The mechanism behind superconductivity suppression induced by Pr substitutions in YBa2Cu3O7-δ (YBCO) has been a mystery since its discovery: in spite of being isovalent to Y3+ with a small magnetic moment, it is the only rare-earth element that has a dramatic impact on YBCO's superconducting properties. Using angle-resolved photoemission spectroscopy (ARPES) and DFT+[Formula: see text] calculations, we uncover how Pr substitution modifies the low-energy electronic structure of YBCO. Contrary to the prevailing Fehrenbacher-Rice (FR) and Liechtenstein-Mazin (LM) models, the low-energy electronic structure contains no signature of any f-electron hybridization or additional f-state Fermi surface sheets. Yet, strong electron doping is observed primarily on the antibonding Fermi surface. Meanwhile, we reveal major electronic structure modifications to Cu-derived states with increasing Pr substitution: a pronounced CuO2 bilayer decoupling and enhanced hopping along the CuO chain, implying indirect electron-release pathways beyond simple 4f state ionization. Our results challenge the long-standing FR/LM mechanism, and establish Pr substituted YBCO as a potential platform for exploring correlation-driven phenomena in coupled 1D-2D systems.

Symmetry properties of the order parameter are among the most fundamental characteristics of a superconductor. The pairing symmetry of recently discovered heavy fermion superconductor UTe2 featuring an exceedingly large upper critical field has attracted a great deal of attention. Even though it is widely believed that UTe2 possesses an odd-parity, spin-triplet pairing symmetry, direct evidence for it is lacking, especially at zero or low magnetic fields. We report here the selection-rule results of Josephson coupling between In, an s-wave superconductor, and UTe2. The orientation dependence of the Josephson coupling suggests very strongly that UTe2 possess an odd-parity pairing state of B_1u in zero magnetic fields. We also report the formation of Andreev surface bound states on the (1-10) surface of UTe2.

At temperatures much lower than its superconducting critical temperature $T_c$ of 2.1 K, the heavy fermion superconductor UTe$_2$ has a remarkable phase diagram of magnetic field $H$ vs. angles $\phi$ and $\theta$ at which $H$ is tilted away from the $b$-axis toward the $a$- and $c$-axes, respectively, in the orthorhombic unit cell. The phase diagram appears to contain three superconducting phases: (1) a low field superconducting phase SC$_{\mathrm{LF}}$ extending over all values of $\phi$ and $\theta$ with an upper critical field $H_{c2}$ with a maximum value of 15 T at $\phi = \theta = 0^\circ$; (2) a high field superconducting phase SC$_{\mathrm{HF}}$ located in a region between $\phi \approx 7^\circ$ and $\theta \approx 4^\circ$ in fields from $H_{c2\mathrm{LF}}$ of the SC$_{\mathrm{LF}}$ phase and the metamagnetic transition at $H_m$ at $\sim 35$ T marking the onset of the magnetic field polarized FP phase: and (3) a SC$_{\mathrm{FP}}$ superconducting phase that resides entirely within the FP phase in a pocket of superconductivity extending from $\theta \approx 20^\circ$ to $40^\circ$ in fields from $\sim 40$ T to above 60 T. In this work, we studied the $H$ vs $\theta$ phase diagram at a base temperature of $\sim 0.6$ K as a function of Th concentration $x$ in U$_{1-x}$Th$_x$Te$_2$ pseudobinary compounds for $0.5\% \lesssim x \lesssim 4.7\%$. We find that for all values of $x$ within this range, the SC$_{\mathrm{LF}}$ phase is retained with a reduced value of $H_{c2}$ of $\sim 10$ T at $\phi = \theta = 0^\circ$ for $x = 4.7\%$, while the SC$_{\mathrm{HF}}$ phase is suppressed. The SC$_{\mathrm{FP}}$ and FP phases are unaffected to values of $x = 2\%$ but are completely suppressed in the region $x = 2.5$ to $4.7\%$ where the residual resistance ratio RRR has decreased from $\sim 14$ at $x = 1.5\%$ to values of $\sim 3$, indicating a significant increase in disorder.

