arXiv subjects

K. Prokes

Publications and source records attributed to K. Prokes.

At least 19 recordsLinked to original sources

arXiv2021

Structural and magnetic properties of the new quantum magnet BaCuTe$_2$O$_6$

We investigate the structural and magnetic properties of the new quantum magnet BaCuTe$_2$O$_6$. This compound is synthesized for the first time in powder and single crystal form. Synchrotron X-ray and neutron diffraction reveal a cubic crystal structure (P4$_1$32) where the magnetic Cu$^{2+}$ ions form a complex network. Physical properties measurements suggest the presence of antiferromagnetic interactions with a Curie-Weiss temperature of -33K, while long-range magnetic order occurs at the much lower temperature of ~6.3K. The magnetic structure, solved using neutron diffraction, reveals antiferromagnetic order along chains parallel to the a, b and c crystal axes. This is consistent with the magnetic excitations which resemble the multispinon continuum typical of the spin-1/2 Heisenberg antiferromagnetic chain. A consistent intrachain interaction value of ~34K is achieved from the various techniques. Finally the magnetic structure provides evidence that the chains are coupled together in a non-colinear arrangement by a much weaker antiferromagnetic, frustrated hyperkagome interaction.

cond-mat.str-el↗

arXiv2020

High temperature tetragonal crystal structure of UPt$_2$Si$_2$

High temperature crystal structure of UPt$_2$Si$_2$ determined using single-crystal neutron diffraction at 400 K is reported. It is found that the crystal structure remains of the primitive tetragonal CaBe$_2$Ge$_2$ type with the space group P4/$n m m. Anisotropic displacement factors of the Pt atoms at the 2a (3/4 1/4 0) and Si atoms at the 2c (1/4 1/4 z) Wyckoff sites are found to be anomalously large.

cond-mat.str-el↗

arXiv2020

Noncollinear magnetic structure in U2Pd2In at high magnetic fields

We report an unexpected magnetic-field-driven magnetic structure in the 5f-electron Shastry- Sutherland system U2Pd2In. This phase develops at low temperatures from a noncollinear antiferromagnetic ground state above the critical field of 25.8 T applied along the a-axis. All U moments have a net magnetic moment in the direction of the applied field, described by a ferromagnetic propagation vector qF = (0 0 0) and an antiferromagnetic component described by a propagation vector qAF = (0 0.30 1/2 ) due to a modulation in the direction perpendicular to the applied field. We conclude that this surprising noncollinear magnetic structure is due to a competition between the single-ion anisotropy trying to keep moments, similar to the ground state, along the [110]-type directions, Dzyaloshinskii-Moryia interaction forcing them to be perpendicular to each other and application of the external magnetic field attempting to align them along the field direction.

cond-mat.str-el↗

arXiv2019

Field-induced phases in a heavy-fermion U(Ru$_{0.92}$Rh$_{0.08}$)$_{2}$Si$_{2}$ single crystal

We report the high-field induced magnetic phases and phase diagram of a high quality \urxrs~single crystal prepared using a modified Czochralski method. Our study, that combines high-field magnetization and electrical resistivity measurements, shows for fields applied along the $c$-axis direction three field-induced magnetic phase transitions at $μ_{0} H_{c1}$ = 21.60 T, $μ_{0} H_{c2}$ = 37.90 T and $μ_{0} H_{c3}$ = 38.25 T, respectively. In agreement with a microscopic up-up-down arrangement of the U magnetic moments the phase above $H_{c1}$ has a magnetization of about one third of the saturated value. In contrast the phase between $H_{c2}$ and $H_{c3}$ has a magnetization that is a factor of two lower than above the $H_{c3}$, where a polarized Fermi-liquid state with a saturated moment $M_{s}$ $\approx$ 2.1 $μ_{B}$/U is realized. Most of the respective transitions are reflected in the electrical resistivity as sudden drastic changes. Most notably, the phase between $H_{c1}$ and $H_{c2}$ exhibits substantially larger values. As the temperature increases, transitions smear out and disappear above $\approx$ 15 K. However, a substantial magnetoresistance is observed even at temperatures as high as 80 K. Due to a strong uniaxial magnetocrystalline anisotropy a very small field effect is observed for fields apllied perpendicular to the $c$-axis direction.

cond-mat.str-el↗

arXiv2018

Magnetic-field-induced incommensurate to collinear spin order transition in NiBr$_{2}$

