arXiv subjects

Manuel Brando

Publications and source records attributed to Manuel Brando.

At least 19 recordsLinked to original sources

arXiv2026

Field-tuned quasiparticles and electronic structure in heavy-fermion YbNi4P2

We study the Fermi surface topology and quasiparticle properties in the heavy fermion compound YbNi4P2 at high magnetic fields using quantum oscillation measurements. We observe a large decrease of the quasiparticle mass with increasing field and demonstrate good qualitative agreement with the single-ion Kondo model. At the putative Lifshitz transition at 17 T, we observe a sudden change of quantum oscillation frequencies suggesting an abrupt change of the electronic structure and/or quasiparticle characteristics. Our results demonstrate the ability to tune the electronic structure and provide input for theoretical models of YbNi4P2 and correlated electron systems in high magnetic fields.

cond-mat.str-el↗

arXiv2026

Entropy mapping under uniaxial pressure utilizing the elastocaloric effect

Uniaxial pressure is a powerful tuning parameter for quantum materials, but conventional thermodynamic probes such as specific heat are difficult to realize in the constrained geometries of strain apparatus. We develop a quantitative analysis framework for a.c. elastocaloric effect measurements that enable the reconstruction of the absolute entropy and hence specific heat across complex phase diagrams. The absolute accuracy is achieved by combining measurements in the strong coupling regime at low frequencies with high signal-to-noise measurements in the quasi-adiabatic regime at high frequencies. Applying the approach to the correlated superconductor Sr$_2$RuO$_4$, we obtain an absolute entropy map across the phase diagram including across phase transitions deep into the superconducting state. We demonstrate that from such data one can derive the absolute specific heat which is currently not possible through other approaches. This data reinforces the finding that the quenching of entropy within the superconductor Sr$_2$RuO$_4$ is strongest at the critical strain consistent with the superconducting gap being maximized at the Van Hove singularity (VHs). Furthermore, we demonstrate that, although $\Delta c /(\gamma T) $ does increase at the VH strain, this increase is much weaker than previously inferred from more indirect caloric experiments.

cond-mat.supr-con↗

arXiv2026

Incommensurate modulation with $Q=0$ A-type Antiferromagnetic Order in CeRh$_2$As$_2$ revealed by NQR studies

We performed $^{75}$As nuclear quadrupole resonance (NQR) and nuclear magnetic resonance (NMR) measurements on a higher-quality single-crystalline CeRh$_2$As$_2$, a heavy-fermion superconductor exhibiting multiple superconducting (SC) phases under magnetic fields along the $c$ axis. This SC multiphase is believed to originate from staggered Rashba spin-orbit coupling associated with locally broken inversion symmetry. In addition to superconductivity, CeRh$_2$As$_2$ exhibits phase I below $T_0\sim0.5$ K and an antiferromagnetic (AFM) state below $T_{N}\sim 0.25$ K in the early-stage samples. In the higher-quality sample, the AFM transition becomes more pronounced, and $T_{N}$ increases to nearly coincide with $T_{SC}$. The NQR spectra at the As(1) site imply an internal field with an incommensurate distribution, indicating a two-dimensional incommensurate modulation of the magnetic structure superimposed on a $Q=0$ A-type AFM component. Moreover, a pronounced decrease in the NQR intensity at $T_0$ well-above $T_{N}$ and an abrupt increase in the internal field at $T_{N}$ suggest the emergence of a slowly fluctuating AFM order at $T_0$ which becomes static at $T_{N}$.

cond-mat.supr-con↗

arXiv2025

Conventional $s$-wave Superconductivity in LaRh$_2$As$_2$; the Analog without the 4$f$ Electrons of CeRh$_2$As$_2$