In 1975 Edwards and Anderson introduced a new paradigm that interacting quenched systems, such as a spin-glass, have a phase transition in which long time memory of spatial patterns is realized without spatial correlations. We show here that the information about the time-dependent correlations above the spin-glass transition are embedded in the four spin correlations of the intensity of speckle pattern. This encodes the spin-orientation memory and can be measured by the technique of resonant magnetic x-ray photon correlation spectroscopy (RM- XPCS). We have implemented this method to observe and accurately characterize the critical slowing down of the spin orientation fluctuations in the classic metallic spin glass alloy Cu_1-xMn_x over time scales of 2 sec. to 2 × 10^4 secs. Remarkably the divergence of the correlation time as a function of temperature is consistent with the Vogel-Vulcher law, universally used to characterize the viscous relaxation time in structural glasses. Our method also opens the way for studying phase transitions in systems such as spin ices, quantum spin liquids, the structural glass transition, as well as possibly provide new perspectives on the multifarious problems in which spin-glass concepts have found applications.

At temperatures much lower than its superconducting critical temperature T_c of 2.1 K, the heavy fermion superconductor UTe_2 has a remarkable phase diagram of magnetic field H vs. angles ϕ and θ at which H is tilted away from the b-axis toward the a- and c-axes, respectively, in the orthorhombic unit cell. The phase diagram appears to contain three superconducting phases: (1) a low field superconducting phase SC_LF extending over all values of ϕ and θ with an upper critical field H_c2 with a maximum value of 15 T at ϕ = θ = 0^∘; (2) a high field superconducting phase SC_HF located in a region between ϕ≈ 7^∘ and θ≈ 4^∘ in fields from H_c2LF of the SC_LF phase and the metamagnetic transition at H_m at ∼ 35 T marking the onset of the magnetic field polarized FP phase: and (3) a SC_FP superconducting phase that resides entirely within the FP phase in a pocket of superconductivity extending from θ≈ 20^∘ to 40^∘ in fields from ∼ 40 T to above 60 T. In this work, we studied the H vs θ phase diagram at a base temperature of ∼ 0.6 K as a function of Th concentration x in U_1-xTh_xTe_2 pseudobinary compounds for 0.5%≲ x ≲ 4.7%. We find that for all values of x within this range, the SC_LF phase is retained with a reduced value of H_c2 of ∼ 10 T at ϕ = θ = 0^∘ for x = 4.7%, while the SC_HF phase is suppressed. The SC_FP and FP phases are unaffected to values of x = 2% but are completely suppressed in the region x = 2.5 to 4.7% where the residual resistance ratio RRR has decreased from ∼ 14 at x = 1.5% to values of ∼ 3, indicating a significant increase in disorder.

At temperatures T much lower than its superconducting critical temperature Tc = 2.1 K, the heavy fermion superconductor UTe2 has a unique phase diagram of magnetic field H vs. φ and θ, angles H is tilted from the b-axis toward the a- and c-axes, respectively, of its orthorhombic unit cell. The phase diagram contains three distinct superconducting phases: SC1 in which φ and θ extend from 0 to 90° and H ≤ ~15 T; SC2 for φ ≤ ~7°, θ ≤ ~4° and ~15 T ≤ H ≤ Hm = ~35 T, the onset of the magnetic field polarized (FP) phase, and SCFP which resides entirely within the FP phase in a pocket of superconductivity extending from θ ≈ 20° to 40° and from ~40 T to above 60 T. We studied the evolution of the H vs. θ phase diagram for Th concentrations 0.005 ≤ x ≤ 0.047 in the U1-xThxTe2 system at ~0.6 K. Within this range of x values, SC1 extends over 0 ≤ θ ≤ 90° and H ≤ ~10 T for x = 0.047, while SC2 is suppressed. The SCFP and FP phases are unaffected to x = 0.02 but are completely suppressed in the region x = 0.025 to 0.047 where the residual resistance ratio RRR ~3 indicates a significant amount of disorder. These results complement recent studies of nonsuperconducting disordered UTe2 single crystals in which the SC1 and SC2 phases are absent, but the FP and so-called "orphan" SCFP phases are retained.

We report results of magnetization and 19F NMR measurements in the normal state of as-grown vacuum- annealed LaO0.5F0.5BiS2. The magnetization is dominated by a temperature-independent diamagnetic component and a field- and temperature-dependent paramagnetic contribution M mu (H, T ) from a <<^>> 1000 ppm concentration of local moments, an order of magnitude higher than can be accounted for by measured rare-earth impurity concentrations. M mu (H, T ) can be fit by the Brillouin function B J ( x ) or, perhaps more realistically, a two-level tanh(x) model for magnetic Bi 6p ions in defect crystal fields. Both fits require a phenomenological Curie-Weiss argument x = mu eff H / ( T + T w ), Tw approximate to 1.7 K. There is no evidence for magnetic order down to 2 K, and the origin of Tw is not clear. 19F frequency shifts, linewidths, and spin-lattice relaxation rates are consistent with purely dipolar 19 F / defect-spin interactions. The defect-spin correlation time t c ( T ) obtained from 19F spin-lattice relaxation rates obeys the Korringa relation t c T = const, indicating the relaxation is dominated by conduction- band fluctuations.

