arXiv subjects

Julian G. Sereni

Publications and source records attributed to Julian G. Sereni.

8 recordsLinked to original sources

arXiv2026

II. Exploring the role of the Crystal Electric Field in the vicinity of a Quantum Critical Point

Very low temperature thermodynamic properties of the Yb$M_{5-x}X_x$ (with $M$= Ni, Cu, and $X$= Cd, Mg, Au, Zn, Ag) cubic compounds are analyzed covering a broad range of behavior between magnetic and Fermi-liquid ground states GS using the {\it chemical doping}: $ζ$ as control parameter. This allows to gain insight into the evolution of the GS behavior including a quantum critical point QCP. Doniach-Lavagna phase diagram limitations are improved by taking into account crystal electric field CEF splittings. Three regions are recognized as a function of $ζ$: i) a magnetic one with long range magnetic order and $T_{ord}\propto T_N \approx 1$\,K, that weakens the interactions between $0.8\,K \geq T_m\geq 0.4$\,K, exhibiting very low Kondo temperature $T_K^{GS}$ in respective doublets GS. Then, for $T_Q\leq 0.4$\,K, quantum fluctuations start to dominate the scenario with the specific heat $C_{4f}/T(T\geq T_Q)$ showing $T$ power law dependencies, and a very heavy-fermion {\it plateau} below $T_Q$. ii) beyond the QCP the typical Non-Fermi-Liquid logarithmic $T$ dependence: $C_{4f}/T \propto \ln(T/T_0)$, with traces of magnetic order. At the non-magnetic limit: iii) the alloys behave as valence-fluctuation systems with growing $T_K$ that overcomes the CEF splitting. With this experimental information, a realistic phase diagram can be drawn around the QCP where the scenario is dominated by low lying quantum fluctuations, without $C_{4f}/T|_{Lim T\to 0}$ divergences but a clear drop entering into the non-magnetic phase.

cond-mat.str-el↗

arXiv2024

Magnetic phase diagram of EuPdSn_2

A magnetic phase diagram for EuPdSn_2 is constructed based on magnetization, DC-susceptibility and specific heat as a function of applied field, and neutron diffraction results at zero field. The contribution of the ferromagnetic FM and antiferromagnetic AFM components to the measured magnetic susceptibility can be separated and weighted as a function of the applied field B by taking the temperature dependence of the neutron counts from the FM phase at B=0 as a reference. In the magnetically ordered phase, the temperature dependence of the specific heat does not follow the mean-field prediction for the Eu^{2+} J=7/2 quantum number rather that for J=5/2. This deviation and the consequent discrepancy for the entropy is discussed in the context of a non-Zeeman distribution of the eight fold ground state. Comparing the respective variations of magnetization M(T,B) and entropy S(T,B), the thermodynamic relationship dM/dT=dS/dB=0 is observed for the ranges: 0<T<10K and 0<B<0.4T.

cond-mat.str-el↗

arXiv2021

The magnetic field induced ferromagnetism in EuPd$_2$Sn$_4$ novel compound

We report on crystal structure, magnetic and thermal physical properties of EuPd$_2$Sn$_4$ stannide. From the magnetic susceptibility measurements a divalent state of Eu rare earth element was determined together with an antiferromagnetic (AFM) order at 11 K. By applying magnetic field, the magnetic behavior variation reveals a complex magnetic structure. Above a critical field Bc a ferromagnetic (FM)-like character starts to prevail. AFM and FM regimes are separated by a metamagnetic region. Heat capacity measurements confirm this behavior.

cond-mat.str-el↗

arXiv2018

Low temperature thermomagnetic properties of very heavy fermions suitable for adiabatic demagnetization refrigeration

With the aim of improving the performance of classical paramagnetic salts for adiabatic refrigeration processes at the sub-Kelvin range, relevant thermodynamic parameters of some new Yb-based intermetallic compounds are analyzed and compared. Two alternative potential applications are recognized, like those requiring fixed temperature reference points to be reached applying low intensity magnetic fields and those requiring controlled thermal drift for temperature dependent studies. Different thermomagnetic entropy S(T,B) trajectories were identified depending on respective specific heat behaviors at very low temperature. To gain insight into the criteria to be used for a proper choice of suitable materials in respective applications, some simple relationships are proposed to facilitate a comparative description of their magnetocaloric behavior, including the referent Cerium-Magnesium-Nitride (CMN) salt in these comparisons

cond-mat.mtrl-sci↗

arXiv2017

First observation of Ce volume collapse in CeN

On the occasion of the 80th anniversary of the first observation of Ce volume collapse in CeN a remembrance of the implications of that transcendent event is presented, along with a review of the knowledge of Ce physical properties available at that time. Coincident anniversary corresponds to the first proposal for Ce as a mix valence element, motivating to briefly review how the valence instability of Ce was investigated since that time.

