arXiv subjects

I. Sheikin

Publications and source records attributed to I. Sheikin.

At least 19 recordsLinked to original sources

arXiv2026

Field-induced antiferromagnetic transition in CeIrIn$_5$

We report low-temperature $^{115}$In nuclear magnetic resonance (NMR) study of the prototypical heavy-fermion compound CeIrIn$_5$ in high magnetic fields applied close to the crystallographic $c$ axis. For this orientation, a field-induced transition was previously reported to take place at about 28 T. Although we do not observe any change of the NMR spectrum above the transition, the intensity of the NMR lines drastically decreases as a consequence of a considerable shortening of the $T_2$ relaxation time. In addition, $1/T_1$ shows a pronounced maximum at the transition. Taking into account previous high-field de Haas-van Alphen results in conjunction with band-structure calculations, our NMR results are most naturally explained by the field-induced transition into an antiferromagnetic state with the propagation vector $\mathbf{Q} = (1/2, 1/2, 0)$ and magnetic moments aligned antiferromagnetically along the $c$ axis. This makes CeIrIn$_5$ a unique case where the application of the magnetic field induces an ordered state with moments antiferromagnetically aligned along the field direction.

cond-mat.str-el↗

arXiv2025

Exotic magnetic phase diagram and extremely robust antiferromagnetism in Ce$_2$RhIn$_8$

The antiferromagnetic heavy-fermion compound Ce$_2$RhIn$_8$ belongs to the same family and bears similarities with the well-studied prototypical material CeRhIn$_5$, which demonstrates a unique behavior under applied magnetic field. Here, we report specific-heat measurements on a high-quality single crystal of Ce$_2$RhIn$_8$ in magnetic fields up to 35 T applied along both principal crystallographic directions. When the magnetic field is applied along the $a$ axis of the tetragonal crystal structure, two additional field-induced antiferromagnetic phases are observed, in agreement with previous reports. One of them is confined in a small area of the field-temperature phase diagram. The other one, which develops above $\sim$2.5 T, is very robust against the field. Its transition temperature increases with field, reaches a maximum at $\sim$12 T, and then starts to decrease. However, it tends to saturate towards the highest field of our measurements. For a field applied along the $c$ axis, the N\'{e}el temperature, $T_N$, initially decreases with field, as expected for a typical antiferromagnet. Surprisingly, an additional phase emerges above $\sim$10 T. It is most likely to be of the same origin as its counterpart observed above 2.5 T for the other field orientation. This phase is also unusually robust: Its transition temperature increases all the way up to 35 T, where it exceeds the zero field $T_N =$~2.85 K. Finally, for both field directions, the phase diagrams contain rarely observed tricritical points of second-order phase transitions.

cond-mat.str-el↗

arXiv2025

Quasi-two-dimensional Fermi surfaces of the antiferromagnet U$_2$RhIn$_8$ revealed by de Haas-van Alphen measurements

We report temperature-dependent Hall effect and low-temperature de Haas-van Alphen (dHvA) effect measurements of the antiferromagnetic heavy-fermion compound U$_2$RhIn$_8$. Temperature dependence of the Hall resistivity suggests a considerable reduction of the carrier density in the antiferromagnetic phase. The observed angular dependence of the dHvA frequencies suggests the existence of three almost ideally two-dimensional Fermi surfaces one of which is quite large. The measured effective masses range from 2$m_0$ to 14$m_0$ for the field applied along the $c$ axis. Local density approximation band-structure calculations performed for the paramagnetic ground state reveal more three-dimensional Fermi surfaces than those observed in the experiment. On the other hand, Fermi surfaces obtained for the antiferromagnetic ground state by band folding are more two dimensional. These calculations account reasonably well for the experimental results assuming a slight modification of the calculated Fermi surfaces.