Superconductor LaRh$_2$As$_2$ has the same crystal structures as CeRh$_2$As$_2$, which exhibits superconducting (SC) multiphase in the $c$-axis magnetic field. Although the SC transition temperatures $T_c$ are similar, around 0.3 K, LaRh$_2$As$_2$ shows conventional type-II superconductivity with a small upper critical field $H_{c2}\sim$ 10 mT. At present, the SC properties of LaRh$_2$As$_2$ have not been clarified yet. We performed $^{75}$As-nuclear quadrupole resonance (NQR) measurements on LaRh$_2$As$_2$ to investigate the SC properties and gap structure. $1/T_1$ shows a clear coherence peak just below $T_c$ and an exponential decrease at lower temperatures, suggesting full-gap $s$-wave superconductivity. The numerical calculations based on an $s$-wave SC model reveal an SC gap size of $\Delta(0)/k_{B}T_{c} \sim 1.48$, consistent with the weak-coupling $s$-wave superconductivity. These results suggest that the 4$f$ electrons in CeRh$_2$As$_2$ not only enhance the orbital limiting field but also contribute to the formation of unconventional superconductivity with SC multiphase.

cond-mat.supr-con↗

arXiv2025

Low-temperature thermal conductivity of the substrate material YAlO3 and its unconventional sister compound YbAlO3

We present thermal conductivity data on single crystals of YAlO3 and YbAlO3 for temperatures between 2 K and 300 K and the heat current along b and c. Both materials are very good thermal conductors in the investigated temperature range. The thermal conductivity in these electrical insulators is due to phonons. The effect of Y-Yb exchange is found to be rather small despite the considerable difference in density and average atomic mass. For YAlO3 we find a moderate thermal conductivity anisotropy with weak temperature dependence and a ratio of c to b direction between at most 1 and 2.2. It is discussed with regard to the velocities of sound and relevant scattering processes. For YbAlO3 the small crystal size limits the precision of absolute thermal conductivity values and does not allow drawing conclusions on the anisotropy. Our results on YAlO3 confirm that the material is suitable for applications requiring a good thermal conductivity at temperatures down to liquid helium, such as lasers, substrates, and detectors.

cond-mat.mtrl-sci↗

arXiv2025

Field-induced magnetic phases in the Kitaev candidate Na$_3$Co$_2$SbO$_6$

We report a rich anisotropic magnetic phase diagram of Na$_3$Co$_2$SbO$_6$, a previously proposed cobaltate Kitaev candidate, based on field- and temperature-dependent magnetization, specific heat, and magnetocaloric effect studies. At low temperatures, our experiments uncover a low-lying $j_{\textrm{eff}} = \frac{1}{2}$ state with an antiferromagnetic ground state and pronounced in-plane versus out-of-plane anisotropy. The experimentally identified magnetic phases are theoretically characterized through classical Monte Carlo simulations within an extended Kitaev-Heisenberg model with additional ring exchange interactions. The resulting phase diagram reveals a variety of exotic field-induced magnetic phases, including double-$\textbf{q}$, $\frac{1}{3}$-AFM, zigzag, and vortex phases.

cond-mat.str-el↗

arXiv2025

Unusual magnetic order in Eu$_{10}$Hg$_{55}$

In solid-state compounds, the valence of europium can sometimes be mixed -- which is especially favored in structures with several positions for the europium atoms. In this work, we study the Eu-based intermetallic noncentrosymmetric system Eu$_{10}$Hg$_{55}$ which has 65 atoms per unit cell and 4 distinct crystallographic positions for europium and 17 positions for mercury. Our detailed analysis of magnetism of large single crystals suggests that europium in Eu$_{10}$Hg$_{55}$ might be present in two valence states, resulting in a fragile magnetic ground state. Due to the cage-like structure with a large distance between the Eu atoms, those atoms are weakly ferromagnetically coupled and Eu$_{10}$Hg$_{55}$ orders at low temperatures, below $T_{1} = 5.5$ K, with a subsequent spin re-orientation at $T_{2} = 4.3$ K. There is no sign of magnetic frustration. Interestingly, the magnetic ordering of europium sub-lattices results in a magnetization pole reversal with a weak ferrimagnetic ground state. Additional magnetic phases can be induced by application of a modest external magnetic field.

