Large-moment Antiferromagnetic Phase Research Articles

Isoelectronic perturbations to f-d-electron hybridization and the enhancement of hidden order in URu2Si2

Electrical resistivity measurements were performed on single crystals of URu2-x Os x Si2 up to x = 0.28 under hydrostatic pressure up to P = 2 GPa. As the Os concentration, x, is increased, 1) the lattice expands, creating an effective negative chemical pressure Pch(x); 2) the hidden-order (HO) phase is enhanced and the system is driven toward a large-moment antiferromagnetic (LMAFM) phase; and 3) less external pressure Pc is required to induce the HO→LMAFM phase transition. We compare the behavior of the T(x, P) phase boundary reported here for the URu2-x Os x Si2 system with previous reports of enhanced HO in URu2Si2 upon tuning with P or similarly in URu2-x Fe x Si2 upon tuning with positive Pch(x). It is noteworthy that pressure, Fe substitution, and Os substitution are the only known perturbations that enhance the HO phase and induce the first-order transition to the LMAFM phase in URu2Si2 We present a scenario in which the application of pressure or the isoelectronic substitution of Fe and Os ions for Ru results in an increase in the hybridization of the U-5f-electron and transition metal d-electron states which leads to electronic instability in the paramagnetic phase and the concurrent formation of HO (and LMAFM) in URu2Si2 Calculations in the tight-binding approximation are included to determine the strength of hybridization between the U-5f-electron states and the d-electron states of Ru and its isoelectronic Fe and Os substituents in URu2Si2.

Phase diagram of URu2–xFexSi2 in high magnetic fields

Electrical transport measurements were performed on URu2 - x Fe x Si2 single-crystal specimens in high magnetic fields up to 45 T (DC fields) and 60 T (pulsed fields). We observed a systematic evolution of the critical fields for both the hidden-order (HO) and large-moment antiferromagnetic (LMAFM) phases and established the 3D phase diagram of T-H-x In the HO phase, H/H0 scales with T/T0 and collapses onto a single curve. However, in the LMAFM phase, this single scaling relation is not satisfied. Within a certain range of x values, the HO phase reenters after the LMAFM phase is suppressed by the magnetic field, similar to the behavior observed for URu2Si2 within a certain range of pressures.

Phase diagram and thermal expansion measurements on the system URu2−xFexSi2

Thermal expansion, electrical resistivity, magnetization, and specific heat measurements were performed on URu2-xFexSi2 single crystals for various values of Fe concentration x in both the hidden-order (HO) and large-moment antiferromagnetic (LMAFM) regions of the phase diagram. Our results show that the paramagnetic (PM) to HO and LMAFM phase transitions are manifested differently in the thermal expansion coefficient. The uniaxial pressure derivatives of the HO/LMAFM transition temperature T0 change dramatically when crossing from the HO to the LMAFM phase. The energy gap also changes consistently when crossing the phase boundary. In addition, for Fe concentrations at xc ≈ 0.1, we observe two features in the thermal expansion upon cooling, one that appears to be associated with the transition from the PM to the HO phase and another one at lower temperature that may be due to the transition from the HO to the LMAFM phase.

Evolution of critical pressure with increasing Fe substitution in the heavy-fermion systemURu2−xFexSi2

