Abstract
Randomness and frustration are believed to be two crucial criteria for the formation of spin glass state. However, the spin freezing occurs in some well-ordered crystals below the related ...temperature
T
f
due to the instability of each spin state, which induces the variation of either magnetic moment value or exchange energy. Here we explore the new mechanism of the in-site originated disorder in antiferromagnets Gd
0.73
La
0.27
B
6
and GdB
6
, which is caused by the random mutual shifts of Gd
3+
spins from the centrally symmetrical positions in the regular cubic lattice. The universal scaling of ESR linewidth temperature dependencies to the power law
ΔH
(
T
) ~ ((
T
−
T
D
)
/T
D
)
α
with
α
= − 1.1 ± 0.05 in the paramagnetic phase of both compounds demonstrates the identity of the origin of magnetic randomness. In Gd
0.73
La
0.27
B
6
the resulting random spin configurations freeze at
T
f
≈ 10.5 K where the maximum of magnetization is observed. Below
T
f
the splitting of ZFC and FC magnetization curves takes place as well as the magnetic state depends on the antecedent sample history. In the case of GdB
6
the coherent displacement of Gd ions compete with these random shifts forming an antiferromagnetic (AFM) phase at
T
N
= 15.5 K, which prevails over the spin freezing at
T
f
≈ 13 K, expected from the ESR data. The observation of the hysteresis of the ESR spectrum in the AFM phase suggests that its properties may be determined by the competition of two types of AFM orders, which results in formation of stable magnetic domains with nonequivalent positions of AFM Gd pairs at
T
< 10 K.
Full text
Available for:
IZUM, KILJ, NUK, PILJ, PNG, SAZU, UL, UM, UPUK
Abstract
Introducing of topological insulator concept for fluctuating valence compound – samarium hexaboride – has recently initiated a new round of studies aimed to clarify the nature of the ground ...state in this extraordinary system with strong electron correlations. Here we discuss the data of magnetic resonance in the pristine single crystals of SmB
6
measured in 60 GHz cavity experiments at temperatures 1.8–300 K. The microwave study as well as the DC resistivity and Hall effect measurements performed for the different states of SmB
6
110 surface prove definitely the existence of the layer with metallic conductivity increasing under lowering temperature below 5 K. Four lines with the g-factors g ≈ 2 are found to contribute to the ESR-like absorption spectrum that may be attributed to intrinsic paramagnetic centers on the sample’s surface, which are robust with respect to the surface treatment. The temperature dependence of integrated intensity
I
(
T
) for main paramagnetic signal is found to demonstrate anomalous critical behavior
I
(
T
) ~ (
T
*
−
T
)
ν
with characteristic temperature
T
*
= 5.34 ± 0.05 K and exponent ν = 0.38 ± 0.03 indicating possible magnetic transition at the SmB
6
110 surface. Additional resonant magnetoabsorption line, which may be associated with either donor-like defects or cyclotron resonance mode corresponding to the mass
m
c
~ 1.2
m
0
, is reported.
Full text
Available for:
IZUM, KILJ, NUK, PILJ, PNG, SAZU, UL, UM, UPUK
The magnetic properties of the topological Kondo insulator SmB
6
are studied in the temperature range
T
< 10 K in an applied magnetic field up to 5 T. The analysis of the nonlinear part of the field ...dependence of the magnetization indicates that the nonlinear contribution to the magnetization at low temperatures is in general related not only to localized magnetic moments but also to the Pauli paramagnetism of surface electrons. It has been shown for the first time that a decrease in temperature in the range of
T*
∼ 5.5–6 K gives rise to the crossover between two modes determining the magnetic properties of SmB
6
. In the range
T
>
T
*, the magnetization qualitatively corresponds to the model of decay of Kondo singlets and is determined by the Pauli paramagnetism, which includes both the linear and nonlinear contributions. In the range
T
<
T
*, the Pauli paramagnetism becomes supplemented in the threshold manner by the contribution of localized magnetic moments. The performed estimate of effective magnetic moments arising at
T
<
T
* gives anomalously high values μ* ∼ (7–14) μ
B
ion, far exceeding the values μ* ∼ (3–5) μ
B
expected for a separate magnetic Sm
3+
ion. This is a signature of the possible spin-polaron nature of localized magnetic moments in the topological Kondo insulator SmB
6
.
Full text
Available for:
DOBA, EMUNI, FIS, FZAB, GEOZS, GIS, IJS, IMTLJ, IZUM, KILJ, KISLJ, MFDPS, NLZOH, NUK, OBVAL, OILJ, PILJ, PNG, SAZU, SBCE, SBJE, SBMB, SBNM, SIK, UILJ, UKNU, UL, UM, UPUK, VKSCE, ZAGLJ
Antiferromagnetic resonance in GdB6 exhibits a series of anomalous features such as jump of the resonance position at TN = 15.5 K and resonance evolution to four line structure with further ...temperature decrease. Here we report the study of temperature behaviour of antiferromagnetic resonance modes v(H0) in AFM phase. We analyzed v(H0) dependencies at several temperatures T < TN within the model of an antiferromagnet with easy anisotropy axis. In the result we have demonstrated that AFMR behaviour is determined by two following factors: (i) abrupt change of the gyromagnetic ratio γ at TN, which remains constant (corresponding to g-factor g = 1.27) in AFM phase and (ii) jump of zero field frequency v0 (from v0 ≈ 0 to v0 ≈ 20 GHz at TN with its further growth to v0 ≈ 35 GHz at T = 1.9 K.