cond-mat.mtrl-sci↗

arXiv2025

Odd-parity superconductivity underpinned by antiferromagnetism in heavy fermion metal YbRh$_2$Si$_2$

Topological superconductors are essential elements of the periodic table of topological quantum matter. However, the relevant odd-parity spin-triplet superconductors are rare. We report high-resolution measurements of the complex electrical impedance of YbRh$_2$Si$_2$ down to 0.4 mK, that reveal the presence of several superconducting states, suppressed differently by magnetic field, both Pauli-limited and beyond the Pauli limit. Superconductivity is abruptly switched off at the critical field of the primary antiferromagnetic order. The onset of electro-nuclear spin density wave order enhances the superconductivity, which we account for by the simultaneous formation of a spin-triplet pair density wave. Together these observations provide compelling evidence for odd-parity superconductivity, and its underpinning by antiferromagnetism, and allow us to identify the topological helical state.

cond-mat.supr-con↗

arXiv2025

Magnetic Phase Diagram of YbRh\textsubscript{2}Si\textsubscript{2}: the Influence of Hyperfine Interactions

We report the determination of the magnetic phase diagram of the heavy fermion metal YbRh\textsubscript{2}Si\textsubscript{2} in magnetic fields up to 70\,mT applied perpendicular to the crystallographic c-axis. By a combination of heat capacity, magneto-caloric, and magneto-resistance measurements we map two antiferromagnetic phases: the electronic AFM1 below 70\,mK and electro-nuclear AFM2 below 1.5\,mK. The measurements extend into the microkelvin regime to explore the quantum phase transitions in this system. We demonstrate how the hyperfine interaction significantly modifies the phase diagram and the putative field-tuned quantum critical point. The determination of the rich magnetic properties of YbRh\textsubscript{2}Si\textsubscript{2} is essential to understanding the interplay of the two magnetic orders and superconductivity in this compound.

cond-mat.str-el↗

arXiv2024

Appearance of $c$-axis magnetic moment in odd-parity antiferromagnetic state in CeRh$_2$As$_2$ revealed by $^{75}$As-NMR

CeRh$_2$As$_2$ shows the superconducting (SC) multiphase under the $c$-axis magnetic field, which is considered to originate from local inversion symmetry breaking at the Ce site. We reported that the antiferromagnetic (AFM) order is inside the SC phase and that the AFM state disappears at the transition field to the high-field SC phase. However, the magnetic structure in the AFM state has not been clarified yet. In this study, we performed $^{75}$As-NMR measurements in the SC phase in $H\parallel [110]$ to identify the magnetic structure. Comparing the NMR linewidth with $H \parallel c$, we found that the internal magnetic field is oriented to the $c$ axis. This suggests a $q = 0$ $A$-type AFM with the moments parallel to the $c$ axis. We also observed the reduction of the spin susceptibility, which indicates spin-singlet superconductivity in the low-field SC phase. This study provides an important clue to clarify the correlation between the SC multiphase, magnetism, and local inversion symmetry breaking.

cond-mat.supr-con↗

arXiv2024

Physical properties of the ferromagnetic quantum critical system YbNi$_4$(P$_{1-x}$As$_{x}$)$_2$

