ВЗАИМОДЕЙСТВИЕ СТОЛБЧАТЫХ ФУНДАМЕНТОВ С МЁРЗЛЫМ ГРУНТОМ

The frozen soils’ stress-strain state varies with time due to the internal soil rheological processes. Those processes become active within the thawing period of the active soil layer causing increase in settlement of the engineering structures’ foundations. Hence, creep processes and thawing of frozen soils should be considered when designing the transportation facilities for regions of the Far North and Siberia.The objective of the research is to develop a procedure for evaluating the variation in time of the stress-strain state of the frozen soil under the post footing of a bridge pier’s foundation considering the frozen soil creep and thawing. The interaction of the bridge pier post foundations and frozen silt-loam soil is modelled and studied. The research is based on the example of an existing overpass over the M‑56 Lena motor road situated at Amga–Samyrdah stage of Tommot–Yakutsk section of the Berkakit–Tommot–Yakutsk railway line. This overpass has piers with post foundations. The above railway line is in the area of hard frozen soils.The study focuses on changes in principal normal compressive stresses with the course of time, as well as on the frozen soil movements under the post footing. The time allotted for the above system behavior study is limited to five months. There are two design cases: a) considering the frozen soil thawing up to a depth range of 1,5 to 4 m; b) without considering the frozen soil thawing.The research has shown that the thawing of the frozen soil up to a comparatively low depth as compared to natural level results in a significant increase (by 2÷2,5 times) in the values of post foundation settlement as compared with the design case without thawing. At the same time, it was found that small values of thawing have a subtle effect on the frozen soil’s stress state under the post footing. Besides, all reviewed design cases (with / without thawing revealed that stress of the frozen soil under the post footing decreases with time (stress relaxation).Известно, что напряжённо-деформированное состояние мёрзлых грунтов изменяется во времени вследствие протекающих в них реологических процессов [1; 2]. Указанные процессы активизируются в период оттаивания деятельного слоя грунта, что приводит к увеличению осадки фундаментов искусственных сооружений. Вследствие этого учёт процессов ползучести и оттаивания мёрзлых грунтов является важной задачей при проектировании транспортных конструкций в районах Крайнего Севера и Сибири.Целью исследования является развитие методики оценки изменения во времени напряжённо-деформированного состояния в мёрзлом грунте под подошвой столба фундамента мостовой опоры c учётом ползучести и оттаивания мёрзлого грунта.Моделируется и исследуется взаимодействие столбчатых фундаментов мостовых опор c пылеватым глинистым мёрзлым грунтом. Исследование проводится на примере реально существующего путепровода через автодорогу М‑56 «Лена», находящегося на перегоне Амга–Самырдах железнодорожного участка Томмот–Якутск железнодорожной линии Беркакит–Томмот–Якутск и имеющего опоры с фундаментами столбчатого типа. Указанная железнодорожная линия расположена в районе твердомёрзлых грунтов.Исследуется изменение с течением времени главных сжимающих нормальных напряжений, а также перемещений в мёрзлом грунте под подошвой столба. Отрезок времени, в течение которого исследуется поведение вышеописанной системы, ограничен пятью месяцами. Рассматриваются два расчётных варианта: а) с учётом оттаивания мёрзлого грунта на различные величины (от 1,5 м до 4 м); б) без учёта оттаиваниямёрзлого грунта.Показано, что оттаивание мёрзлого грунта даже на сравнительно небольшую величину от уровня природного рельефа приводит к существенному увеличению значений осадки столбчатого фундамента по сравнению с расчётным вариантом, когда оттаивание отсутствует (в 2÷2,5 раза). Вместе с тем установлено, что небольшие величины оттаивания оказывают малозаметное влияние на напряжённое состояние в мёрзлом грунте под подошвой столба. Кроме того, во всех рассмотренных расчётных вариантах (с учётом и без учёта оттаивания) зафиксировано снижение напряжений в мёрзлом грунте под подошвой столба с течением времени (релаксация напряжений).

Using the de Haas-van Alphen effect to map out the closed three-dimensional Fermi surface of natural graphite

The Fermi surface of graphite has been mapped out using de Haas van Alphen (dHvA) measurements at low temperature with in-situ rotation. For tilt angles $\theta>60^{\circ}$ between the magnetic field and the c-axis, the majority electron and hole dHvA periods no longer follow the $\cos(\theta)$ behavior demonstrating that graphite has a 3 dimensional closed Fermi surface. The Fermi surface of graphite is accurately described by highly elongated ellipsoids. A comparison with the calculated Fermi surface suggests that the SWM trigonal warping parameter $\gamma_3$ is significantly larger than previously thought

Magnetoresistance oscillations in GaAs/AlGaAs superlattices subject to in-plane magnetic fields

The MBE-grown GaAs/AlGaAs superlattice with Si-doped barriers has been used to study a 3D-2D transition under the influence of the in-plane component of applied magnetic field. The longitudinal magnetoresistance data measured in tilted magnetic fields have been interpreted in terms of a simple tight-binding model. The data provide values of basic parameters of the model and make it possible to reconstruct the superlattice Fermi surface and to calculate the density of states for the lowest Landau subbands. Positions of van Hove singularities in the DOS agree excellently with magnetoresistance oscillations, confirming that the model describes adequately the magnetoresistance of strongly coupled semiconductor superlattices.Comment: 4 pages, 3 figures, elsart/PHYEAUTH macros; presented on the EP2DS-16 Conference in Albuquerque, New Mexico USA. To be published in Physica

Quasi-two-dimensional Fermi surfaces of the heavy-fermion superconductor Ce2_2PdIn8_8

We report low-temperature de Haas-van Alphen (dHvA) effect measurements in magnetic fields up to 35 T of the heavy-fermion superconductor Ce$_2$PdIn$_8$. The comparison of the experimental results with band-structure calculations implies that the 4$f$ electrons are itinerant rather than localized. The cyclotron masses estimated at high field are only moderately enhanced, 8 and 14 $m_0$, but are substantially larger than the corresponding band masses. The observed angular dependence of the dHvA frequencies suggests quasi-two-dimensional Fermi surfaces in agreement with band-structure calculations. However, the deviation from ideal two dimensionality is larger than in CeCoIn$_5$, with which Ce$_2$PdIn$_8$ bears a lot of similarities. This subtle distinction accounts for the different superconducting critical temperatures of the two compounds.Comment: accepted to Phys. Rev.

