Schottky-like Anomaly Research Articles

The Effect of Magnetic Field on the Thermodynamic Properties of Three-Dimensional GaAs Quantum Dot

The problem of parabolically single three-dimensional semiconductor GaAs quantum dot in the presence of an external magnetic field and including the spin of the electron is studied using canonical formalism. The energy spectrum is obtained as a function of temperature and magnetic field. It is observed that the system tends to absorb the largest amount of heat at critical values of the magnetic field which depends on the temperature. A schottky-like anomaly in heat capacity is observed at low temperature, while at high temperature it converges rapidly and saturates. It was also found that the entropy is inversely proportional to the magnetic field strength, also entropy showed exciting behavior at low temperature, a shoulder is observed which becomes curvier when the magnetic field decreases. However, at high temperature, entropy increases monotonically with temperature and becomes essentially independent of the magnetic field. The substantial effect of magnetic field on the magnetic properties of quantum dot is observed at low temperature. It is shown that, magnetization increases significantly at low temperature and shows strong dependence on magnetic field, while its magnitude increases as the temperature increases and reduces to zero regardless the value of magnetic field strength. In addition, at low temperature susceptibility peaks and is found to be paramagnetic and at high temperature the diamagnetic state is found to be a favored state of the system and susceptibility becomes completely independent of magnetic field. It is also shown that for a fixed value of temperature, susceptibility increases rapidly with magnetic field and beyond a critical value of magnetic field, the increase becomes much slower, susceptibility exhibits a crossing behavior, and magnetization shows a significant drop as the magnetic field increases. So far, no study has been made in literature on the effect of magnetic field on the thermal and magnetic properties of three-dimensional quantum dot.

Thermodynamic signatures of short-range magnetic correlations in UTe2

The normal-state out of which unconventional superconductivity in UTe$_2$ emerges is studied in detail using a variety of thermodynamic and transport probes. Clear evidence for a broad Schottky-like anomaly with roughly R ln 2 entropy around $T^{*} \approx 12$K is observed in all measured quantities. Comparison with high magnetic field transport data allows the construction of an $H\text{-}T$ phase diagram resembling that of the ferromagnetic superconductor URhGe. The low field electronic Gr\"uneisen parameter of $T^{*}$ and that of the metamagnetic transition at $H_m \approx 35$T are comparable pointing to a common origin of both phenomena. Enhanced Wilson and Korringa ratios suggests that the existence of short range ferromagnetic fluctuations cannot be ruled out.

Structural and physical properties of trilayer nickelates R4Ni3O10 ( R=La , Pr, and Nd)

We investigate the low temperature structural and physical properties of the trilayer nickelates R4Ni3O10 (R = La, Pr and Nd) using resistivity, thermopower, thermal conductivity, specific heat, high-resolution synchrotron powder X-ray diffraction and thermal expansion experiments. We show that all three compounds crystallize with a monoclinic symmetry, and undergo a metal-to-metal (MMT) transition at 135 K (La), 156 K (Pr) and 160 K (Nd). At MMT, the lattice parameters show distinct anomalies; however, without any lowering of the lattice symmetry. Unambiguous signatures of MMT are also seen in magnetic and thermal measurements, which suggest a strong coupling between the electronic, magnetic and structural degrees of freedom in these nickelates. Analysis of thermal expansion yields hydrostatic pressure dependence of MMT in close agreement with experiments. We show that the 9-fold coordinated Pr ions in the rocksalt (RS) layers have a crystal field (CF) split doublet ground state with possible antiferromagnetic ordering at 5 K. The Pr ions located in the perovskite block (PB) layers with 12-fold coordination, however, exhibit a non-magnetic singlet ground state. The CF ground state of Nd in both RS and PB layers is a Kramers doublet. Heat capacity of R = Nd shows a Schottky-like anomaly near35 K, and an upturn below T = 10 K suggesting the presence of short-range correlations between the Nd moments. However, no signs of long-range ordering could be found down to 2 K despite a sizeable theta_p ~ -40 K. The strongly suppressed magnetic long-range ordering in both R = Pr and Nd suggests the presence of strong magnetic frustration in these compounds. The low-temperature resistivity shows a T^0.5 dependence. No evidence for the heavy fermion behavior could be found in any of the three compounds.