We report on single crystal growth and physical properties of the quantum critical Kondo-lattice system YbNi$_4$(P$_{1-x}$As$_{x}$)$_2$ with $0\leq x\leq 1$ which hosts a ferromagnetic quantum critical point at $x \approx 0.1$. We performed measurements of the magnetization, electrical resistivity, thermopower, heat capacity, and resonant X-ray emission spectroscopy. Arsenic substitution leads to a homogeneous increase of the unit-cell volume, with well-defined As-concentrations in large parts of the single crystals. All data consistently show that with increasing $x$ the Kondo temperature increases, while the magnetic anisotropy observed at low $x$ fully vanishes towards $x=1$. Consequently, at low temperatures, the system shows a crossover from pronounced non-Fermi liquid behaviour for $x \leq 0.2$ to a Fermi liquid behavior for $x > 0.2$ with weak correlations. There is a continuous change in Yb valence from nearly trivalent at low $x$ to a slightly lower value for $x = 0.6$, which correlates with the Kondo temperature. Interestingly, specific heat measurements at very low temperatures show that $C/T$ strongly increases towards lower $T$ for $x = 0.13$ and $x = 0.2$ with a very similar power law. This suggests that in YbNi$_4$(P$_{1-x}$As$_{x}$)$_2$ a quantum critical region rather than a quantum critical point might exist.

cond-mat.str-el↗

arXiv2024

Precise Calorimetry of Small Metal Samples Using Noise Thermometry

We describe a compact calorimeter that opens ultra-low temperature heat capacity studies of small metal crystals in moderate magnetic fields. The performance is demonstrated on the canonical heavy Fermion metal YbRh2Si2. Thermometry is provided by a fast current sensing noise thermometer. This single thermometer enables us to cover a wide temperature range of interest from 175 $\mu$K to 90 mK with temperature independent relative precision. Temperatures are tied to the international temperature scale with a single point calibration. A superconducting solenoid surrounding the cell provides the sample field for tuning its properties and operates a superconducting heat switch. Both adiabatic and relaxation calorimetry techniques, as well as magnetic field sweeps, are employed. The design of the calorimeter results in an addendum heat capacity which is negligible for the study reported. The keys to sample and thermometer thermalisation are the lack of dissipation in the temperature measurement and the steps taken to reduce the parasitic heat leak into the cell to the tens of fW level.

physics.ins-det↗

arXiv2024

Coexistence of local magnetism and superconductivity in the heavy-fermion CeRh$_2$As$_2$ revealed by $\mu$SR studies

The superconducting (SC) state ($T_\mathrm{c}$ = 0.3 K) of the heavy-fermion compound CeRh$_2$As$_2$, which undergoes an unusual field-induced transition to another high-field SC state, emerges from an unknown ordered state below $T_\mathrm{o}$ = 0.55 K. While an electronic multipolar order of itinerant Ce-4$f$ states was proposed to account for the $T_\mathrm{o}$ phase, the exact order parameter has not been known to date. Here, we report on muon spin relaxation ($\mu$SR) studies of the magnetic and SC properties in CeRh$_2$As$_2$ single crystals at low temperatures. We reveal a magnetic origin of the $T_\mathrm{o}$ order by identifying a spontaneous internal field below $T_\mathrm{o}$ = 0.55 K. Furthermore, we find evidence of a microscopic coexistence of local magnetism with bulk superconductivity. Our findings open the possibility that the $T_\mathrm{o}$ phase involves both dipole and higher order Ce-4$f$ moment degrees of freedom and accounts for the unusual non-Fermi liquid behavior.

cond-mat.str-el↗

arXiv2024

Metallic local-moment magnetocalorics as a route to cryogenic refrigeration

Commercial adiabatic demagnetisation refrigerators still employ the same hydrated salts that were first introduced over 85 years ago. The inherent limitations of these insulating magnetocalorics - poor thermal conductivity at sub-Kelvin temperatures, low entropy density, corrosiveness - can be overcome by a new generation of rare-earth based metallic magnetocalorics. Here, we present the metallic magnetocaloric YbNi1.6 Sn as an attractive alternative to conventional refrigerants. YbNi1.6Sn retains high entropy into the 100 mK regime and avoids the noble metal constituents of alternative refrigerants. Demagnetisation tests demonstrate that YbNi1.6Sn enables economical and durable alternatives to traditional cooling devices for temperatures reaching below 120 mK. We find that the magnetocaloric properties of this material are facilitated by unusually small Kondo and RKKY interactions, which position YbNi1.6Sn in the extreme local moment limit on the generalised Kondo lattice phase diagram.