cond-mat.str-el↗

arXiv2022

Field-angle dependence reveals odd-parity superconductivity in CeRh$_2$As$_2$

CeRh$_2$As$_2$ is an unconventional superconductor with multiple superconducting phases and $T_\mathrm{c} = 0.26$ K. When $H\parallel c$, it shows a field-induced transition at $μ_0H^* = 4$ T from a low-field superconducting state SC1 to a high-field state SC2 with a large critical field of $μ_0H_\mathrm{c2} = 14$ T. In contrast, for $H\perp c$, only the SC1 with $μ_0H_\mathrm{c2} = 2$ T is observed. A simple model based on the crystal symmetry was able to reproduce the phase-diagrams and their anisotropy, identifying SC1 and SC2 with even and odd parity superconducting states, respectively. However, additional orders were observed in the normal state which might have an influence on the change of the superconducting state at $H^*$. Here, we present a comprehensive study of the angle dependence of the upper critical fields using magnetic ac-susceptibility, specific heat and torque on single crystals of CeRh$_2$As$_2$. The experiments show that the state SC2 is strongly suppressed when rotating the magnetic field away from the $c$ axis and it disappears for an angle of 35$^{\circ}$. This behavior agrees perfectly with our extended model of a pseudospin triplet state with $\vec{d}$ vector in the plane and hence allows to nail down that SC2 is indeed the suggested odd-parity state.

cond-mat.supr-con↗

arXiv2021

Possible quadrupole density wave in the superconducting Kondo lattice CeRh2As2

CeRh2As2 has recently been reported to be a rare case of multi-phase unconventional superconductor [S. Khim et al., arXiv:2101.09522] close to a quantum critical point (QCP). Here, we present a comprehensive study of its normal state properties and of the phase (I) below To ~ 0.4 K which preempts superconductivity at Tc = 0.26 K. The 2nd-order phase transition at To presents signatures in specific heat and thermal expansion, but none in magnetization and ac-susceptibility, indicating a non-magnetic origin of phase I. In addition, an upturn of the in-plane resistivity at To points to a gap opening at the Fermi level in the basal plane. Thermal expansion indicates a strong positive pressure dependence of To , dTo/dp = 1.5 K/GPa, in contrast to the strong negative pressure coefficient observed for magnetic order in Ce-based Kondo lattices close to a QCP. Similarly, an in-plane magnetic field shifts To to higher temperatures and transforms phase I into another non-magnetic phase (II) through a 1st-order phase transition at about 9 T. Using renormalized band structure calculations, we found that the Kondo effect (TK ~ 30 K) leads to substantial mixing of the excited crystalline-electric-field (CEF) states into the ground state. This allows quadrupolar degrees of freedom in the resulting heavy bands at the Fermi level which are prone to nesting. The huge sensitivity of the quadrupole moment on hybridization together with nesting would cause an unprecedented case of phase transition into a quadrupole-density-wave (QDW) state at a temperature To << TK , which would explain the nature of phase I and II.

cond-mat.str-el↗

arXiv2021

Origin of the 30 T transition in CeRhIn$_5$ in tilted magnetic fields

We present a comprehensive ultrasound study of the prototypical heavy-fermion material CeRhIn$_5$, examining the origin of the enigmatic 30 T transition. For a field applied at 2$^\circ$ from the $c$ axis, we observed two sharp anomalies in the sound velocity, at $B_m \approx$ 20 T and $B^* \approx$ 30 T, in all the symmetry-breaking ultrasound modes at low temperatures. The lower-field anomaly corresponds to the well-known first-order metamagnetic incommensurate-to-commensurate transition. The higher-field anomaly takes place at 30 T, where an electronic-nematic transition was previously suggested to occur. Both anomalies, observed only within the antiferromagnetic state, are of similar shape, but the corresponding changes of the ultrasound velocity have opposite signs. Based on our experimental results, we suggest that a field-induced magnetic transition from a commensurate to another incommensurate antiferromagnetic state occurs at $B^*$. With further increasing the field angle from the $c$ axis, the anomaly at $B^*$ slowly shifts to higher fields, broadens, and becomes smaller in magnitude. Traced up to 30$^\circ$ from the $c$ axis, it is no longer observed at 40$^\circ$ below 36 T.

cond-mat.str-el↗

arXiv2021

Specific heat of CeRhIn$_5$ in high magnetic fields: Magnetic phase diagram revisited

