arXiv subjects

Milan Klicpera

Publications and source records attributed to Milan Klicpera.

13 recordsLinked to original sources

arXiv2026

Spin-orbit-entangled $J_{\rm eff}=\frac{1}{2}$ magnetism and unconventional spin freezing in the bond-disordered pyrochlore antiferromagnet NaCdCo$_2$F$_7$

Bond disorder in frustrated pyrochlore antiferromagnets can give rise to fundamentally different quantum ground states depending on the nature of the local magnetic moments. Here, we show that the bond-disordered $J_{\rm eff}=\frac{1}{2}$ pyrochlore antiferromagnet NaCdCo$_2$F$_7$ realizes an unconventional spin-glass-like state with continued dynamics, in stark contrast to its isostructural $S=\frac{1}{2}$ NaCdCu$_2$F$_7$ counterpart. High-field magnetization and Co $L_{2,3}$-edge XAS/XMCD establish spin-orbit-entangled $J_{\rm eff}=\frac{1}{2}$ Co$^{2+}$ moments with a substantial unquenched orbital contribution, consistent with local $XY$ anisotropy seen in the isostructural Na$A''$Co$_2$F$_7$ ($A''$ = Ca, Sr) analogues. $\mu$SR and $^{23}$Na NMR measurements reveal progressive slowing of spin fluctuations below $\sim10$ K, culminating in a partially frozen state with persistent low-temperature dynamics that deviates from a canonical spin glass. Comparison with the isostructural bond-disordered pyrochlore NaCdCu$_2$F$_7$, which realizes a random-singlet state, reveals a fundamentally different response of spin-orbit-entangled Co$^{2+}$ moments to bond disorder. These results identify spin-orbit coupling as a key ingredient governing the fate of bond-disordered frustrated pyrochlore magnets.

cond-mat.str-el↗

arXiv2024

Pressure-tuned magnetism and conductivity in pyrochlore iridates Lu2Ir2O7 and Er2Ir2O7

A2Ir2O7 iridates were proven to crystallise in the geometrically frustrated pyrochlore structure, which remains stable upon rare-earth cation substitution, temperature variation, and external pressure application. However, the change of interatomic distances and local distortions in the lattice frequently leads to complex electronic properties. The low-temperature behaviour in light-A iridates has been thoroughly investigated, including its evolution with pressure. The present pressure study reports the electrical transport and magnetotransport properties in heavy rare-earth Lu2Ir2O7 and Er2Ir2O7. Both compounds reveal a semiconductor-to-insulator transition induced by the antiferromagnetic ordering of the all-in-all-out (AIAO) type in the Ir sublattice. The transition monotonously shifts to a higher temperature under applied pressure by approximately 20 K at 3 GPa. As the transition in resistivity originates in the antiferromagnetic order, the latter is expected to be enhanced with the applied pressure as well. Upon cooling the compound in a magnetic field, the AIAO/AOAI domain structure with non-zero net magnetic moment is formed, mirroring itself in an asymmetric term in the magnetoresistance of Lu2Ir2O7. The application of pressure then enhances the asymmetric term. The same behaviour is proposed for the whole heavy rare-earth A2Ir2O7 series (A = Gd - Lu), although with magnetoresistance features masked significantly by a stronger response of magnetic A cations.