cond-mat.str-el↗

arXiv2024

Investigation of the hyperfine coupling constant of locally noncentrosymmetric heavy-fermion superconductor CeRh$_{2}$As$_{2}$

We performed $^{75}$As-NMR measurements in $H\parallel ab$ to investigate the normal-state magnetic properties of CeRh$_2$As$_2$, a recently-discovered heavy-fermion superconductor. We compared the NMR Knight shift $K$ with the magnetic susceptibility $\chi_{ab}$, and estimated the hyperfine coupling constant $A_{\mathrm{hf}}$ from the slope of the $K-\chi$ plot. We observed that the magnitude of $A_{\mathrm{hf},ab}$ at the As(1) site changes at around 20 K owing to emerging the heavy-fermion state, which was also observed in $A_{\mathrm{hf}}$ at the As(2) site and in $H\parallel c$. The sign of $A_{\mathrm{hf},ab}$ at the As(1) site is negative in low temperature. These are important for the analysis of the NMR results of CeRh$_2$As$_2$ in the superconducting state.

cond-mat.supr-con↗

arXiv2023

Exposing the odd-parity superconductivity in CeRh$_2$As$_2$ with hydrostatic pressure

Odd-parity superconductivity is a fundamentally interesting but rare state of matter with a potential for applications in topological quantum computing. Crystals with staggered locally noncentrosymmetric structures have been proposed as platforms where a magnetic field can induce a transition between even- and odd-parity superconducting (SC) states. The strongly correlated superconductor CeRh$_2$As$_2$ with the critical temperature $T_{\mathrm{c}}\approx0.4\,\mathrm{K}$ is likely the first example material showing such a phase transition, which occurs at the magnetic field $\mu_{0}H^{*}=4\,\mathrm{T}$ applied along the crystallographic $c$ axis. CeRh$_2$As$_2$ also undergoes a phase transition of an unknown origin at $T_{0}=0.5\,\mathrm{K}$. By subjecting CeRh$_2$As$_2$ to hydrostatic pressure and mapping the resultant changes to the SC phase diagrams we investigated how the lattice compression and changes to the electronic correlations affect the stability and relative balance of the two SC states. The abnormally high in-plane upper critical field becomes even higher close to a quantum critical point of the $T_{0}$ order. Remarkably, the SC phase-switching field $H^{*}$ is drastically reduced under pressure, dropping to $0.3\,\mathrm{T}$ at $2.7\,\mathrm{GPa}$. This result signals an apparent strengthening of the local noncentrosymmetricity and forecasts a possible stabilization of the putative odd-parity state down to zero field, hitherto not considered by theoretical models.

cond-mat.supr-con↗

arXiv2023

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↗

arXiv2023

Parity transition of spin-singlet superconductivity using sub-lattice degrees of freedom

Recently, a superconducting (SC) transition from low-field (LF) to high-field (HF) SC states was reported in CeRh$_2$As$_2$, indicating the existence of multiple SC states. It has been theoretically noted that the existence of two Ce sites in the unit cell, the so-called sub-lattice degrees of freedom owing to the local inversion symmetry breaking at the Ce sites, can lead to the appearance of multiple SC phases even under an interaction inducing spin-singlet superconductivity. CeRh$_2$As$_2$ is considered as the first example of multiple SC phases owing to this sub-lattice degree of freedom. However, microscopic information about the SC states has not yet been reported. In this study, we measured the SC spin susceptibility at two crystallographically inequivalent As sites using nuclear magnetic resonance for various magnetic fields. Our experimental results strongly indicate a spin-singlet state in both SC phases. In addition, the antiferromagnetic phase, which appears within the SC phase, only coexists with the LF SC phase; there is no sign of magnetic ordering in the HF SC phase. The present work reveals unique SC properties originating from the locally noncentrosymmetric characteristics.

cond-mat.supr-con↗