Dynamical splitting of cubic crystal field levels in rare-earth cage compounds

The Crystalline Electric Field (CEF) influence is usually described by considering an ideally symmetrical rare-earth site. In the case of cage compounds, ample excursions of the rare-earth inside the cage require an adapted CEF description. A corrective, position dependent, CEF term accounts for the deviation from the perfect symmetry. In the paramagnetic range, a CEF level with orbital degeneracy thus acquires a width reflecting the rare-earth spatial distribution. In the case, frequent in cubic systems, of an orbitally degenerate CEF ground state at the center, this width introduces an additional energy scale, influential at low temperature. A spherical simplification allows to identify the major consequences of a cage-split ground multiplet: a Schottky-like anomaly appears in the specific heat with associated reduction of the magnetic entropy and alteration of the magnetic properties. Concomitantly, a centrifugal Jahn-Teller effect develops that expands the distribution of the magnetic ion and softens the rattling phonons. These effects are confronted with anomalous paramagnetic properties of rare-earth cage compounds, notably rare-earth filled skutterudites and hexaborides.

Thermodynamic properties of quadrupolar states in the frustrated pyrochlore magnet Tb2Ti2O7

The low-temperature thermodynamic properties of the frustrated pyrochlore Tb2+xTi2−xO7+y have been studied using the single crystal of x = 0.005 sitting in a long range ordered phase in the x–T phase diagram. We observed that the specific heat exhibits a minimum around 2 K and slightly increases on cooling, similar to a Schottky-like anomaly for canonical spin ices. A clear specific-heat peak observed at Tc = 0.53 K is ascribable to the phase transition to a quadrupolar state, which contributes to a relatively large change in entropy, S ≃ 2.7 J K−1mol−1. However, it is still smaller than R ln 2 for the ground state doublet of the Tb ions. The entropy release persists to higher temperatures, suggesting strong fluctuations associated with spin ice correlations above Tc. We discuss the field dependence of the entropy change for H ‖ [111] and H ‖ [001].

Thermodynamic properties of superconducting and non-superconducting Pr2Ba4Cu7O[formula omitted] compounds with metallic double chains

To examine the thermodynamic properties of Pr2Ba4Cu7O15-δ compounds with metallic CuO double chains, we measured the specific heats of superconducting and non-superconducting polycrystalline samples at low temperatures (1.8–40 K) under various magnetic fields (up to 9 T). In the as-sintered non-superconducting sample, a λ-like enhancement in the specific heat measurement appeared near the antiferromagnetic transition temperature TN=17 K. In contrast, the reduced superconducting sample with Tcon=26.5 K exhibited no obvious superconducting anomaly in its specific heat data, but a Schottky-like broad maximum appeared at low temperatures. The Schottky-like anomaly was attributed to low-lying quasi-triplet splitting of Pr3+ ions under the crystal field effect.

Characteristics of a granular electronic system in Heusler-type Fe2+xV1−xAl

We report comprehensive measurements of the magnetic, transport, and thermal properties of the Heusler-type compound Fe2+xV1−xAl, with x values near the ferromagnetic quantum critical point, xc ∼ 0.05. At T ∼ 60 K, a prominent Schottky-like anomaly appeared in the specific heat; this anomaly was correlated with a smaller pseudo-gap formation in magnetic susceptibility, magnetoresistance, and thermoelectric power. Furthermore, a magnetic anomaly observed in the magnetic susceptibility and resistivity at T ∼ 4 K was suppressed significantly by applying a magnetic field. A magnetically inhomogeneous phase arose below T ∼ 60 K, which appeared to consist of ferromagnetic and paramagnetic clusters.

Thermodynamic Measurements of Fe-Rh Alloys

FeRh undergoes an unusual antiferromagnetic-to-ferromagnetic (AFM-FM) transition just above room temperature (T(AFM>FM)) that can be tuned or even completely suppressed with small changes in composition. The underlying temperature-dependent entropy difference between the competing AFM and FM states that drives this transition is measured by specific heat as a function of temperature from 2 to 380 K on two nearly equiatomic epitaxial Fe-Rh films, one with a ferromagnetic ground state (Fe-rich) and the other with an antiferromagnetic ground state (Rh-rich). The FM state shows an excess heat capacity near 100 K associated with magnetic excitations that are not present in the AFM state. The integrated entropy and enthalpy differences between the two alloys up to T(AFM>FM) agree with the previously measured entropy of the transition (ΔS = 17 ± 3 J/kg/K) and yield a T=0 energy difference of 3.4 J/g, consistent with literature calculations and experimental data; this agreement supports the use of the Fe-rich FM sample as a proxy for the (unstable) FM state of the AFM Rh-rich sample. From the low-temperature specific heat, along with sound velocity and photoemission measurements, the lattice contribution to the difference (ΔS(latt) = -33 ± 9 J/kg/K) and electronic contribution (ΔS(el) = 8 ± 1 J/kg/K) to the difference in entropy are calculated, from which the excess heat capacity in the FM phase and the resulting entropy difference are shown to be dominated by magnetic fluctuations (ΔS(mag) = 43 ± 9 J/kg/K). The excess magnetic heat capacity is dominated by the magnetic heat capacity of the FM phase, which can be fit to a Schottky-like anomaly with an energy splitting of 16 ± 1 meV and a multiplicity of 1 per unit cell.

