Using high-energy-resolution inelastic x-ray scattering, we observe anomalous softening and damping of the transverse acoustic phonon in UPt_{2}Si_{2} as the system is cooled toward the charge density wave (CDW) transition temperature T_{CDW}. The phonon exhibits a marked Kohn-type anomaly around the CDW wave vector Q_{CDW} and becomes overdamped within a finite momentum range already well above T_{CDW}. The dispersion anomaly is consistent with potential Fermi surface nesting, which together with the extended phonon collapse indicates strong electron-phonon coupling. The transition temperature estimated from the phonon softening is markedly lower than T_{CDW}, consistent with a primarily electronic instability rather than a phonon-driven transition. Our results establish UPt_{2}Si_{2} as a prime example of a strongly correlated electron CDW system with exceptionally strong electron-phonon coupling driving phonon softening and collapse.

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.

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.

Thermoelectricity is a direct conversion between heat energy and electrical power. Thermoelectric generators provide a green approach to energy production but many materials that exhibit thermoelectric phenomena are not efficient enough to compete with other forms of renewable energy. In recent years, the 1-2-20 class of materials has displayed advanced thermoelectric properties, through large Seebeck coefficients and figures of merit. In this study, different rare earth elements were substituted in an effort to enhance the overall thermoelectric properties of these compounds. Using the molten metal flux growth technique, we synthesized crystals of A1-xBxIr2Zn20 (A, B = Ce, Sm, Yb). We characterized the stoichiometry using energy-dispersive x-ray spectroscopy and the structure using x-ray diffraction for each specimen. Detailed thermodynamic and electrical transport properties are investigated in order to determine the effect of chemical substitution on the rare earth site has on the thermoelectric properties.

Strongly correlated f-electron systems are known to host exotic quantum states, such as quantum criticality, complex order parameters, and unconventional superconductivity. However, the appearance of these exotic states is difficult to predict, making the study of quantum critical behavior challenging, especially in ferromagnetic materials. Herein, we report a structure-property map for Ce2M3X5 (M = transition metal; X = main group element) that aids in the targeted design of materials likely to exhibit quantum criticality. Guided by this map, we report on the synthesis of single-crystalline Ce2Ru3Ge5 and provide, for the first time, magnetic susceptibility, heat capacity, resistivity, and magnetoresistance measurements on single crystals. We observe a weak ferromagnetic-like response at 7.5 K, which is contrasted with the bulk ferromagnetic ordering that appears in polycrystalline samples. Non-Fermi liquid behavior is seen in the temperature dependent electrical resistivity and heat capacity of the single crystals, suggesting proximity to a ferromagnetic quantum critical point without chemical or physical pressure. Given the contrast with previous reports of polycrystals, these results lead us to propose that single crystalline Ce2Ru3Ge5 is intrinsically tuned into the vicinity of a ferromagnetic quantum critical point.

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 study the Hall effect, AC magnetic susceptibility ( χ ac ), and magnetic force microscopy of the uniaxial ferromagnet CeRu 2 Ga 2 B with a centrosymmetric crystal structure. We observe a finite topological Hall effect (THE) within the ordered phase, before the magnetization is polarized by applied field. By comparing the field dependences of the area fraction of the magnetic bubbles, the derivative of χ ac , and the THE signal, we deduce that the magnetic bubbles in CeRu 2 Ga 2 B evolve from the trivial to topological spin texture with field. Our findings enable the expansion of the search for magnetic materials hosting topological spin textures to include uniaxial ferromagnets and open a new possibility to tailor the topological spin texture.

For some intermetallic compounds containing lanthanides, structural transitions can result in intermediate electronic states between trivalency and tetravalency; however, this is rarely observed for praseodymium compounds. The dominant trivalency of praseodymium limits potential discoveries of emergent quantum states in itinerant 4 f 1 systems accessible using Pr 4+ -based compounds. Here, we use in situ powder x-ray diffraction and in situ electron energy-loss spectroscopy (EELS) to identify an intermetallic example of a dominantly Pr 4+ state in the polymorphic system Pr 2 Co 3 Ge 5 . The structure-valence transition from a nearly full Pr 4+ electronic state to a typical Pr 3+ state shows the potential of Pr-based intermetallic compounds to host valence-unstable states and provides an opportunity to discover previously unknown quantum phenomena. In addition, this work emphasizes the need for complementary techniques like EELS when evaluating the magnetic and electronic properties of Pr intermetallic systems to reveal details easily overlooked when relying on bulk magnetic measurements alone.