The triangular spin lattice of NiBr$_{2}$ is a canonical example of a frustrated helimagnet that shows a temperature-driven phase transition from a collinear commensurate antiferromagnetic structure to an incommensurate spin helix on cooling. Employing neutron diffraction, bulk magnetization, and magnetic susceptibility measurements, we have studied the f\hspace*{.5pt}ield-induced magnetic states of the NiBr$_{2}$ single crystal. Experimental f\hspace*{.5pt}indings enable us to recapitalize the driving forces of the spin spiral ordering in the triangular spin-lattice systems, in general. Neutron diffraction data conf\hspace*{.5pt}irms, at low temperature below T$_{\rm m}$ = 22.8(1) K, the presence of diffraction satellites characteristic of an incommensurate magnetic state, which are symmetrically arranged around main magnetic reflections that evolve just below T$_{\rm N}$ = 44.0(1) K. Interestingly, a f\hspace*{.5pt}ield-induced transition from the incommensurate to commensurate spin phase has been demonstrated that enforces spin helix to restore the high temperature compensated antiferromagnetic structure. This spin reorientation can be described as a spin-flop transition in the (\hbox{$a$--$b$}) basal plane of a triangular spin lattice system. These f\hspace*{.5pt}indings offer a new pathway to control the spin helix in incommensurate phases that are currently considered having high technical implications in the next-generation data storage devices.

cond-mat.mtrl-sci↗

arXiv2018

Colossal magnetoresistance in a Mott insulator via magnetic field-driven insulator-metal transition

We present a new type of colossal magnetoresistance (CMR) arising from an anomalous collapse of the Mott insulating state via a modest magnetic field in a bilayer ruthenate, Ti-doped Ca$_3$Ru$_2$O$_7$. Such an insulator-metal transition is accompanied by changes in both lattice and magnetic structures. Our findings have important implications because a magnetic field usually stabilizes the insulating ground state in a Mott-Hubbard system, thus calling for a deeper theoretical study to reexamine the magnetic field tuning of Mott systems with magnetic and electronic instabilities and spin-lattice-charge coupling. This study further provides a model approach to search for CMR systems other than manganites, such as Mott insulators in the vicinity of the boundary between competing phases.

cond-mat.str-el↗

arXiv2017

Magnetic structure in U(Ru0.92Rh0.08)2Si2 single crystal studied by neutron diffraction in static magnetic fields up to 24 T

We report the high-field induced magnetic phase in single crystal of U(Ru0.92Rh0.08)2Si2. Our neutron study combined with high-field magnetization, shows that the magnetic phase above the first metamagnetic transition at Hc1 = 21.6 T has an uncompensated commensurate antiferromagnetic structure with propagation vector Q2 = ( 2/3 0 0) possessing two single-Q domains. U moments of 1.45 (9) muB directed along the c axis are arranged in an up-up-down sequence propagating along the a axis, in agreement with bulk measurements. The U magnetic form factor at high fields is consistent with both the U3+ and U4+ type. The low field short-range order that emerges from the pure URu2Si2 due to Rh-doping is initially strengthened by the field but disappears in the field-induced phase. The tetragonal symmetry is preserved across the transition but the a axis lattice parameter increases already at low fields. Our results are in agreement with itinerant electron model with 5f states forming bands pinned in the vicinity of the Fermi surface that is significantly reconstructed by the applied magnetic field.

cond-mat.str-el↗

arXiv2016

Electronic properties of a heavy-fermion U(Ru0.92Rh0.08)2Si2 single crystal

We report the crystal structure and highly-anisotropic magnetic, transport and thermal properties of an exceptionally good single crystal of U(Ru0.92Rh0.08)2Si2 prepared using a modified Czochralski method. Our study, that also includes neutron diffraction results, shows all the heavy-fermion signatures of pristine URu2Si2 , however, the superconductivity, hidden order and remanent weak antiferromagnetic orders are absent. Instead, the ground state of the doped system can be classified as a spin liquid that preserves the heavy-fermion character. U(Ru0.92Rh0.08)2Si2 exhibits a short-range magnetic order distinguished by reflections of a Lorentzian profile at qIII = (1/2 1/2 1/2) positions that disappear above approx. 15 K. The short-range order seems to be a precursor of a long-range magnetic order that occurs with higher Rh concentration. We indicate that these short-range fluctuations involve, at least partially, inelastic scattering processes.

cond-mat.str-el↗

arXiv2014

Coexistence of different magnetic moments in CeRuSn probed by polarized neutrons

We report on the spin densities in CeRuSn determined at elevated and at low temperatures using polarized neutron diffraction. At 285 K, where the CeRuSn crystal structure, commensurate with the CeCoAl type, contains two different crystallographic Ce sites, we observe that one Ce site is clearly more susceptible to the applied magnetic field whereas the other is hardly polarizable. This finding clearly documents that distnictly different local environment of the two Ce sites causes the Ce ions to split into magnetic Ce3+ and non-magnetic Ce(4-delta)+ valence states. With lowering the temperature, the crystal structure transforms to a structure incommensurately modulated along the c axis. This leads to new inequivalent crystallographic Ce sites resulting in a re-distribution of spin densities. Our analysis using the simplest structural approximant shows that in this metallic system Ce ions co-exist in different valence states. Localized 4f states that fulfill the third Hund's rule are found to be close to the ideal Ce3+ state (at sites with the largest Ce-Ru interatomic distances) whereas Ce(4-delta)+ valence states are found to be itinerant and situated at Ce sites with much shorter Ce-Ru distances. The similarity to the famous alpha-gamma transition in elemental cerium is discussed.