CeRhIn$_5$ is a prototypical antiferromagnetic heavy-fermion compound, whose behavior in a magnetic field is unique. A magnetic field applied in the basal plane of the tetragonal crystal structure induces two additional phase transitions. When the magnetic field is applied along, or close to, the $c$ axis, a new phase characterized by a pronounced in-plane electronic anisotropy emerges at $B^* \approx$ 30 T, well below the critical field, $B_c \simeq$ 50 T, to suppress the antiferromagnetic order. The exact origin of this new phase, originally suggested to be an electronic-nematic state, remains elusive. Here we report low-temperature specific-heat measurements in CeRhIn$_5$ in high static magnetic fields up to 36 T applied along both the $a$ and $c$ axes. For fields applied along the $a$ axis, we confirmed the previously suggested phase diagram, and extended it to higher fields. This allowed us to observe a triple point at $\sim$ 30 T, where the first-order transition from an incommensurate to commensurate magnetic structure merges into the onset of the second-order antiferromagnetic transition. For fields applied along the $c$ axis, we observed a small but distinct anomaly at $B^*$, which we discuss in terms of a possible field-induced transition, probably weakly first-order. We further suggest that the transition corresponds to a change of magnetic structure. We revise magnetic phase diagrams of CeRhIn$_5$ for both principal orientations of the magnetic field based entirely on thermodynamic anomalies.

cond-mat.str-el↗

arXiv2021

Robust Fermi-Surface Morphology of CeRhIn$_5$ across the Putative Field-Induced Quantum Critical Point

We report a comprehensive de Haas--van Alphen (dHvA) study of the heavy-fermion material CeRhIn$_5$ in magnetic fields up to 70~T. Several dHvA frequencies gradually emerge at high fields as a result of magnetic breakdown. Among them is the thermodynamically important $β_1$ branch, which has not been observed so far. Comparison of our angule-dependent dHvA spectra with those of the non-$4f$ compound LaRhIn$_5$ and with band-structure calculations evidences that the Ce $4f$ electrons in CeRhIn$_5$ remain localized over the whole field range. This rules out any significant Fermi-surface reconstruction, either at the suggested nematic phase transition at $B^{*}\approx$ 30~T or at the putative quantum critical point at $B_c \simeq$ 50~T. Our results rather demonstrate the robustness of the Fermi surface and the localized nature of the 4$f$ electrons inside and outside of the antiferromagnetic phase.

cond-mat.str-el↗

arXiv2020

Pre-saturation phase in the frustrated ferro-antiferromagnet Pb$_2$VO(PO$_4$)$_2$

Magnetization, magnetic torque, neutron diffraction and NMR experiments are used to map out the $H$$-$$T$ phase diagram of the prototypical quasi-two-dimensional ferro-antiferromagnet Pb$_2$VO(PO$_4$)$_2$ in magnetic fields up to 27 T. When the field is applied perpendicular to the axis of magnetic anisotropy, a new magnetic state emerges through a discontinuous transition and persists in a narrow field range just below saturation. The measured NMR spectra suggest a complex and possibly incommensurate magnetic order in that regime.

cond-mat.str-el↗

arXiv2020

Magnetic structure of Cd-doped CeIrIn$_5$

We report the magnetic structure of nominally 10% Cd-doped CeIrIn$_5$, CeIr(In$_{0.9}$Cd$_{0.1}$)$_5$, determined by elastic neutron scattering. Magnetic intensity was observed only at the ordering wave vector $Q_{AF} = (1/2,1/2,1/2)$, commensurate with the crystal lattice. A staggered moment of 0.47(3)$μ_B$ at 1.8 K resides on the Ce ion. The magnetic moments are found to be aligned along the crystallographic $c$ axis. This is further confirmed by magnetic susceptibility data, which suggest the $c$ axis to be the easy magnetic axis. The determined magnetic structure is strikingly different from the incommensurate antiferromagnetic ordering of the closely related compound CeRhIn$_5$, in which the magnetic moments are antiferromagnetically aligned within the tetragonal basal plane.