The Nd–Mn exchange interaction, low temperature specific heat and magnetism of Nd2/3Ca1/3MnO3

The low temperature specific heat and magnetic characteristics of Nd2/3Ca1/3MnO3 perovskite are studied in a wide range of magnetic fields (up to 9T). Temperature dependent specific heat data show a broadened Schottky-like anomaly below 20K caused by splitting of the Nd3+ ions ground-state doublet in the effective molecular field Hex, determined by exchange interaction between Nd and Mn spin systems supplemented by an applied external magnetic field. Existence of the splitting at zero magnetic field and expressed field dependence is the evidence of a strong exchange coupling between Nd and Mn magnetic subsystems. The Nd-ions magnetic ordering leads to an additional contribution to the magnetic moment of the system below 30K, producing anomalies of the magnetic loss and field-cooled and zero-field-cooled magnetizations. The observed broadened Schottky-like anomalies are fitted for each applied magnetic field by the sum of three Schottky functions. Applied magnetic field extends the anomaly region and shifts it to higher temperatures. Splitting of the higher crystal field Kramers doublets gives an additional contribution to the heat capacity in magnetic fields. The ground state doublet g-factors g|| and g⊥ were estimated to be 3.4 and 2.2, respectively, and Hex was estimated to be 9T. The Nd3+ ions magnetic moment estimated from the magnetization data agrees with the value obtained from the specific heat data.

Crystal-field and Nd-Mn exchange interaction in Nd2/3Ca1/3MnO3

A study of the low field magnetization and specific heat in magnetic fields up to 9 T of Nd2/3Ca1/3MnO3 perovskite in the 2-30 K temperature range has been done. All the specific heat data show broadened Schottky-like anomaly below 20 K. We suppose that such a behavior originates from the Nd magnetic ordering caused by the splitting of the Nd3+ ions ground-state doublet (GSD) in the effective molecular field Hex of Mn spin system supplemented by an applied external magnetic field. The zero field GSD splitting is an evidence of a strong exchange coupling between Nd and Mn magnetic subsystems. The Nd-ions magnetic ordering introduces an additional contribution to the ferromagnetic moment producing anomalies of the field-cooled and zero-field-cooled magnetizations of the system below 28 K. The broadened Schottky-like anomalies found are fitted for every field by a set of three Schottky functions. Applied magnetic field extends the anomaly region and shifts it to the higher temperatures. Splitting of the higher crystal field Kramers doublets gives an additional contribution to the heat capacity under magnetic fields. The GSD g-factors g|| and g⊥ was estimated as 3.4 and 2.2, respectively, and Hex as 9 T.

Low temperature heat capacity of Nd2/3Ca1/3MnO3 and (Nd0.9 Y0.1)2/3Ca1/3MnO3 manganites: transformation of the Schottky-like anomaly

Effect of Y-doping on interactions in the spin system of the colossal magnetoresistance compound Nd2/3Ca1/3MnO3 has been studied by the thermal relaxation heat capacity and neutron diffraction techniques in the 2 — 30 K temperature range. In 5 K temperature region a broadened Schottky-like specific heat anomaly Csch(T) has been foDnd both in the parent Nd2/3Ca1/3MnO3 and in the doped (Nd0.9Y0.1)2/3Ca1/3MnO3 compounds. The experimental data were successfully fitted by a set of three two-level Schottky contributions, implying a broadening of the exited level in the system with randomly distributed Y ions. Replacement of 10 % Nd ions by nonmagnetic Y ions leads to the about 10 % enlargement of the splitting of the ground state doublet of Nd3+ ion.

Low temperature transport and specific heat studies ofNd1−xPbxMnO3 single crystals

Electrical transport and specific heat properties ofNd1−xPbxMnO3 singlecrystals for 0.15≤x≤0.5 have been studied in the low temperature regime. The resistivity in the ferromagnetic insulating (FMI)phase for x≤0.3 has an activated character. The dependence of the activation gapΔ ondoping x has been determined and the critical concentration for thezero-temperature metal–insulator transition is determined asxc≈0.33. For a metallicsample with x = 0.42, a conventional electron–electron (e–e) scattering term is found in the low temperature electrical resistivity, although the Kadowaki–Woodsratio is found to be much larger for this manganite than for a normal metal.There is a resistivity minimum observed around 60 K for a metallic sample withx = 0.5. The effect is attributed to weak localization and can be described by a negativeT1/2 weak-localization contribution to resistivity for a disordered three-dimensional electronsystem. The specific heat data have been fitted to contributions from free electrons(γ), spinexcitations (β3/2), lattice and a Schottky-like anomaly related to the rare-earth magnetism of the Nd ions. The valueof γ is larger than for normal metals, which is ascribed to magnetic ordering effects involvingNd. Also, the Schottky-like anomaly appears broadened and weakened suggestinginhomogeneous molecular fields at the Nd-sites.

