arXiv subjects

Robert Borth

Publications and source records attributed to Robert Borth.

2 recordsLinked to original sources

arXiv2024

Pressure-tuned quantum criticality in the locally non-centrosymmetric superconductor CeRh$_2$As$_2$

The unconventional superconductor CeRh$_2$As$_2$ (critical temperature $T_{\mathrm{c}}\approx0.4\,\mathrm{K}$) displays an exceptionally rare magnetic-field-induced transition between two distinct superconducting (SC) phases, proposed to be states of even and odd parity of the SC order parameter, which are enabled by a locally noncentrosymmetric structure. The superconductivity is preceded by a phase transition of unknown origin at $T_{0}\approx0.5\,\mathrm{K}$. Electronic low-temperature properties of CeRh$_2$As$_2$ show pronounced non-Fermi-liquid behavior, indicative of a proximity to a quantum critical point (QCP). The role of quantum fluctuations and normal state orders for the superconductivity in a system with staggered Rashba interaction is currently an open question, pertinent to explaining the occurrence of two-phase superconductivity. In this work, using measurements of resistivity and specific heat under hydrostatic pressure, we show that the $T_{0}$ order vanishes completely at a modest pressure of $P_{0}=0.5\,\mathrm{GPa}$, revealing a QCP. In line with the quantum criticality picture, the linear temperature dependence of the resistivity at $P_{0}$ evolves into a Fermi-liquid quadratic dependence as quantum critical fluctuations are suppressed by increasing pressure. Furthermore, the domelike behavior of $T_{\mathrm{c}}$ around $P_{0}$ implies that the fluctuations of the $T_{0}$ order are involved in the SC pairing mechanism.

cond-mat.str-el↗

arXiv2019

Strange metal behavior in a pure ferromagnetic Kondo lattice

The strange metal phases found to develop in a wide range of materials near a quantum critical point (QCP), have posed a long-standing mystery. The frequent association of strange metals with unconventional superconductivity and antiferromagnetic QCPs has led to a belief that they are highly entangled quantum states. Ferromagnets, by contrast are regarded as an unlikely setting for strange metals, for they are weakly entangled and their QCPs are often interrupted by competing phases or first order phase transitions. Here, we provide compelling evidence that the stoichiometric heavy fermion ferromagnet CeRh$_6$Ge$_4$ becomes a strange metal at a pressure-induced QCP: specific heat and resistivity measurements demonstrate that the FM transition is continuously suppressed to zero temperature revealing a strange metal phase. We argue that strong magnetic anisotropy plays a key role in this process,injecting entanglement, in the form of triplet resonating valence bonds (tRVBs) into the ordered ferromagnet. We show that the singular transformation from tRVBs into Kondo singlets that occurs at the QCP causes a jump in the Fermi surface volume: a key driver of strange metallic behavior. Our results open up a new direction for research into FM quantum criticality, while also establishing an important new setting for the strange metal problem. Most importantly, strange metallic behavior at a FM quantum critical point suggests that it is quantum entanglement rather than the destruction of antiferromagnetism that is the common driver of the many varied examples of strange metallic behavior.

cond-mat.str-el↗