Measurements of electrical resistivity, $\ensuremath{\rho}(T)$, were performed under quasihydrostatic pressure up to $P\ensuremath{\sim}2.2$ GPa to determine the pressure dependence of the so-called hidden order (HO) and large-moment antiferromagnetic (LMAFM) phases for the ${\mathrm{URu}}_{2\ensuremath{-}x}{\mathrm{Fe}}_{x}{\mathrm{Si}}_{2}$ system with $x$ = 0.025, 0.05, 0.10, 0.15, and 0.20. As the Fe concentration ($x$) is increased, we observed that a smaller amount of external pressure, ${P}_{c}$, is required to induce the HO $\ensuremath{\rightarrow}$ LMAFM phase transition. A critical pressure of ${P}_{c}\ensuremath{\sim}1.2$ GPa at $x=0.025$ reduces to ${P}_{c}\ensuremath{\sim}0$ at $x=0.15$, suggesting the ${\mathrm{URu}}_{2\ensuremath{-}x}{\mathrm{Fe}}_{x}{\mathrm{Si}}_{2}$ system is fully expressed in the LMAFM phase for $x\phantom{\rule{4pt}{0ex}}\ensuremath{\ge}\phantom{\rule{4pt}{0ex}}{x}_{c}^{*}=0.15$, where ${x}_{c}^{*}$ denotes the ambient pressure critical concentration of Fe. Using a bulk modulus calculation to convert $x$ to chemical pressure, ${P}_{\mathrm{ch}}(x)$, we consistently found that the induced HO $\ensuremath{\rightarrow}$ LMAFM phase transition occurred at various combinations of ${x}_{c}$ and ${P}_{c}$ such that ${P}_{\mathrm{ch}}({x}_{c})+{P}_{c}\phantom{\rule{4pt}{0ex}}\ensuremath{\approx}1.5$ GPa, where ${x}_{c}$ denotes those critical concentrations of Fe that induce the HO $\ensuremath{\rightarrow}$ LMAFM phase transition for the ${\mathrm{URu}}_{2\ensuremath{-}x}{\mathrm{Fe}}_{x}{\mathrm{Si}}_{2}$ compounds under pressure. We performed exponential fits of $\ensuremath{\rho}(T)$ in the HO and LMAFM phases in order to determine the pressure dependence of the energy gap, $\mathrm{\ensuremath{\Delta}}$, that opens over part of the Fermi surface in the transition from the paramagnetic (PM) phase to the HO/LMAFM phase at the transition temperature, ${T}_{0}$. The change in the pressure variation of $\mathrm{\ensuremath{\Delta}}(P)$ at the HO $\ensuremath{\rightarrow}$ LMAFM phase transition is consistent with the values of ${P}_{c}$ determined from the ${T}_{0}(P)$ phase lines at the PM $\ensuremath{\rightarrow}$ HO/LMAFM transition.

Enhancement of the hidden order/large moment antiferromagnetic transition temperature in the URu2−xOsxSi2 system

Electrical resistivity, specific heat, and magnetization measurements on the URuOsSi system suggest a phase transition from the ‘hidden order’ phase to another unidentified phase that is likely to be a large moment antiferromagnetic phase. It is noteworthy that the hidden order/large moment antiferromagnetic phase boundary is enhanced from 17.5 K at = 0 to 50 K at = 1. However, as increases, the gap opening in the Fermi surface due to the hidden order phase transition, deduced from electrical resistivity and specific heat measurements, decreases. This study reveals that both Fe and Os isoelectronic substitutions for Ru in URuSi yield an enhancement of . In contrast to the URuFeSi system, where the unit cell volume decreases with , in the URuOsSi system, the unit cell volume increases with . Thus the enhancement of the hidden order/large moment antiferromagnetic transition temperature cannot be solely due to an increase in chemical pressure.