Zintl phases have potential applications as thermoelectric materials for power generation and cooling owing to their complex crystal structures and unique electronic properties. We carried out reactions of silicon with barium and strontium in excess Mg/Zn flux to synthesize (Ba/Sr)(5+x)Mg19-xSi12 Zintl phases, investigating the effect of varying Ba/Sr ratio on site mixing and thermoelectric properties. (Ba/Sr)(5+x)Mg19-xSi12 compounds with 0 < x < 3 are charge-balanced Zintl phases which adopt the hexagonal Ho5Ni19P12 structure type (space group P62m). Density of states calculations indicate that these materials are semimetals. Single-crystal X-ray diffraction data and elemental analysis for Ba5Mg19Si12, Ba4.86Sr2.94Mg16.20Si12, Ba3.63Sr4.20Mg16.17Si12, Ba1.93Sr5.99Mg16.08Si12, and Sr7.82Mg16.18Si12 show occupation of barium and strontium cations in Ho sites, while strontium mixes with magnesium on a specific Ni site. Powder XRD data of products show that they are single phase throughout the sample. Thermoelectric measurements indicate that increasing strontium content and mixing on three cation sites decreases thermal conductivity; it is hypothesized that improved overall thermoelectric behavior is likely due to the rattling of the Sr cations in their positions.

The phenomenon of temperature induced magnetization-reversal, giving rise to negative magnetization (NM)in several rare-earth chromates, has been revisited through magnetization studies on Gd1-xYxCrO3. The studyre-examines the well accepted explanation of NM, i.e., the antiparallel polarization of the paramagnetic rare-earth(R3+) moments against the weak magnetization (MCr) of the canted antiferromagnetically ordered Cr3+momentsubsystem and compares the results with a relatively new explanation invoking frustration of the Gd3+momentsubsystem at temperatures as high as 170 K [Phys. Rev. B99, 014422 (2019)]. Keeping in view the highlylocalized nature of f electrons the invoked frustration appears unphysical. We find that the magnitude of theNM increases with increasing concentration of nonmagnetic Y3+ion. This is attributed to increased antiparallelpolarization of the Gd3+moments against increasedMCrdue to structural modifications in Gd1-xYxCrO3withincreasing Y3+content. This observation strongly contradicts the interpretation for NM based on the frustrationof Gd3+moments. The temperature induced negative to positive magnetization jump (TMJ) is observed and isattributed to the minimization of the Zeeman-energy against an energy barrier. A phenomenological comparativestudy of the modified Curie-Weiss fitting to M(T) indicates that TMJ in polycrystalline Gd1-xYxCrO3is anoutcome of cascaded individual flips of Gd3+moments occurring over random sites having homogeneouslydistributed barrier energy. In our analysis the M(T) data is fitted using modified Curie-Weiss treatingMCrto betemperature dependent, i.e.,MCr(T)=M(0).[1-(T/TC)alpha]beta

New stuffed lanthanide borocarbides were grown from reactions in lanthanide-rich eutectic melts, exploring the use of C14H10 (anthracene), C12F10 (decafluorobiphenyl), and LnCl(3) as sources of interstitial hydride, fluoride, and chloride ions. Ln(3)BC(2)X(n) (Ln = La, Ce, Pr; X = H, F) are analogues of the orthorhombic Ca3C3Cl2 structure type, featuring borocarbide units in place of the allenylide anions and small anions (X = H and F) in place of the chloride anions, partially occupying octahedral and tetrahedral interstitial sites surrounded by lanthanide cations. Single-crystal neutron diffraction data for La3BC2H1.69 confirm that hydrogen is present in both interstitial sites, with site-splitting in the octahedral site. Magnetic studies and resistivity measurements indicate that La3BC2H1.69 shows superconductivity with a T-c near 4.5 K. Pr3BC2Hx shows ferromagnetic ordering below 55 K, while the fluoride analogue Pr3BC2F0.71 is paramagnetic. Attempts to incorporate chlorine into the interstitial sites lead to a structure change to the La3BC2Br2 structure type.

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.

Thermoelectric devices are both solid-state heat pumps and energy generators. Having a reversible process without moving parts is of high importance for applications in remote locations or under extreme conditions. Yet, most thermoelectric devices have a rather limited energy conversion efficiency due to the natural competition between high electrical conductivity and low thermal conductivity, both being essential conditions for achieving a high energy conversion efficiency. Heavy-fermion compounds YbT2Zn20 (T = Co, Rh, Ir) have been reported to be potential candidate materials for thermoelectric applications at low temperatures. Motivated by this result, we applied chemical substitution studies on the transition metal site in order to optimize the charge carrier concentration as well as promote more efficient phonon scatterings. Here, we present the latest investigation on the Ni-doped specimens YbCo2-xNixZn20, where enhanced thermoelectric figure of merit values have been obtained.