The article presents the results of research in the field of computational X-ray tomography, obtained within the framework of the scientific school, by Doctor of Engineering, Corresponding Member of ...the Russian Academy of Sciences V.L. Arlazarov, on artificial intelligence. The field of computed tomography, which is relatively young for the school, arose as a result of a combination of combinatorial optimization approaches, training neural network models, image processing, and solving the problem of stopping information acquisition. Thanks to the accumulated experience, four areas of research can be distinguished: fast algorithms for tomographic reconstruction, scanning protocols for monitored tomographic reconstruction, neural network approaches to reconstruction, as well as methods for suppressing artifacts and distortions that occur in tomographic reconstructions. This article reflects the main successes and achievements obtained in these areas, which are demonstrated using the example of specific applied solutions.
Full text
Available for:
EMUNI, FIS, FZAB, GEOZS, GIS, IJS, IMTLJ, KILJ, KISLJ, MFDPS, NLZOH, NUK, OILJ, PNG, SAZU, SBCE, SBJE, SBMB, UKNU, UL, UM, UPUK, VKSCE, ZAGLJ
Abstract
Electron spin resonance (ESR) in strongly correlated metals is an exciting phenomenon, as strong spin fluctuations in this class of materials broaden extremely the absorption line below the ...detection limit. In this respect, ESR observation in CeB
6
provides a unique chance to inspect Ce
3+
magnetic state in the antiferroquadrupole (AFQ) phase. We apply the original high frequency (60 GHz) experimental technique to extract the temperature and angular dependences of
g
-factor, line width and oscillating magnetization. Experimental data show unambiguously that the modern ESR theory in the AFQ phase considering the Γ
8
ground state of Ce
3+
ion completely fails to predict both the
g
-factor magnitude and its angular dependence. Alignment of the external magnetic field along 100 axis induces a strong (more than twofold) broadening of ESR line width with respect to the other crystallographic directions and results also in the anomalous temperature dependences of the
g
-factor and oscillating magnetization. In this experimental geometry the latter parameter surprisingly exceeds total static magnetization by 20% at
T
* ~ 2.5 K. We argue that the unusual physical picture of ESR in CeB
6
may be strongly affected by spin fluctuations and dynamic collective effects predominantly pronounced in 100 direction.
Full text
Available for:
IZUM, KILJ, NUK, PILJ, PNG, SAZU, UL, UM, UPUK
The low temperature (
T
< 5 K) electron spin resonance in topological Kondo insulator SmB
6
is investigated. It is shown that spin relaxation time of the paramagnetic centers responsible for low ...temperature electron spin resonance in SmB
6
is about ~ 10
−8
s, which exceeds by five orders of magnitude the widely accepted spin and charge fluctuation time ~ 10
−13
s in the homogeneous mixed valence state of this material. This discrepancy may be attributed to the formation of the specific paramagnetic centers in the sample surface–spin polarons.
Full text
Available for:
EMUNI, FIS, FZAB, GEOZS, GIS, IJS, IMTLJ, KILJ, KISLJ, MFDPS, NLZOH, NUK, OBVAL, OILJ, PNG, SAZU, SBCE, SBJE, SBMB, SBNM, UKNU, UL, UM, UPUK, VKSCE, ZAGLJ
The monitored tomographic reconstruction (MTR) with optimized photon flux technique is a pioneering method for X-ray computed tomography (XCT) that reduces the time for data acquisition and the ...radiation dose. The capturing of the projections in the MTR technique is guided by a scanning protocol built on similar experiments to reach the predetermined quality of the reconstruction. This method allows achieving a similar average reconstruction quality as in ordinary tomography while using lower mean numbers of projections. In this paper, we, for the first time, systematically study the MTR technique under several conditions: reconstruction algorithm (FBP, SIRT, SIRT-TV, and others), type of tomography setup (micro-XCT and nano-XCT), and objects with different morphology. It was shown that a mean dose reduction for reconstruction with a given quality only slightlyvaries with choice of reconstruction algorithm, and reach up to 12.5 % in case of micro-XCT and 8.5 % for nano-XCT. The obtained results allow to conclude that the monitored tomographic reconstruction approach can be universally combined with an algorithm of choice to perform a controlled trade-off between radiation dose and image quality. Validation of the protocol on independent common ground truth demonstrated a good convergence of all reconstruction algorithms within the MTR protocol.
The temperature dependence of the conductivity of the topological Kondo-insulator (TKI) Sm
1 –
x
Yb
x
B
6
is investigated in the temperature range 2 <
T
< 300 K for compositions with
x
≤ 0.024. It is ...found that the ytterbium impurity most strongly affects the low-temperature (
T
< 20 K) electrical conductivity: when changing from
x
= 0 to
x
= 0.024, the activation energy of bulk conductivity decreases by 1.8 times from 4 to 2.2 meV, and the 2D surface conductivity increases by six times. The separation made for the contributions to the conductivity enables us to establish that the one-parameter scaling model can be used for describing the conductivity of 2D surface states in the Sm
1 –
x
Yb
x
B
6
TKI in which both the interaction with phonons and the electron–electron scattering effects are taken into account.
Full text
Available for:
DOBA, EMUNI, FIS, FZAB, GEOZS, GIS, IJS, IMTLJ, IZUM, KILJ, KISLJ, MFDPS, NLZOH, NUK, OBVAL, OILJ, PILJ, PNG, SAZU, SBCE, SBJE, SBMB, SBNM, SIK, UILJ, UKNU, UL, UM, UPUK, VKSCE, ZAGLJ
It is shown that the magnetic phase diagram of substitutional solid solutions exhibits two quantum critical points: , which corresponds to the disappearance of long-range spiral magnetic order, and , ...which marks the suppression of the phase with short-range magnetic order. It is established that the microscopic reason for the observed complicated quantum critical behavior is the evolution of the Fermi surface structure driven by an increase in iron concentration.
PDF