cond-mat.str-el↗

arXiv2024

Unique magnetic structure of the vdW antiferromagnet VBr$_3$

VBr$_3$ is a van der Waals antiferromagnet below the Néel temperature of 26.5 K with a saturation moment of 1.2 mB/f.u. above the metamagnetic transitions detected in the in-plane and out-of-plane directions. To reveal the AFM structure of VBr$_3$ experimentally, we performed a single-crystal neutron diffraction study on a large high-quality crystal. The collected data confirmed a slight monoclinic distortion of the high-temperature rhombohedral structure below 90 K. The magnetic structure was, nevertheless, investigated within the R-3 model. The antiferromagnetic structure propagation vector k = (1, 0, 0.5) was revealed. In an attempt to determine the magnetic structure, 72 non-equivalent magnetic reflections were recorded. The experimental data were confronted with the magnetic space groups dictated by the R-3 lattice symmetry and propagation vector. The best agreement between the experimental data and the magnetic structure model was obtained for the space group P-1.1'_c. The magnetic unit cell of the proposed unique antiferromagnetic structure with periodicity 6c is built from two identical triple layers antiferromagnetically coupled along the c axis. Each triple layer comprises a Néel antiferromagnetic monolayer sandwiched between two antiferromagnetically coupled ferromagnetic monolayers.

cond-mat.mtrl-sci↗

arXiv2024

Robust pinned magnetisation in A2Ir2O7 iridates, the case of Er2Ir2O7 and Lu2Ir2O7 flux-grown single crystals

Reliable and profound studies of actual magnetic domain structure in rare-earth A2Ir2O7 pyrochlore iridates are frequently limited by insufficient sample quality or lack of single crystals. We report the magnetic properties of the for-the-first-time synthesised Lu2Ir2O7 and Er2Ir2O7 single-crystals. The paper is focused on the robust ferromagnetic component of magnetisation present in the material with the antiferromagnetically ordered state of the all-in-all-out (AIAO) type, and is discussed in the framework of AIAO and AOAI domains and interfaces on the geometrically frustrated lattice.

cond-mat.mtrl-sci↗

arXiv2024

Robustness of the pyrochlore structure in rare-earth A2Ir2O7 iridates and pressure-induced structural transformation in IrO2

A comprehensive study of the structural properties of the heavily investigated rare-earth A2Ir2O7 series under extreme conditions is presented. The series is covered by studying iridates with A = Pr, Sm, Dy-Lu. Temperature- and pressure-dependent synchrotron X-ray powder diffraction experiments reveal robustness of the pyrochlore structure throughout the rare-earth series, down to 4 K and up to 20 GPa. The thermal expansivity and pressure compressibility are determined, including Debye temperature, bulk modulus and Grüneisen parameter. Temperature and pressure evolution of the fractional coordinate of oxygen at 48f Wyckoff position, the sole free atomic position parameter, is investigated and discussed regarding the antiferromagnetic ordering of the Ir magnetic moments. In addition, the pressure evolution of the crystal structure of an IrO2 minority phase is followed. The tetragonal rutile-type structure is orthorhombically distorted at 15 GPa, and the orthorhombic structure is not fully stabilised up to 20 GPa.

cond-mat.mtrl-sci↗

arXiv2022

Nd:YAG single crystals grown by floating zone method in laser furnace

We report on single crystal growth of laser material Nd:YAG widely used in the applications by innovative crucible-free floating zone method implemented in an advanced laser optical furnace. We have optimized the parameters for the production of high-quality single crystals of the size typical for laser tubes. To reduce the strain and improve machinability, we have developed an afterheater to thermalize the grown part of a single crystal below the hot zone, which is a technique unavailable in common mirror furnaces. The high quality of the single crystals was verified by Laue diffraction and the internal strain was documented by neutron diffraction. The absorption spectrum corresponds with the parameters of the commercially used material produced by Czochralski method. The presented methodology for the single crystal growth by floating zone method with laser heating is applicable for the preparation of other high-quality single crystals of oxide-based materials, particularly in an oxidizing environment unattainable in commonly used crucible methods.

cond-mat.mtrl-sci↗

arXiv2020

The magnetic behaviour of Dy$_2$Ir$_2$O$_7$ -- beyond the mean field approximation