High pressure x-ray diffraction up to 30 GPa and resonant emission x-ray spectroscopy and partial fluorescence yield x-ray absorption spectroscopy up to 52 GPa were used to study how the structural and electronic properties of UTe$_2$ evolve with pressure at room temperature. An orthorhombic to tetragonal phase transition was observed to occur between 5 and 7 GPa, with a large volume collapse of nearly 11% and a nearest U-U distance increase by about 4%. This lower to higher symmetry transition suggests less 5f electron participation in bonding when the weakly correlated superconducting phase in the tetragonal structure of UTe$_2$ appears. Beyond 7 GPa, no new structural transitions were found up to 30 GPa. The resonant x-ray emission spectra clearly demonstrate an intermediate valence of U, nearly +3.74 at 1.8 GPa and room temperature, and reveal that the U valence shifts towards 4+, passes through a peak at 2.8 GPa, and then decreases towards 3+ and settles down to a nearly constant value above 15 GPa. These experiments reveal that some fundamental structural and valence changes occur in UTe2 at relatively low pressures, which could be responsible for the interplay between unconventional superconductivity, magnetic ordering, and weakly correlated superconductivity that is manifested in the temperature-pressure phase diagram of UTe2.

We report measurements of magnetization, specific heat, and thermal expansion performed on As-deficient MnAs single crystals (MnAs_0.968). Ferromagnetic order is observed near T_C ≃ 306 K on warming and T_C ≃ 302 K on cooling, which is consistent with previously-reported values for stoichiometric MnAs samples. In contrast, the second-order structural phase transition is observed at T_S ≃ 353 K, which is nearly 50 K lower than in the stoichiometric compound. We observe differences in the thermal expansion of our samples when compared to reports of stoichiometric MnAs including: (1) the ∼1.5 (2) the lattice parameters perpendicular to the basal plane exhibit a discontinuous jump of ∼1.1 T_C, and (3) thermal expansion perpendicular to the basal plane for T_C ≤ T ≤ 315 K is negative rather than positive. We also observe a correlation between the ratio of hexagonal lattice parameters, c/a, and T_S, strongly suggesting that the degree of structural anisotropy in MnAs could play an important role in tuning T_S.

Single-crystalline FeSi samples with a conducting surface state were studied under high pressure and magnetic field by means of electrical resistance measurements to explore how the bulk semiconducting state and the surface state are tuned by the application of pressure. We found that the energy gap associated with the semiconducting bulk phase begins to close abruptly at a critical pressure of similar to 10 GPa and the bulk material becomes metallic with no obvious sign of any emergent phases or non-Fermi liquid behavior in temperature dependent electrical resistance in the neighborhood of the critical pressure above 3 K. Moreover, the metallic phase appears to remain at near-ambient pressure upon release of the pressure. Interestingly, the hysteresis in the electrical resistance vs magnetic field curve associated with the magnetically ordered conducting surface state decreases with pressure and vanishes at the critical pressure, while the slope of the electrical resistance vs magnetic field curve, which has a negative value for pressure below the critical pressure, decreases in magnitude with pressure and changes sign at the critical pressure. Thus the conducting surface state and the corresponding two-dimensional magnetic order collapse at the critical pressure where the energy gap of the bulk material starts to close abruptly, revealing the connection between the conducting surface state and the semiconducting bulk state in FeSi.

The application of pressure as well as the successive substitution of Ru with Fe in the hidden order (HO) compound URu2Si2 leads to the formation of the large-moment antiferromagnetic phase. Here, we investigate the substitution series URu2-xFexSi2 from x = 0.0 to 2.0 by U 4 f core-level photoelectron spectroscopy and observe nonmonotonic changes in the spectra. The initial increase and subsequent decrease in the spectral weight of the 4 f core-level satellite with increasing x stands for a nonmonotonic 5 f filling across the substitution series. The competition of chemical pressure and increase in the density of states at the Fermi energy, both due to substitution of Ru with Fe, can explain such behavior. An extended Doniach phase diagram including the x dependence of the density of states is proposed. Also in URu2-xFexSi2 the ground state is a singlet or quasidoublet state consisting of two singlets. Hence, the formation of magnetic order in the URu2-xFexSi2 substitution series must be explained within a singlet magnetism model.