cond-mat.str-el↗

arXiv2019

Fermi-surface topology of the heavy-fermion system Ce$_{2}$PtIn$_{8}$

Ce$_{2}$PtIn$_{8}$ is a recently discovered heavy-fermion system structurally related to the well-studied superconductor CeCoIn$_{5}$. Here, we report on low-temperature de Haas-van Alphen-effect measurements in high magnetic fields in Ce$_{2}$PtIn$_{8}$ and Pr$_{2}$PtIn$_{8}$. In addition, we performed band-structure calculations for localized and itinerant Ce-$4f$ electrons in Ce$_{2}$PtIn$_{8}$. Comparison with the experimental data of Ce$_{2}$PtIn$_{8}$ and of the $4f$-localized Pr$_{2}$PtIn$_{8}$ suggests the itinerant character of the Ce-$4f$ electrons. This conclusion is further supported by the observation of effective masses in Ce$_{2}$PtIn$_{8}$, which are strongly enhanced with up to 26 bare electron masses.

cond-mat.str-el↗

arXiv2018

Fermi surface properties of the bifunctional organic metal $κ$-(BETS)$_2$Mn[N(CN)$_2$]$_3$ near the metal--insulator transition

We present detailed studies of the high-field magnetoresistance of the layered organic metal $κ$-(BETS)$_2$\-Mn\-[N(CN)$_2$]$_3$ under a pressure slightly above the insulator-metal transition. The experimental data are analysed in terms of the Fermi surface properties and compared with the results of first-principles band structure calculations. The calculated size and shape of the inplane Fermi surface are in very good agreement with those derived from Shubnikov-de Haas oscillations as well as the classical angle-dependent magnetoresistance oscillations. A comparison of the experimentally obtained effective cyclotron masses with the calculated band masses reveals electron correlations significantly dependent on the electron momentum. The momentum- or band-dependent mobility is also reflected in the behavior of the classical magnetoresistance anisotropy in a magnetic field parallel to layers. Other characteristics of the conducting system related to interlayer charge transfer and scattering mechanisms are discussed based on the experimental data. Besides the known high-field effects associated with the Fermi surface geometry, new pronounced features have been found in the angle-dependent magnetoresistance, which might be caused by coupling of the metallic charge transport to a magnetic instability in proximity to the metal-insulator phase boundary.

cond-mat.str-el↗

arXiv2018

Fermi surface reconstruction and dimensional topology change in Nd-doped CeCoIn$_5$

We performed low-temperature de Haas-van Alphen (dHvA) effect measurements on a Ce$_{1-x}$Nd$_x$CoIn$_5$ series, for x = 0.02, 0.05, 0.1, and 1, down to T = 40 mK using torque magnetometry in magnetic felds up to 35 T. Our results indicate that a Fermi-surface (FS) reconstruction occurs from a quasi-two-dimensional (2D) topology for Nd-2% to a rather three-dimensional (3D) for Nd-5%, thus reducing the possibility of perfect FS nesting. The FS evolves further with increasing Nd content with no observed divergence of the effective mass between Nd-2% and 10%, consistent with the crossing of a spin density wave (SDW) type of quantum critical point (QCP). Our results elucidate the origin of the Q-phase observed at the 5% Nd-doping level [Raymond et al., J. Phys. Soc. Jpn. 83, 013707 (2014)].

cond-mat.str-el↗

arXiv2017

Quasi-two-dimensional Fermi surfaces with localized $f$ electrons in the layered heavy-fermion compound CePt$_2$In$_7$

We report measurements of the de Haas-van Alphen effect in the layered heavy-fermion compound CePt$_2$In$_7$ in high magnetic fields up to 35 T. Above an angle-dependent threshold field, we observed several de Haas-van Alphen frequencies originating from almost ideally two-dimensional Fermi surfaces. The frequencies are similar to those previously observed to develop only above a much higher field of 45 T, where a clear anomaly was detected and proposed to originate from a change in the electronic structure [M. M. Altarawneh et al., Phys. Rev. B 83, 081103 (2011)]. Our experimental results are compared with band structure calculations performed for both CePt$_2$In$_7$ and LaPt$_2$In$_7$, and the comparison suggests localized $f$ electrons in CePt$_2$In$_7$. This conclusion is further supported by comparing experimentally observed Fermi surfaces in CePt$_2$In$_7$ and PrPt$_2$In$_7$, which are found to be almost identical. The measured effective masses in CePt$_2$In$_7$ are only moderately enhanced above the bare electron mass $m_0$, from 2$m_0$ to 6$m_0$.