cond-mat.str-el↗

arXiv2014

Valence modulations in CeRuSn

CeRuSn exhibits an extraordinary room temperature structure at 300~K with coexistence of two types of Ce ions, namely trivalent Ce$^{3+}$ and intermediate valent Ce$^{(4-δ)+}$, in a metallic environment. The ordered arrangement of these two Ce types on specific crystallographic sites results in a doubling of the unit cell along the $c$-axis with respect to the basic monoclinic CeCoAl-type structure. Below room temperature, structural modulation transitions with very broad hysteresis have been reported from measurements of various bulk properties. X-ray diffraction revealed that at low temperatures the doubling of the CeCoAl type structure is replaced by a different modulated ground state, approximating a near tripling of the basic CeCoAl cell. The transition is accompanied by a significant contraction of the $c$ axis. We present new x-ray absorption near-edge spectroscopy data at the Ce L$_{3}$ absorption edge, measured on a freshly cleaved surface of a CeRuSn single crystal. In contrast to a previous report, the new data exhibit small but significant variations as function of temperature that are consistent with a transition of a fraction of Ce$^{3+}$ ions to the intermediate valence state, analogous to the $γ\rightarrow α$ transition in elemental cerium, when cooling through the structural transitions of CeRuSn. Such results in a valence-modulated state.

cond-mat.str-el↗

arXiv2013

Phase diagram with enhanced spin-glass region of mixed Ising / XY magnets LiHo$_{x}$Er$_{1-x}$F$_{4}$

We present the experimental phase diagram of LiHo$_x$Er$_{1-x}$F$_4$, a dilution series of dipolar-coupled model magnets.The phase diagram was determined using a combination of AC susceptibility and neutron scattering. Three unique phases in addition to the Ising ferromagnet LiHoF$_4$ and the XY anti-ferromagnet LiErF$_4$ have been identified. Below $x=0.86$ an embedded spin-glass phase is observed, where a spin-glass exists within the ferromagnetic structure. Below $x=0.57$ an Ising spin-glass is observed consisting of frozen needle-like clusters. For $x \sim 0.3 - 0.1$ an antiferromagnetically coupled spin-glass occurs. A reduction of $T_C(x)$ for the ferromagnet is observed which disobeys the mean-field predictions that worked for LiHo$_x$Y$_{1-x}$F$_4$.

cond-mat.str-el↗

arXiv2010

Anomalous spin distribution in the superconducting ferromagnet UCoGe studied by polarized neutron diffraction

We report a polarized neutron diffraction study conducted to reveal the nature of the weak ferromagnetic moment in the superconducting ferromagnet UCoGe. We find that the ordered moment in the normal phase in low magnetic fields (B // c) is predominantly located at the U atom and has a magnitude of about 0.1 muB at 3 T, in agreement with bulk magnetization data. By increasing the magnetic field the U moment grows to about 0.3 muB in 12 T and most remarkably, induces a substantial moment (about 0.2 muB) on the Co atom directed antiparallel to the U moment. The anomalous polarizability of the Co 3d orbitals is unique among uranium intermetallics and might reflect the proximity to a magnetic quantum critical point of UCoGe in zero field.

cond-mat.str-el↗

arXiv2010

Evidence from neutron diffraction for superconductivity in the stabilized tetragonal phase of CaFe2As2 under uniaxial pressure

CaFe2As2 single crystals under uniaxial pressure applied along the c axis exhibit the coexistence of several structural phases at low temperatures. We show that the room-temperature tetragonal phase is stabilized at low temperatures for pressures above 0.06 GPa, and its weight fraction attains a maximum in the region where superconductivity is observed under applied uniaxial pressure. Simultaneous resistivity measurements strongly suggest that this phase is responsible for the superconductivity in CaFe2As2 found below 10 K in samples subjected to nonhydrostatic pressure conditions.

cond-mat.supr-con↗

arXiv2010

From (pi, 0) magnetic order to superconductivity with (pi, pi) magnetic resonance in Fe1.02(Te1-xSex)