The Ln(n+1)M(n)X(3n+1) (Ln = lanthanide, M = transition metal, and X = tetrel) homologous series provides a platform to study collective phenomena in quantum materials. In this work, we compare the crystal growth, structure, and magnetic properties of the n = 3 members of the Prn+1MnGe3n+1 (M = Fe, Co) analogues, Pr4Fe3Ge10 (a = 4.3207 (10) & Aring;, b = 35.523 (8) & Aring;, c = 4.2982 (15) & Aring;, and V = 659.7 (3) & Aring;(3)) and Pr4Co3Ge10 (a = 4.3091 (12) & Aring;, b = 35.750 (9) & Aring;, c = 4.2807 (11) & Aring;, and V = 659.4 (3) & Aring;(3)). We determined that the ideal flux growth conditions for each compound are highly dependent on the concentration of Sn flux and quench temperature. Pr4Fe3Ge10 orders ferromagnetically at 10 K along the c-direction while Pr4Co3Ge10 orders antiferromagnetically at 16 K along the b-direction. For both compounds, we observed a magnetic moment higher than that expected for only Pr3+ ions (3.58 mu (B)/Pr), implying that the transition metal ions contribute to magnetic ordering (3.91, 3.48, and 3.69 mu (B)/Pr for Pr4Fe3Ge10, and 3.76, 4.04, and 3.83 mu (B)/Pr for Pr4Co3Ge10 measured along the a-, b-, and c-directions, respectively). Moreover, the zero-field M & ouml;ssbauer spectrum obtained at 4.2 K for Pr4Fe3Ge10 demonstrates that the iron sites participate in magnetic ordering.

The compounds LnAuAl4Ge2 (Ln = lanthanide) form in a structure that features two-dimensional triangular lattices of Ln ions that are stacked along the crystalline c axis. Together with crystal electric field effects, magnetic anisotropy, and electron-mediated spin exchange interactions, this sets the stage for the emergence of strongly correlated spin and electron phenomena. Here we investigate SmAuAl4Ge2, which exhibits weak paramagnetism that strongly deviates from conventional Curie-Weiss behavior. Complex antiferromagnetic ordering emerges at TN1 = 13.2 K and TN2 = 7.4 K, where heat capacity measurements show that these transitions are first and second order, respectively. These measurements also reveal that the Sommerfeld coefficient is not enhanced compared to the nonmagnetic analog YAuAl4Ge2, consistent with the charge carrier quasiparticles exhibiting typical Fermi liquid behavior. The temperature-dependent electrical resistivity follows standard metallic behavior, but linear magnetoresistance unexpectedly appears within the ordered state. We compare these results to other LnAuAl4Ge2 materials, which have already been established as localized f -electron magnets that are hosts for interesting magnetic and electronic phases. From this, SmAuAl4Ge2 emerges as a complex quantum spin metal, inviting further investigations into its properties and the broader family of related materials.

The synthesis and characterization of Ce4Fe3Ge10, n = 3, member of the homologous series Ln(n+1)M(n)X(3n+1) (Ln = lanthanides, M = transition metal, X = tetrel), is reported. The structure can be modeled with the Cmcm space group adopting the Eu2Ni2-x Sn-5 structure type, with lattice parameters of a = 4.3323 (15) & Aring;, b = 35.507 (9) & Aring;, and c = 4.3069 (12) & Aring;. Members of the series for n > 2 consist of stacking of ordered (CeNiSi2 type) and disordered (BaNiSn3/AuCu3 type) subunits with the latter acting as a "spacer" between CeNiSi2 subunits. Although neither CeFeGe3 nor CeFe0.63Ge2 order magnetically down to 2 K, Ce4Fe3Ge10 is an antiferromagnet below 3.6 K. To rationalize the emergent magnetism: (i) we established the Kondo- and RKKY-interaction dominant regions for the Ce analogues adopting the BaNiSn3 and CeNiSi2 by creating an electronic landscape for each and (ii) mapped the strained subunits, due to stacking, within the series. We established that the CeFeGe3 subunit within Ce4Fe3Ge10 contracts and is located within the Kondo-interaction dominant region, while the CeFeGe(2 )subunit expands and is in the RKKY-interaction dominant region.

There is an ongoing interest in kagome materials because they offer tunable platforms at the intersection of magnetism and electron correlation. Herein, we examine single crystals of new kagome materials, LnxCo3(Ge1-ySny)3 (Ln = Y, Gd; y = 0.11, 0.133), which were produced using the Sn flux-growth method. Unlike many of the related chemical analogues with the LnM6X6 formula (M = transition metal and X = Ge, Sn), the Y and Gd analogues crystallize in a hybrid YCo6Ge6/CoSn structure, with Sn substitution. While the Y analogue displays temperature-independent paramagnetism, magnetic measurements of the Gd analogue reveal a magnetic moment of 8.48 μB, indicating a contribution from both Gd and Co. Through anisotropic magnetic measurements, the direction of Co-magnetism can be inferred to be in plane with the kagome net, as the Co contribution is only along H//a. Crystal growth and structure determination of YxCo3(Ge,Sn)3 and GdxCo3(Ge,Sn)3, two new hybrid kagome materials of the CoSn and YCo6Ge6 structure types. Magnetic properties, heat capacity, and resistivity on single crystals are reported.