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تسجيل مستخدم جديدMagnetocaloric effect and magnetic cooling near a field-induced quantum-critical point
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The presence of a quantum critical point (QCP) can significantly affect the thermodynamic properties of a material at finite temperatures T. This is reflected, e.g., in the entropy landscape S(T, r) in the vicinity of a QCP, yielding particularly strong variations for varying the tuning parameter r such as pressure or magnetic field B. Here we report on the determination of the critical enhancement of $ delta S / delta B$ near a B-induced QCP via absolute measurements of the magnetocaloric effect (MCE), $(delta T / delta B)_S$, and demonstrate that the accumulation of entropy around the QCP can be used for efficient low-temperature magnetic cooling. Our proof of principle is based on measurements and theoretical calculations of the MCE and the cooling performance for a Cu$^{2+}$-containing coordination polymer, which is a very good realization of a spin-1/2 antiferromagnetic Heisenberg chain - one of the simplest quantum-critical systems.
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We present a comprehensive experimental and theoretical investigation of the thermodynamic properties: specific heat, magnetization and thermal expansion in the vicinity of the field-induced quantum critical point (QCP) around the lower critical fiel
d $H_{c1} approx 2$,T in DTN . A $T^{3/2}$ behavior in the specific heat and magnetization is observed at very low temperatures at $H=H_{c1}$ that is consistent with the universality class of Bose-Einstein condensation of magnons. The temperature dependence of the thermal expansion coefficient at $H_{c1}$ shows minor deviations from the expected $T^{1/2}$ behavior. Our experimental study is complemented by analytical calculations and Quantum Monte Carlo simulations, which reproduce nicely the measured quantities. We analyze the thermal and the magnetic Gr{u}neisen parameters that are ideal quantities to identify QCPs. Both parameters diverge at $H_{c1}$ with the expected $T^{-1}$ power law. By using the Ehrenfest relations at the second order phase transition, we are able to estimate the pressure dependencies of the characteristic temperature and field scales.
In metals near a quantum critical point, the electrical resistance is thought to be determined by the lifetime of the carriers of current, rather than the scattering from defects. The observation of $T$-linear resistivity suggests that the lifetime o
nly depends on temperature, implying the vanishing of an intrinsic energy scale and the presence of a quantum critical point. Our data suggest that this concept extends to the magnetic field dependence of the resistivity in the unconventional superconductor BaFe$_2$(As$_{1-x}$P$_{x}$)$_2$ near its quantum critical point. We find that the lifetime depends on magnetic field in the same way as it depends on temperature, scaled by the ratio of two fundamental constants $mu_B/k_B$. These measurements imply that high magnetic fields probe the same quantum dynamics that give rise to the $T$-linear resistivity, revealing a novel kind of magnetoresistance that does not depend on details of the Fermi surface, but rather on the balance of thermal and magnetic energy scales. This opens new opportunities for the investigation of transport near a quantum critical point by using magnetic fields to couple selectively to charge, spin and spatial anisotropies.
Detailed anisotropic (H$parallel$ab and H$parallel$c) resistivity and specific heat measurements were performed on online-grown YbPtIn and solution-grown YbPt$_{0.98}$In single crystals for temperatures down to 0.4 K, and fields up to 140 kG; H$paral
lel$ab Hall resistivity was also measured on the YbPt$_{0.98}$In system for the same temperature and field ranges. All these measurements indicate that the small change in stoichiometry between the two compounds drastically affects their ordering temperatures (T$_{ord}approx3.4$ K in YbPtIn, and $sim2.2$ K in YbPt$_{0.98}$In). Furthermore, a field-induced quantum critical point is apparent in each of these heavy fermion systems, with the corresponding critical field values of YbPt$_{0.98}$In (H$^{ab}_c$ around 35-45 kG and H$^{c}_capprox120$ kG) also reduced compared to the analogous values for YbPtIn (H$^{ab}_capprox60$ kG and H$^{c}_c>140$ kG)
Precision measurements of the Hall effect have been carried out for both archetypal heavy fermion compound - CeCu6 and exemplary solid solutions CeCu6-xAux (x= 0.1 and 0.2) with quantum critical behavior. The experimental results have been obtained b
y technique with a sample rotation in magnetic field in the temperature range 1.8-300K. The experiment revealed a complex activation type dependence of the Hall coefficient RH(T) in CeCu6 with activation energies Ea1/kB = 110K and Ea2/kB = 1.5K in temperature ranges 50-300K and 3-10K, respectively. Microscopic parameters of charge carriers transport (effective masses, relaxation time) and localization radii ap1,2* of heavy fermions (ap1*(T>50K)= 1.7 A and ap2*(T<20K)= 14 A) were estimated for CeCu6. The second angular harmonic contribution has been established in the Hall voltage of CeCu5.9Au0.1 and CeCu6 at temperatures below T*=24K. A hyperbolic type divergence of the second harmonic term in Hall effect RH2(T)= C(1/T-1/T*) at low temperatures is found to be accompanied with a power-law behavior RH(T)= T -0.4 of the main contribution in the Hall coefficient for CeCu5.9Au0.1 compound with quantum critical behavior.
The criticality-enhanced magnetocaloric effect (MCE) near a field-induced quantum critical point (QCP) in the spin systems constitutes a very promising and highly tunable alternative to conventional adiabatic demagnetization refrigeration. Strong flu
ctuations in the low-$T$ quantum critical regime can give rise to a large thermal entropy change and thus significant cooling effect when approaching the QCP. In this work, through efficient and accurate many-body calculations, we show there exists a significant inverse MCE(iMCE) in the spin-1 quantum chain materials(CH$_3$)$_4$NNi(NO$_2$)$_3$ (TMNIN) and NiCl$_2$-4SC(NH$_2$)$_2$ (DTN), where DTN has substantial low-$T$ refrigeration capacity while requiring only moderate magnetic fields. The iMCE characteristics, including the adiabatic temperature change $Delta T_{rm ad}$, isothermal entropy change $Delta S$, differential Gruneisen parameter, and the entropy change rate, are obtained with quantum many-body calculations at finite temperature. The cooling performance, i.e., the efficiency factor and hold time, of the two compounds is also discussed. Based on the many-body calculations on realistic models for the spin-chain materials, we conclude that the compound DTN constitutes a very promising and highly efficient quantum magnetic coolant with pronounced iMCE properties. We advocate that such quantum magnets can be used in cryofree refrigeration for space applications and quantum computing environments.
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