Analysis of low temperature specific heat in Nd 0.5Sr 0.5MnO 3 and R 0.5Ca 0.5MnO 3 (R=Nd, Sm, Dy and Ho) compounds

We report on the specific heat C( T) of doped manganites Nd 0.5Sr 0.5MnO 3, Nd 0.5Ca 0.5MnO 3, Sm 0.5Ca 0.5MnO 3, Dy 0.5Ca 0.5MnO 3 and Ho 0.5Ca 0.5MnO 3 in the temperature range 2⩽ T⩽300 K using modified rigid ion model (MRIM). The present specific heat results are in general satisfactory agreement with experimental data except at very low temperatures (i.e. T⩽12 K). Also a sharp peak observed in the experimental results for these compounds around 5 K could not be revealed by our computed results as they arise due to Schottky-like anomaly. Besides, we have reported the cohesive and the thermal properties of these compounds. The results obtained by us are discussed in detail.

Specific heat at low temperatures and magnetic measurements inNd0.5Sr0.5MnO3andR0.5Ca0.5MnO3(R=Nd,Sm, Dy, and Ho) samples

We studied the magnetization as a function of temperature and magnetic field in the compounds Nd0.5Sr0.5MnO3, Nd0.5Ca0.5MnO3, Sm0.5Ca0.5MnO3, Dy0.5Ca0.5MnO3 and Ho0.5Ca0.5MnO3. Ferromagnetic, antiferromagnetic and charge ordering transition in our samples agreed with previous reports. The intrinsic magnetic moments of Nd3+, Sm3+, Dy3+ and Ho3+ ions experienced a short range order at low temperatures. We also did specific heat measurements with applied magnetic fields between 0 and 9 T and temperatures between 2 and 30 K in all five samples. Below 10 K the specific heat measurements evidenced a Schottky-like anomaly for all samples. However, we could not successfully fit the curves to either a two level nor a distribution of two-level Schottky anomaly.

Study of magnetic and specific heat measurements at low temperatures in Nd 0.5Sr 0.5MnO 3, Nd 0.5Ca 0.5MnO 3

The magnetization at low temperatures for Nd 0.5Sr 0.5MnO 3 and Nd 0.5Ca 0.5MnO 3 samples showed a rapid increase with decreasing temperatures, contrary to a La 0.5Ca 0.5MnO 3 sample. Specific heat measurement at low temperatures showed a Schottky-like anomaly for the first two samples. However, there is no a straight forward correlation between the intrinsic magnetic moment of the Nd 3+ ions and the Schottky-like anomaly.

The effect ofannealing conditions on the physical properties ofNd0.67Sr0.33MnO3-δ

A study of the influence of annealing in atmospheres with definedoxygen partial pressures (1.5×10-6 Pa ⩽ PO2 ⩽ 10+5 Pa) on the electrical resistivity ρ(T),magnetic susceptibility χ(T), and heat capacity cp(T) ofceramic samples of Nd0.67Sr0.33MnO3-δ is presented. ρ(T) at 1000 °C showsno appreciable change for 10+2 Pa ⩽ PO2 ⩽ 10+5 Pa. For samples annealed in air a sharp peak isfound in cp(T) and in ρ(T) at the ferromagnetic transition(TC≈238 K). The insulator-metal transitionis observed below TC as a broad peak in ρ(T).Annealing at lower PO2 leads to an increase of ρwhereas TC decreases only by ≈2 K.Annealing at PO2 ⩽ 6.3×10-2 Pa suppressesTC and results in spin canting and a vanishing of the peakin cp(T). The specific heat (2-300 K) for samples annealed inair and in Ar/5% H2 is determined and thermodynamicstandard values are calculated. The problem of the separation ofcp(T) into lattice, electronic, and Mn and Nd magneticcontributions is discussed. cp(T < 20 K) is dominated by aSchottky-like anomaly from the Zeeman-split ground-state doublet ofNd3+. A large quasi-linear term is mimicked by abroadening of the Schottky-like anomaly and by magneticcontributions from Mn in oxygen-deficient samples. The results arediscussed in the context of oxygen deficiency δ, carrierconcentration, defect chemistry, and disorder.