cond-mat.str-el↗

arXiv2017

The role of the entropy in the ground state formation of magnetically frustrated systems within their quantum critical regime

A systematic modification of the entropy trajectory $S_m(T)$ is observed at very low temperature in magnetically frustrated systems as a consequence of the constraint $S_m\geq 0$ imposed by the third law of thermodynamics. The lack of magnetic order allows to explore some unusual thermodynamic properties by tracing the physical behavior of real systems. The most relevant findings are: i) a common $C_m/T|_{T\to 0} \approx 7$\,J/molK$^2$ 'plateau' in at least five Yb-based very-heavy-fermions VHF compounds; ii) quantitative and qualitative differences between VHF and standard non-Fermi-liquids. iii) Entropy-bottlenecks governing the change of $S_m(T)$ trajectories in a continuous transition into alternative ground states that exhibits third order characteristics. An empirical analysis of the possible $S_m(T\to 0)$ dependencies according to the $\partial ^2 S_m/\partial T^2$ derivative is also preformed. Altogether, this work can be regarded as an empirical application of the third law of thermodynamics.

cond-mat.str-el↗

arXiv2012

Thermodynamic analysis of the Quantum Critical behavior of Ce-lattice compounds

A systematic analysis of low temperature magnetic phase diagrams of Ce compounds is performed in order to recognize the thermodynamic conditions to be fulfilled by those systems to reach a quantum critical regime and, alternatively, to identify other kinds of low temperature behaviors. Based on specific heat ($C_m$) and entropy ($S_m$) results, three different types of phase diagrams are recognized: i) with the entropy involved into the ordered phase ($S_{MO}$) decreasing proportionally to the ordering temperature ($T_{MO}$), ii) those showing a transference of degrees of freedom from the ordered phase to a non-magnetic component, with their $C_m(T_{MO})$ jump ($ΔC_m$) vanishing at finite temperature, and iii) those ending in a critical point at finite temperature because their $ΔC_m$ do not decrease with $T_{MO}$ producing an entropy accumulation at low temperature. Only those systems belonging to the first case, i.e. with $S_{MO}\to 0$ as $T_{MO}\to 0$, can be regarded as candidates for quantum critical behavior. Their magnetic phase boundaries deviate from the classical negative curvature below $T\approx 2.5$\,K, denouncing frequent misleading extrapolations down to T=0. Different characteristic concentrations are recognized and analyzed for Ce-ligand alloyed systems. Particularly, a pre-critical region is identified, where the nature of the magnetic transition undergoes significant modifications, with its $\partial C_m/\partial T$ discontinuity strongly affected by magnetic field and showing an increasing remnant entropy at $T\to 0$. Physical constraints arising from the third law at $T\to 0$ are discussed and recognized from experimental results.

cond-mat.str-el↗

arXiv2011

Atypical thermodynamic behavior of Ce compounds in the vicinity of zero temperature Critical Points

A systematic analysis of thermodynamic properties performed on Ce-base exemplary compounds allows to identify different types of behaviors as the system approaches the quantum critical region. They are recognized in the respective magnetic phase boundaries (T_{N,C}(x)) as a change from the classical negative curvature to a linear composition ($x$) dependence, the occurrence of a critical point or the evanescence at finite temperature under pressure at finite temperature. In the first case, an anomalous reduction of the entropy S_m respect to the S_m = RLn2 value (expected for the usual Ce-magnetic doublet ground state) is observed around x_{cr}, and analyzed taking profit of detailed studies performed on CeIn_{3-x}Sn_x alloys. As expected from Maxwell relations, the volume variation V_0(x) at T=>0 also shows a non-monotonous behavior around x_{cr}. Different regimes in the entropy variation of the ordered phase (S_{MO}) are recognized. Only in the former case S_{MO}=> 0 continuously as T_{N,C}=>0, whereas in the second S_{MO}(x,B) remains constant till a first order transition occurs. In the third case, the degrees of freedom of the MO phase are progressively transferred to the heavy fermion component as indicated by the decreasing C_m(T_N) jump which vanishes at finite temperature.

cond-mat.str-el↗