A central research topic in condensed matter physics is the understanding of the evolution of various phases and phase transitions under different tuning parameters such as temperature, magnetic field and pressure. To explore the pressure-induced evolution of the magnetism and Fermi surface of the heavy fermion antiferromagnet YbPtBi, we performed tunnel diode oscillator based measurements under pressure at low temperatures in high magnetic fields. Our results reveal that the magnetic order strengthens and the Fermi surface shrinks as the pressure increases, which are consistent with typical observations for Yb-based heavy fermion compounds. In addition, an anomalous change in the quantum oscillation amplitudes is observed above 1.5 GPa, and determining the origin requires further study. 

Pressure Evolution of the Magnetism and Fermi Surface of YbPtBi Probed by a Tunnel Diode Oscillator Based Method

We report a study of isoelectronic chemical substitution in the recently discovered quantum critical ferromagnet ${\mathrm{CeRh}}_{6}{\mathrm{Ge}}_{4}$. Upon silicon doping, the ferromagnetic ordering temperature of ${\mathrm{CeRh}}_{6}{({\mathrm{Ge}}_{1\ensuremath{-}x}{\mathrm{Si}}_{x})}_{4}$ is continuously suppressed, and no transition is observed beyond ${x}_{c}\ensuremath{\approx}0.125$. Non-Fermi-liquid behavior with $C/T\ensuremath{\propto}\mathrm{ln}({T}^{*}/T$) is observed close to ${x}_{c}$, indicating the existence of strong quantum fluctuations, while the $T$-linear behavior observed upon pressurizing the parent compound is absent in the resistivity, which is likely a consequence of the disorder induced by silicon doping. Our findings show the effects of disorder on the unusual ferromagnetic quantum criticality in ${\mathrm{CeRh}}_{6}{\mathrm{Ge}}_{4}$, and provide further evidence for understanding the origin of this behavior. 

/pdf/suppression-of-ferromagnetism-and-influence-of-disorder-in-14q904v7.pdf

Suppression of ferromagnetism and influence of disorder in silicon-substituted 
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We report a study of isoelectronic chemical substitution in the recently discovered quantum critical ferromagnet CeRh 6 Ge 4 . Upon silicon-doping, the ferromagnetic ordering temperature of CeRh 6 (Ge 1 − x Si x ) 4 is continuously suppressed, and no transition is observed beyond x c ≈ 0.125. Non-Fermi liquid behavior with C/T ∝ log( T ∗ /T ) is observed close to x c , indicating the existence of strong quantum ﬂuctuations, while the T -linear behavior observed upon pressurizing the parent compound is absent in the resistivity, which appears to be a consequence of the disorder induced by silicon doping. Our ﬁndings provide evidence for the role played by disorder on the unusual ferromagnetic quantum criticality in CeRh 6 Ge 4 , and provides further evidence for understanding the origin of this behavior.

Suppression of ferromagnetism and influence of disorder in silicon-substituted CeRh6Ge4

A real-space formulation of the numerical renormalization group (NRG) procedure is introduced. The real-space construction, dubbed eNRG, is more straightforward than the NRG discretization, and allows a faithful description of the coupling between quantum dots and conduction states even if the design of the couplings is intricate. General features of the two procedures are discussed comparatively. A more specific comparison is then developed, based on computations of the zero-bias transport properties for an Anderson-model description of a quantum wire side-coupled to a single quantum dot. An eNRG computation is shown to reproduce accurately the temperature-dependent electrical conductance for the uncorrelated model in the continuum limit, while significant deviations mark the corresponding NRG computation for thermal energies comparable to the conduction bandwidth. A combination of analytical and numerical results for the transport properties of the correlated model then provides a more exacting check on the accuracy of the eNRG procedure. A recent NRG analysis mapping the transport properties of a single-electron transistor onto universal functions of the temperature scaled by the Kondo temperature is extended to the side-coupled device, on the basis of eNRG reasoning. Numerical results for the electrical conductance, thermopower, and thermal conductance in side-coupled geometry are then shown to agree very well with the mappings. The numerical results are also checked against the thermal dependence of the thermopower measured by K\"ohler et al. [Phys. Rev. B 77, 104412 (2008)] in ${\mathrm{Lu}}_{0.9}^{\phantom{\ifmmode\dagger\else\textdagger\fi{}}}{\mathrm{Yb}}_{0.1}^{\phantom{\ifmmode\dagger\else\textdagger\fi{}}}{\mathrm{Rh}}_{2}^{\phantom{\ifmmode\dagger\else\textdagger\fi{}}}{\mathrm{Si}}_{2}^{\phantom{\ifmmode\dagger\else\textdagger\fi{}}}$, and the remarkably accurate, recent conductance measurements by Xu et al. [Chin. Phys. Lett. 38, 087101 (2021)]. 

Real-space numerical renormalization group computation of transport properties in side-coupled geometry

FERRARI, A. Real-space formulation of the numerical renormalization group method (NRG). 2022. 87p. Dissertation (Master of Science) Instituto de Física de São Carlos, Universidade de Sao Paulo, São Carlos, 2022. The equilibrium transport properties of an elementary nanostructured device with sidecoupled geometry are computed and related to universal functions. The computation relies on a real-space formulation of the numerical renormalization-group (NRG) procedure. The real-space construction, dubbed eNRG, is more straightforward than the NRG discretization and allows more faithful description of the coupling between quantum dots and conduction states. The procedure is applied to an Anderson-model description of a quantum wire side-coupled to a single quantum dot. A gate potential controls the dot occupation. In the Kondo regime, the electrical conductance through this device is known to map linearly onto a universal function of the temperature scaled by the Kondo temperature. Here, the energy moments from which the Seebeck coefficient and the thermal conductance can be computed are shown to map linearly onto universal functions also. The moments and transport properties computed by the eNRG procedure are shown to agree very well with these analytical developments. Algorithms facilitating comparison with experimental results are discussed. As an illustration, one of the algorithms is applied to thermal dependence of the thermopower measured by Köhler in Lu 0.9Yb0.1Rh2Si2, and to resistivity measurements in CeRh6Ge4. 1,2

Real-space formulation of the Numerical Renormalization Group method (NRG)

The heavy fermion ferromagnet CeRh$_6$Ge$_4$ is the first example of a clean stoichiometric system where the ferromagnetic transition can be continuously suppressed by hydrostatic pressure to a quantum critical point. In order to reveal the outcome when the magnetic lattice of CeRh$_6$Ge$_4$ is diluted with non-magnetic atoms, this study reports comprehensive measurements of the physical properties of both single crystal and polycrystalline samples of La$_x$Ce$_{1-x}$Rh$_6$Ge$_4$. With increasing $x$, the Curie temperature decreases, and no transition is observed for $x$ $>$ 0.25, while the system evolves from exhibiting coherent Kondo lattice behaviors at low $x$, to the Kondo impurity scenario at large $x$. Moreover, non-Fermi liquid behavior (NFL) is observed over a wide doping range, which agrees well with the disordered Kondo model for 0.52 $\leq$ $x$ $\leq$ 0.66, while strange metal behavior is revealed in the vicinity of $x_c$ = 0.26.

Ce-site dilution in the ferromagnetic Kondo lattice CeRh$_6$Ge$_4$

Ce-Site Dilution in the Ferromagnetic Kondo Lattice CeRh$_6$Ge$_4$

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Ce-site dilution in the ferromagnetic Kondo lattice CeRh$_6$Ge$_4$

Ce-Site Dilution in the Ferromagnetic Kondo Lattice CeRh$_6$Ge$_4$