The magnetic properties of a pyrochlore iridate Dy$_2$Ir$_2$O$_7$ prepared by the CsCl flux method were investigated by means of specific heat and magnetization measurements. The low temperature behaviour was confirmed to be consistent with previously published results, whereas the high temperature data and their analysis are presented for the first time. At 128 K, a bifurcation of magnetization measured under zero field cooled and field cooled regimes coincides with an onset of the anomaly in specific heat pointing to an ordering of the iridium sublattice. Further, our high-temperature magnetization and specific heat data are interpreted in the frame of the previously determined crystal field scheme for Dy$_2$Ir$_2$O$_7$. The difference between calculations and experimental data, represented by 20 K specific heat anomaly, suggests the behaviour of studied iridate beyond the mean field approximation. This is in contrast with the rest of the heavy rare-earth pyrochlore iridates, where the mean field approximation constitutes an adequate description.

cond-mat.str-el↗

arXiv2020

Spin dynamics in the pyrochlore iridate, Er2Ir2O7, investigated by muSR spectroscopy

Building a full understanding of the complex magnetism in the rare-earth pyrochlore iridates, A2Ir2O7, is an ongoing issue in condensed matter physics. The possibility of two interpenetrating and interacting frustrated magnetic pyrochlore sublattices, connected with the strong spin-orbit coupling of the 5d Ir4+ ion, lends itself to a wide array of potential electronic and magnetic states. In this paper, we present longitudinal field and zero field muSR measurements of Er2Ir2O7. The muSR response of Er2Ir2O7 is dominated by the strong fluctuating fields of the Er3+ (J = 15/2) moments, showing a peak in fitted relaxation rate, λ_2, at T = 15 K . The muSR spectra show just weak signatures of the Ir transition, that were easily suppressed in applied longitudinal field, however full decoupling of the muons from local magnetic fields was not achieved even in applied fields up to 3 kG.

cond-mat.str-el↗

arXiv2020

Pressure induced superconductivity in a CeRhSi$_{3}$ single crystal -- the high pressure study

Pressure induced superconductivity in non-centrosymmetric CeRhSi$_{3}$ and CeIrSi$_{3}$ compounds has attracted significant attention of the scientific community since its discovery 15 years ago. Up-to-date, all reported experimental results were obtained employing the hybrid-cylinder piston pressure cells with a maximum reachable pressure of 3 GPa. Present study focuses on the superconducting state at higher, so far unreported, pressures using the Bridgman anvil cell and a CeRhSi$_{3}$ single crystal synthesized by the Sn-true-flux method. The initial increase of superconducting critical temperature from 0.4 K at 1.1 GPa to 1.1 K at 2.4 GPa is followed by a gradual suppression of SC state upon increasing the pressure above 3.0 GPa, forming a typical dome. The pressure induced superconductivity is expected to be completely suppressed in the pressure region between 4.5 and 5.0 GPa. Temperature dependence of electrical resistivity in constant magnetic fields and high pressures, as well as the magnetoresistance measurements, reveal a large critical field, exceeding 19 T at 0.6 K and 2.4 GPa, sharply decreasing receding the superconductivity dome. The previously reported $\it{T-p}$ and $\it{H-T}$ phase diagrams are completed by our high-pressure data and discussed in the frame of previous results.

cond-mat.supr-con↗

arXiv2019

Mapping the structural transitions controlled by the trilinear coupling in Ca3-xSrxTi2O7

We present the results of the high-temperature neutron and x-ray diffraction experiments on the Ca3-xSrxTi2O7 (x = 0.5, 0.8, 0.85, 0.9) compounds. The ferro- to paraelectric transition in these hybrid improper ferroelectric materials arises from the so-called trilinear coupling. Depending on the Strontium content, various structures and phase transitions, different from theoretical predictions, emerge. The in-situ x-ray powder diffraction indicates a direct ferro- to paraelectric transition between the orthorhombic A21am and tetragonal undistorted I4/mmm phase for x < 0.6. We identified a reduction in the trilinear coupling robustness by increasing the Sr-doping level to lead to the emergence of the intermediate tetragonal P42/mnm phase and the gradual suppression of the orthorhombic phase. The observed character of the structure transitions and the Ca3-xSrxTi2O7 phase diagram are discussed in the framework of theoretical models of other related hybrid improper ferroelectric systems.