cond-mat.str-el↗

arXiv2017

Drastic change of the Fermi surface across the metamagnetic transition in CeRh$_2$Si$_2$

We report high field de Haas-van Alphen (dHvA) effect measurements in CeRh$_2$Si$_2$ both below and above the first-order 26 T metamagnetic transition from an antiferromagnetic to a polarized paramagnetic state. The dHvA frequencies observed above the transition are much higher than those observed below, implying a drastic change of the Fermi-surface size. The dHvA frequencies above the transition and their angular dependence are in good agreement with band-structure calculations for LaRh$_2$Si$_2$, which correspond to CeRh$_2$Si$_2$ with localized $f$ electrons. Given that the $f$ electrons are also localized at low fields in CeRh$_2$Si$_2$, the Fermi-surface reconstruction is due to the suppression of antiferromagnetism and the restoration of the crystallographic Brillouin zone rather than the delocalization of the $f$ electrons. This example suggests that the intuitive notation of "small" and "large" Fermi surfaces commonly used for localized and itinerant $f$ electrons, respectively, requires careful consideration, taking into account the modification of the Brillouin zone in the antiferromagnetic state, when interpreting experimental results.

cond-mat.str-el↗

arXiv2017

Determination of the magnetic structure of CePt$_2$In$_7$ by means of neutron diffraction

The magnetic structure of the heavy fermion antiferromagnet CePt$_2$In$_7$ is determined using neutron diffraction. We find a magnetic wave vector $\mathbf{q}_M = (1/2,1/2,1/2)$, which is temperature independent up to $T_N =$ 5.5 K. A staggered moment of 0.45(1)$μ_B$ at 2 K resides on the Ce ion. The nearest-neighbor moments in the tetragonal basal plane are aligned antiferromagnetically. The moments rotate by 90$^\circ$ from one CeIn$_3$ plane to another along the $c$ axis. A much weaker satellite peak with an incommensurate magnetic wave vector $\mathbf{q}_M = (1/2,1/2,0.47)$ seems to develop at low temperature. However, the experimental data available so far are not sufficient to draw a definitive conclusion about the possible co-existence of commensurate and incommensurate magnetic structures in this material.

cond-mat.str-el↗

arXiv2017

Field induced Lifshitz transition in UPt$_2$Si$_2$: Fermi surface under extreme conditions

We have measured Hall effect, magnetotransport and magnetostriction on the field induced phases of single crystalline UPt$_2$Si$_2$ in magnetic fields up to 60\,T at temperatures down to 50\,mK. For the magnetic field applied along the $c$ axis we observe strong changes in the Hall effect at the phase boundaries. From a comparison to band structure calculations utilizing the concept of a dual nature of the uranium 5$f$ electrons, we find evidence for field induced topological changes of the Fermi surface due to at least one Lifshitz transition. Furthermore, we find a unique history dependence of the magnetotransport and magnetostriction data, indicating that the Lifshitz type transition is of a discontinuous nature, as predicted for interacting electron systems.

cond-mat.str-el↗

arXiv2016

Shubnikov-de Haas oscillations and electronic correlations in the layered organic metal $κ$-(BETS)$_2$Mn[N(CN)$_2$]$_3$

We present magnetoresistance studies of the quasi-two-dimensional organic conductor $κ$-(BETS)$_2$Mn[N(CN)$_2$]$_3$, where BETS stands for bis\-(ethylene\-dithio)\-tetra\-selena\-fulvalene. Under a moderate pressure of 1.4\,kbar, required for stabilizing the metallic ground state, Shubnikov - de Haas oscillations, associated with a classical and a magnetic-breakdown cyclotron orbits on the cylindrical Fermi surface, have been found at fields above 10\,T. The effective cyclotron masses evaluated from the temperature dependence of the oscillation amplitudes reveal strong renormalization due to many-body interactions. The analysis of the relative strength of the oscillations corresponding to the different orbits and its dependence on magnetic field suggests an enhanced role of electron-electron interactions on flat parts of the Fermi surface.

cond-mat.str-el↗