The iron chalcogenide Fe1+y(Te1-xSex) is structurally the simplest of the Fe-based superconductors. Although the Fermi surface is similar to iron pnictides, the parent compound Fe1+yTe exhibits antiferromagnetic order with in-plane magnetic wave-vector (pi, 0). This contrasts the pnictide parent compounds where the magnetic order has an in-plane magnetic wave-vector (pi, pi) that connects hole and electron parts of the Fermi surface. Despite these differences, both the pnictide and chalcogenide Fe-superconductors exhibit superconducting spin resonances around (pi, pi), suggesting a common symmetry for their superconducting order parameter. A central question in this burgeoning field is therefore how (pi, pi) superconductivity can emerge from a (pi, 0) magnetic instability. Here, we report that the magnetic soft mode evolving from the (pi, 0)-type magnetic long-range order is associated with weak charge carrier localization. Bulk superconductivity occurs only as the magnetic mode at (pi, pi) becomes dominant upon doping. Our results suggest a common magnetic origin for superconductivity in iron chalcogenide and pnictide superconductors.

cond-mat.supr-con↗

arXiv2009

Anisotropic magnetic field responses of ferroelectric polarization in a trigonal multiferroic CuFe1-xAlxO2 (x=0.015)

We have investigated magnetic field dependences of a ferroelectric incommensurate-helimagnetic order in a trigonal magneto-electric (ME) multiferroic CuFe1-xAlxO2 with x=0.015, which exhibits the ferroelectric phase as a ground state, by means of neutron diffraction, magnetization and dielectric polarization measurements under magnetic fields applied along various directions. From the present results, we have established the H-T magnetic phase diagrams for the three principal directions of magnetic fields; (i) parallel to the c axis, (ii) parallel to the helical axis, and (iii) perpendicular to the c and the helical axes. While the previous dielectric polarization (P) measurements on CuFe1-xGaxO2 with x=0.035 have demonstrated that the magnetic field dependence of the `magnetic domain structure' results in distinct magnetic field responses of P [S. Seki et al., Phys. Rev. Lett., 103 237601 (2009)], the present study have revealed that the anisotropic magnetic field dependence of the ferroelectric helimagnetic order `in each magnetic domain' can be also a source of a variety of magnetic field responses of P in CuFe1-xAxO2 systems (A=Al, Ga).

cond-mat.str-el↗

arXiv2009

Magnetic field induced flop of cycloidal spin order in multiferroic TbMnO3: The magnetic structure of the P||a phase

Using in-field single crystal neutron diffraction we have determined the magnetic structure of TbMnO3 in the high field P||a phase. We unambiguously establish that the ferroelectric polarization arises from a cycloidal Mn spins ordering, with spins rotating in the ab plane. Our results demonstrate directly that the flop of the ferroelectric polarization in TbMnO3 with applied magnetic field is caused from the flop of the Mn cycloidal plane.

cond-mat.str-el↗

arXiv2008

Lattice collapse and quenching of magnetism in CaFe2As2 under pressure: A single crystal neutron and x-ray diffraction investigation

Single crystal neutron and high-energy x-ray diffraction have identified the phase lines corresponding to transitions between the ambient-pressure tetragonal (T), the antiferromagnetic orthorhombic (O) and the non-magnetic collapsed tetragonal (cT) phases of CaFe2As2. We find no evidence of additional structures for pressures up to 2.5 GPa (at 300 K). Both the T-cT and O-cT transitions exhibit significant hysteresis effects and we demonstrate that coexistence of the O and cT phases can occur if a non-hydrostatic component of pressure is present. Measurements of the magnetic diffraction peaks show no change in the magnetic structure or ordered moment as a function of pressure in the O phase and we find no evidence of magnetic ordering in the cT phase. Band structure calculations show that the transition results in a strong decrease of the iron 3d density of states at the Fermi energy, consistent with a loss of the magnetic moment.

cond-mat.supr-con↗

arXiv2006

Crystal structure and high-field magnetism of La2CuO4

Neutron diffraction was used to determine the crystal structure and magnetic ordering pattern of a La2CuO4 single crystal, with and without applied magnetic field. A previously unreported, subtle monoclinic distortion of the crystal structure away from the orthorhombic space group Bmab was detected. The distortion is also present in lightly Sr-doped crystals. A refinement of the crystal structure shows that the deviation from orthorhombic symmetry is predominantly determined by displacements of the apical oxygen atoms. An in-plane magnetic field is observed to drive a continuous reorientation of the copper spins from the orthorhombic b-axis to the c-axis, directly confirming predictions based on prior magnetoresistance and Raman scattering experiments. A spin-flop transition induced by a c-axis oriented field previously reported for non-stoichiometric La2CuO4 is also observed, but the transition field (11.5 T) is significantly larger than that in the previous work.

cond-mat.str-el↗