnitude and orientation of the magnetic field in relation to the layers is analyzed. It is found that when current flows perpendicular to the layers, the resistance of the specimen strongly depends on the angle ϑ between the normal and the vector of a strong magnetic field. The Kapitza law is shown to hold within a fairly broad range of magnetic fields in the plane of the layers, i.e., the resistance increases linearly with the magnetic field strength. The Hall field proves to be insensitive to the emergence of open sections of the Fermi surface, and the Hall constant in strong magnetic fields is the same for any orientation of the magnetic field and the current. Zh. éksp. Teor. Fiz. 112, 618–627 (August 1997)