Electronic structure theory of the hidden-order materialURu2Si2

We report a comprehensive electronic structure investigation of the paramagnetic (PM), the large moment antiferromagnetic (LMAF), and the hidden order (HO) phases of ${\text{URu}}_{2}{\text{Si}}_{2}$. We have performed relativistic full-potential calculations on the basis of the density-functional theory, employing different exchange-correlation functionals to treat electron correlations within the open $5f$ shell of uranium. Specifically, we investigate---through a comparison between calculated and low-temperature experimental properties---whether the $5f$ electrons are localized or delocalized in ${\text{URu}}_{2}{\text{Si}}_{2}$. The local spin-density approximation (LSDA) and generalized gradient approximation (GGA) are adopted to explore itinerant $5f$ behavior, the GGA plus additional strong Coulomb interaction ($\text{GGA}+U$ approach) is used to approximate moderately localized $5f$ states, and the $5f$-core approximation is applied to probe potential properties of completely localized uranium $5f$ states. We also performed local-density approximation plus dynamical mean-field theory calculations (DMFT) to investigate the temperature evolution of the quasiparticle states at 100 K and above, unveiling a progressive opening of a quasiparticle gap at the chemical potential when temperature is reduced. A detailed comparison of calculated properties with known experimental data demonstrates that the LSDA and GGA approaches, in which the uranium $5f$ electrons are treated as itinerant, provide an excellent explanation of the available low-temperature experimental data of the PM and LMAF phases. We show furthermore that due to a material-specific Fermi-surface instability a large, but partial, Fermi-surface gapping of up to 750 K occurs upon antiferromagnetic symmetry breaking. The occurrence of the HO phase is explained through dynamical symmetry breaking induced by a mode of long-lived antiferromagnetic spin fluctuations. This dynamical symmetry breaking model explains why the Fermi-surface gapping in the HO phase is similar but smaller than that in the LMAF phase and it also explains why the HO and LMAF phases have the same Fermi surfaces yet different order parameters. A suitable order parameter for the HO is proposed to be the Fermi-surface gap, and the dynamic spin-spin correlation function is further suggested as a secondary order parameter.

Evolution of Magnetism and Its Interplay with Superconductivity in Heavy‐fermion UPt3 Doped with Pd

AbstractFor Abstract see ChemInform Abstract in Full Text.

Evolution of magnetism and its interplay with superconductivity in heavy-fermion UPt 3 doped with Pd

The pseudobinary series U(Pt 1− x Pd x ) 3 demonstrates a wealth of magnetic and superconducting properties that are exemplary for heavy-fermion physics. In this paper, I present a survey of recent neutron-diffraction and μSR experiments, conducted to study the evolution of magnetism, and its interplay with superconductivity, in UPt 3 doped with Pd. The small-moment antiferromagnetic order (SMAF) with T N∼6 K reported for pure UPt 3, is robust upon alloying till at least x=0.005. The small ordered moment m( x) grows from 0.018 μ B/U-atom for x=0.00 to 0.048 μ B/U-atom for x=0.005. T N of the SMAF phase does not vary with Pd content. The increase of m( x) correlates with the splitting Δ T c of the superconducting transition and provides evidence for a Ginzburg–Landau scenario for unconventional superconductivity with magnetism as symmetry-breaking field. The absence of a signal of the SMAF phase in zero-field μSR spectra provides strong evidence for a moment fluctuating at a rate >10 MHz. A second large-moment antiferromagnetic phase (LMAF) is found at higher Pd concentrations. For this phase, at optimum doping ( x=0.05) T N,max=5.8 K and m=0.62 μ B/U-atom. The critical Pd concentration for the emergence of the LMAF phase is x c,af∼0.006. At the same Pd content, superconductivity is completely suppressed. The existence of a magnetic quantum critical point in the phase diagram, which coincides with the critical point for superconductivity ( x c,af= x c,sc≈0.006), yields evidence for odd-parity superconductivity mediated by ferromagnetic spin-fluctuations.

Neutron-diffraction study of the evolution of antiferromagnetic order inUPt3doped with Pd

Neutron-diffraction experiments have been carried out on U(Pt1-xPdx)3 single crystals (x<= 0.05). The small-moment antiferromagnetic order (SMAF) reported for pure UPt3 is robust upon doping and persists till at least x= 0.005. The ordered moment grows from 0.018(2) mB/U-atom for pure UPt3 to 0.048(8) mB/U-atom for x= 0.005. The Neel temperature, TN, is approximately 6 K and does not vary with Pd contents. The order parameter for the SMAF has an unusual quasi-linear temperature variation. For x>= 0.01 a large moment antiferromagnetic phase (LMAF) is found. For this phase at optimum doping (x= 0.05) TN attains a maximum value of 5.8 K and the ordered moment equals 0.63(5) mB/U-atom. TN(x) for the LMAF follows a Doniach-type phase diagram. We infer that the antiferromagnetic instability in U(Pt1-xPdx)3 is located in the range 0.5-1 at.% Pd.