We report the results of thermodynamic measurements in external magnetic field of the cubic Ce-based cage compounds CeT2Cd20(T= Ni,Pd). Our analysis of the heat-capacity data shows that the Γ7doublet is the ground state multiplet of the Ce3+ions. Consequently, for the Γ7doublet it can be theoretically shown that the Ruderman-Kittel-Kasuya-Yosida interaction between the localized Ce moments mediated by the conduction electrons, must vanish at temperatures much lower than the energy separating the ground state doublet from the first excited Γ8quartet. Our findings provide an insight as to why no long range order has been observed in these compounds down to temperatures in the milliKelvin range.

The Kondo lattice compound URu2Si2 has attracted much interest during the past several decades because it exhibits an unknown ordered phase (referred to as "hidden order" (HO)) at THO = 17.5 K, as well as unconventional superconductivity below Tc ≈ 1.5 K. Extensive efforts have been made to disentangle these behaviors using chemical substitution, where remarkable continuity has emerged in the resulting phase diagrams. In particular, substitution strategies that produce: (i) enhancement of the hybridization strength (pressure and Ru → isoelectronic Fe or Os substitution) convert hidden order to local moment antiferromagnetism; (ii) electron-like doping (Ru → Co, Rh, Ir, and Pt or Si → P substitution) lead to an abrupt collapse of hidden order and the appearance of Pauli paramagnetism, which are followed by the onset of antiferromagnetic order, and; (iii) hole-like substitution (Ru → Mn, Tc, and Re) lead to suppression and sudden quenching of hidden order, which is interrupted by the occurrence of Pauli paramagnetism before the onset of ferromagnetic order. Here, we extend these studies through a detailed investigation of the effect of Co and Ir substitution at the Ru site on the electrical resistivity, magnetic susceptibility, and specific heat. These measurements are used to construct T − x phase diagrams, which reveal suppression of the HO and superconducting phases with increasing x in both systems. We examine these results in the context of an impurity-based model and find that while the model is partially successful, it is also clear that electronic tuning plays an important role in determining the ground state. Finally, we find that the rate of suppression of the HO energy gap ∆ with x, where ∆ is inferred from the exponential behavior of the specific heat below THO, is larger than the rate of suppression of THO with x, suggesting a possible scenario of a "gapless" HO phase, similar to that previously proposed for the URu2−xRexSi2 system. PACS numbers: 74.70-b, 71.27.+a, 74.62. Bf, 71.10. Ay.

Our group has previously reported the existence of a conducting surface state (CSS) in FeSi, a candidate for a d-electron topological Kondo insulator (TKI), at low temperature. In this paper, we present the electrical transport properties of single crystals of FeSi studied in the phase space of temperature (T ), magnetic field (B), and the angle (theta) between the electrical current and B. The normalized T-dependent electrical resistance (R) of a successively thinned FeSi crystal provides further confirmation of the existence of a CSS. We report that, in the CSS, the magnetoresistance (MR) exhibits a hysteresis loop bounded within +/- 0.5 T, suggesting two-dimensional magnetic ordering. The hysteretic MR is asymmetric in B and anisotropic with respect to theta. Further exploration of R(theta) at a fixed field of 9 T reveals an initial progressive rotation of the axis for twofold rotational symmetry from 2 to 10 K, then a stabilized axis for twofold symmetry until, at T > 40 K, the anisotropy vanishes, coincident with the disappearance of the CSS. These observations point to a possible magnetically ordered surface state that has been reported in similar systems such as FeSi nanofilms and bulk SmB6.

Recently, evidence for a conducting surface state below 19 K was reported for the correlated d-electron small gap semiconductor FeSi. In the work reported herein, the conducting surface state and the bulk phase of FeSi were probed via electrical resistivity measurements as a function of temperature T, magnetic field B to 60 T and pressure P to 7.6 GPa, and by means of a magnetic field modulated microwave spectroscopy (MFMMS) technique. The properties of FeSi were also compared to those of the Kondo insulator SmB6 to address the question of whether FeSi is a d-electron analogue of an f-electron Kondo insulator and, in addition, a topological Kondo insulator. The overall behavior of the magnetoresistance MR of FeSi at temperatures above and below the onset temperature (T_S) 19 K of the conducting surface state is similar to that of SmB6. The two energy gaps, inferred from the resistivity data in the semiconducting regime, increase with pressure up to about 7 GPa, followed by a drop which coincides with a sharp suppression of T_S. This behavior is similar to that reported for SmB6, except that the two energy gaps in SmB6 decrease with pressure before dropping abruptly at T_S. The MFMMS measurements showed a sharp feature at T_S (19 K) for FeSi, but no such feature was observed at T_S 4.5 K for SmB6. The absence of a feature at T_S for SmB6 may be due to experimental issues and will be the subject of a future investigation.