cond-mat.mtrl-sci↗

arXiv2018

Low-energy spin dynamics of orthoferrites AFeO$_3$ (A = Y, La, Bi)

YFeO$_3$ and LaFeO$_3$ are members of the rare-earth orthoferrites family with \textit{Pbnm} space group. Using inelastic neutron scattering, the low-energy spin excitations have been measured around magnetic Brillouin zone center. Splitting of magnon branches and non-zero magnon gap is observed for both compounds, which is similar to the behavior observed in multiferroic BiFeO$_3$. Spin wave calculations which include both Dzyaloshinsky-Moriya interactions and single-ion anisotropy in the spin-Hamiltonian comprehensively accounts for all the experimentally observed features. Our results offer insight into the unifying physics underlying the Fe$^{3+}$-based perovskites as well as their distinguishing characteristics.

cond-mat.str-el↗

arXiv2017

Neutron Diffraction Study on Single-crystalline UAu${_2}$Si$_2$

Magnetic structure of tetragonal UAu$_2$Si$_2$ was investigated by single-crystal neutron diffraction experiments. Below $T_{\rm N}$ = 20 K it orders antiferromagnetically with a propagation vector of $k = (2/3, 0, 0)$ and magnetic moments of uranium ions pointing along the tetragonal $c$-axis. Weak signs of the presence of a ferromagnetic component of magnetic moment were traced out.Taking into account a group theory calculation and experimental results of magnetization and $^{29}$Si-NMR, the magnetic structure is determined to be a squared-up antiferromagnetic structure, with a stacking sequence ($+ + -$) of the ferromagnetic $ac$-plane sheets along the $a$-axis. This result highlights similar magnetic correlations in UAu$_2$Si$_2$ and isostructural URu$_2$Si$_2$.

cond-mat.str-el↗

arXiv2017

Effect of lattice distortion on U magnetic moments in $\mathrm{U_{4}Ru_{7}Ge_{6}}$ studied by polarized neutron diffraction

Small spontaneous lattice distortion of a cubic ferromagnet at temperatures below $T_{\mathrm{C}}$ is expected, but usually a neglected phenomenon. This study is focused on such effect on the example of $\mathrm{U_{4}Ru_{7}Ge_{6}}$. We propose the lattice distortion from the cubic space group to a lower symmetry as a result of our DFT calculations. The strong spin-orbit coupling on the U site plays the most essential role, that should lower the symmetry of the system and give rise to the two different U sites (U1 and U2). We bring convincing experimental evidence for this splitting resulting from our polarized neutron diffraction experiment. Proper treatment of the flipping ratios collected in the ordered state and in the paramagnetic state reveals dramatically different magnetic moments on the U1 and U2 sites. We have reached good agreement of the results from the MAXENT method and from direct fitting of the data. The ratio of the orbital and spin component on the U2 site is lower than for the free $\mathrm{U^{3+}}$ or $\mathrm{U^{4+}}$ ion and points to a strong hybridization between the U $5f$ and Ru $4d$ wave functions. The same ratio on the U1 site is unexpectedly low. This would mean that its orbital component is almost quenched. We have observed the absence of a magnetic moment on the Ru1 site, but a rather large induced moment on the Ru2 site. Very similar results were obtained also in the paramagnetic state in the regime induced by the magnetic field. It points to the intimate coupling of the magnetic ordering and structural distortion as it is possible to split the U site in to two different polarizing paramagnetic U moments above $T_{\mathrm{C}}$ by a strong magnetic field. We propose that the difference between the magnetic moment on the U sites is caused by the change of local point symmetry of the sites that is tightly bound to the role of the strong spin-orbit interaction.

cond-mat.str-el↗