Specific heat of heavy fermion CePd2Si2 in high magnetic fields

We report specific heat measurements on the heavy fermion compound CePd2Si2 in magnetic fields up to 16 T and in the temperature range 1.4-16 K. A sharp peak in the specific heat signals the antiferromagnetic transition at T_N ~ 9.3 K in zero field. The transition is found to shift to lower temperatures when a magnetic field is applied along the crystallographic a-axis, while a field applied parallel to the tetragonal c-axis does not affect the transition. The magnetic contribution to the specific heat below T_N is well described by a sum of a linear electronic term and an antiferromagnetic spin wave contribution. Just below T_N, an additional positive curvature, especially at high fields, arises most probably due to thermal fluctuations. The field dependence of the coefficient of the low temperature linear term, gamma_0, extracted from the fits shows a maximum at about 6 T, at the point where an anomaly was detected in susceptibility measurements. The relative field dependence of both T_N and the magnetic entropy at T_N scales as [1-(B/B_0)^2] for B // a, suggesting the disappearance of antiferromagnetism at B_0 ~ 42 T. The expected suppression of the antiferromagnetic transition temperature to zero makes the existence of a magnetic quantum critical point possible.Comment: to be published in Journal of Physics: Condensed Matte

Anderson lattice with explicit Kondo coupling: general features and the field-induced suppression of heavy-fermion state in ferromagnetic phase

We apply the extended (statistically-consistent, SGA) Gutzwiller-type approach to the periodic Anderson model (PAM) in an applied magnetic field and in the strong correlation limit. The finite-U corrections are included systematically by transforming PAM into the form with Kondo-type interaction and residual hybridization, appearing both at the same time. This effective Hamiltonian represents the essence of \textit{Anderson-Kondo lattice model}. We show that in ferromagnetic phases the low-energy single-particle states are strongly affected by the presence of the applied magnetic field. We also find that for large values of hybridization strength the system enters the so-called \textit{locked heavy fermion state}. In this state the chemical potential lies in the majority-spin hybridization gap and as a consequence, the system evolution is insensitive to further increase of the applied field. However, for a sufficiently strong magnetic field, the system transforms from the locked state to the fully spin-polarized phase. This is accompanied by a metamagnetic transition, as well as by drastic reduction of the effective mass of quasiparticles. In particular, we observe a reduction of effective mass enhancement in the majority-spin subband by as much as 20% in the fully polarized state. The findings are consistent with experimental results for Ce$_x$La$_{1-x}$B$_6$ compounds. The mass enhancement for the spin-minority electrons may also diminish with the increasing field, unlike for the quasiparticles states in a single narrow band in the same limit of strong correlations

Specific heat of single crystal MgB_2: a two-band superconductor with two different anisotropies

Heat-capacity measurements of a 39 microgramm MgB_2 single crystal in fields up to 14 T and below 3 K allow the determination of the low-temperature linear term of the specific heat, its field dependence and its anisotropy. Our results are compatible with two-band superconductivity, the band carrying the small gap being isotropic, that carrying the large gap having an anisotropy of ~ 5. Three different upper critical fields are thus needed to describe the superconducting state of MgB2.Comment: 4 pages, 4 figures - V2: Bibliography updated and some typo corrected. One reference added - V3: version accepted for publication in PRL, changes made in the tex

Magnetic oscillations in a two-dimensional network of compensated electron and hole orbits

The FS of (ET)8Hg4Cl12(C6H5Br)2 can be regarded as a 2D network of compensated electron and hole orbits coupled by magnetic breakthrough. Simultaneous measurements of the interlayer magnetoresistance and magnetic torque have been performed up to 28 T. Magnetoresistance and de dHvA oscillations spectra exhibit frequency combinations typical of such a network. Even though some of the observed magnetoresistance oscillations cannot be interpreted on the basis of neither conventional SdH oscillations nor quantum interference, the temperature and magnetic field (both orientation and magnitude) dependence of all the Fourier components of the dHvA spectra can be consistently accounted for by the LK formula. This behaviour is at variance with that currently reported for compounds illustrating the linear chain of coupled orbits model.Comment: accepted for publication in europhysics Letter

Magnetic Breakdown in the electron-doped cuprate superconductor Nd2−x_{2-x}Cex_xCuO4_4: the reconstructed Fermi surface survives in the strongly overdoped regime

We report on semiclassical angle-dependent magnetoresistance oscillations (AMRO) and the Shubnikov-de Haas effect in the electron-overdoped cuprate superconductor Nd$_{2-x}$Ce$_x$CuO$_4$. Our data provide convincing evidence for magnetic breakdown in the system. This shows that a reconstructed multiply-connected Fermi surface persists, at least at strong magnetic fields, up to the highest doping level of the superconducting regime. Our results suggest an intimate relation between translational symmetry breaking and the superconducting pairing in the electron-doped cuprate superconductors.Comment: 5 pages, 4 figures, submitted to PR