The shape of 3$d$-orbitals often governs the electronic and magnetic properties of correlated transition metal oxides. In the superconducting cuprates, the planar confinement of the $d_{x^2-y^2}$ orbital dictates the two-dimensional nature of the unconventional superconductivity and a competing charge order. Achieving orbital-specific control of the electronic structure to allow coupling pathways across adjacent planes would enable direct assessment of the role of dimensionality in the intertwined orders. Using Cu-$L_3$ and Pr-$M_5$ resonant x-ray scattering and first-principles calculations, we report a highly correlated three-dimensional charge order in Pr-substituted YBa$_2$Cu$_3$O$_{7}$, where the Pr $f$-electrons create a direct orbital bridge between CuO$_2$ planes. With this, we demonstrate that interplanar orbital engineering can be used to surgically control electronic phases in correlated oxides and other layered materials.

In this study, we change the chemical pressure within YbCo 2 Zn 20 by substituting the radially smaller Sc for Yb (Yb 1− x Sc x Co 2 Zn 20 ) in order to investigate the relationship between pressure and the correlated electron ground state of Yb. We find that the behavior of Yb 1− x Sc x Co 2 Zn 20 is divided into two regimes, x ≤ 0.3 and x > 0.3. Resistivity ( ρ ( T )) measurements reveal not only a low-temperature upturn that can be described by a log T -dependence, consistent with the predictions of the single ion Kondo model, but also that the large and robust Kondo contribution to the resistivity observed in the range 0 ≤ x ≤ 0.3, becomes much weaker for 0.3 < x ≤ 1. We determine the Yb valence v Yb from X-ray powder diffraction (XRD) patterns at room temeprature and magnetization ( M ( T )) measurements and observe that v Yb decreases linearly from ∼ 3+ at x = 0 to ∼ 2.7+ at x = 0.3 before stabilizing back at ∼ 3+ for 0.3 < x ≤ 1. We analyze C ( T ) /T for Yb 1− x Sc x Co 2 Zn 20 and find signatures consistent with non-Fermi liquid (NFL) like behavior, suggesting a nearby quantum critical point (QCP). The Yb 1− x Sc x Co 2 Zn 20 system shows a confluence of phenomena typically found in 4 f electron systems including valence fluctuations, the Kondo effect, and heavy fermion behavior.

CeOs 4 Sb 12 , a member of the skutterudite family, has an unusual semimetallic low-temperature L -phase that inhabits a wedge-like area of the field H —temperature T phase diagram. We have conducted measurements of electrical transport and megahertz conductivity on CeOs 4 Sb 12 single crystals under pressures of up to 3 GPa and in high magnetic fields of up to 41 T to investigate the influence of pressure on the different H – T phase boundaries. While the high-temperature valence transition between the metallic H -phase and the L -phase is shifted to higher T by pressures of the order of 1 GPa, we observed only a marginal suppression of the S -phase that is found below 1 K for pressures of up to 1.91 GPa. High-field quantum oscillations have been observed for pressures up to 3.0 GPa and the Fermi surface of the high-field side of the H -phase is found to show a surprising decrease in size with increasing pressure, implying a change in electronic structure rather than a mere contraction of lattice parameters. We evaluate the field-dependence of the effective masses for different pressures and also reflect on the sample dependence of some of the properties of CeOs 4 Sb 12 which appears to be limited to the low-field region.

We perform measurements of the heat capacity as a function of temperature on URu2-xOsxSi2 alloys. Our experimental results show that the critical temperature of the second-order phase transition increases while the value of the Sommerfeld coefficient in the ordered state decreases with an increase in osmium concentration. We also observe an increase in the values of the heat capacity at the critical temperature as well as a broadening of the critical fluctuation region with an increase in x. We analyze the experimental data using the Haule-Kotliar model which, in particular, identifies the "hidden order" transition in the parent material URu2Si2 as a transition to a state with a nonzero hexadecapole moment. We demonstrate that our experimental results are consistent with the predictions of that model.