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We report the magnetic and electronic transport properties of the inversion and time-reversal symmetry breaking Weyl semimetal NdAlGe. This material is analogous to NdAlSi, whose helical magnetism presents a rare example of a Weyl-mediated collective phenomenon, but with a larger spin-orbit coupling. Our neutron diffraction experiments revealed that NdAlGe, similar to NdAlSi, supports an incommensurate Ising spin density wave ($T_{\text{inc}}=6.8$ K) with a small helical spin canting of 3$^\circ$ and a long-wavelength of $\sim$ 35 nm, which transitions to a commensurate ferrimagnetic state below $T_{\text{com}}=5.1$ K. Using small-angle neutron scattering, we showed that the zero-field cooled ferrimagnetic domains form stripes in real space with characteristic length scales of 18 nm and 72 nm parallel and perpendicular to the [110] direction, respectively. Interestingly, for the transport properties, NdAlSi does not exhibit an anomalous Hall effect (AHE) that is commonly observed in magnetic Weyl semimetals. In contrast to NdAlSi, we identify two different AHE regimes in NdAlGe that are respectively governed by intrinsic Berry curvature and extrinsic disorders/spin fluctuations. Our study suggests that Weyl-mediated magnetism prevails in this group of noncentrosymmetric magnetic Weyl semimetals NdAl$X$, but transport properties including AHE are affected by material-specific extrinsic effects such as disorders, despite the presence of prominent Berry curvature., Preprint, 16 pages, 6 main figures, 6 supplementary figures

We report full vector mapping of local magnetization in CeAlSi, a Weyl semimetal in which both inversion and time-reversal symmetries are broken. The vector maps reveal unanticipated features both within domains and at their boundaries. Boundaries between domains form two kinds of walls with distinct topology and therefore different interactions with Weyl fermions. Domain walls aligned along the tetragonal axes, e.g. (100), exhibit emergent chirality forbidden by the bulk space group, while diagonal walls are non-chiral. Within the domains, we observe that the previously reported set of four easy axes aligned along the in-plane diagonals of the tetragonal structure actually split to form an octet with decreasing temperature below the magnetic transition. All the above phenomena are ultimately traced to the noncollinear magnetic structure of CeAlSi., 5+8 pages, 4+3 figures

We use x-ray spectroscopy at the Ir ${L}_{3}/{L}_{2}$ absorption edge to study powder samples of the intercalated honeycomb magnet ${\mathrm{Ag}}_{3}\mathrm{Li}{\mathrm{Ir}}_{2}{\mathrm{O}}_{6}$. Based on x-ray absorption and resonant inelastic x-ray scattering measurements, and exact diagonalization calculations including nearest-neighbor Ir-Ir electron hopping integrals, we argue that the intercalation of Ag atoms results in a delocalized electronic structure with enhanced Ir-O hybridization, departing from the local relativistic ${j}_{\mathrm{eff}}=1/2$ state. We find that the relative orbital contribution to the magnetic moment is increased and the magnetization density is spatially extended and asymmetric in this hybridized state. Our results confirm the importance of metal-ligand hybridization in the magnetism of transition metal oxides and provide empirical guidance for understanding the collective magnetism in intercalated honeycomb iridates.

${\mathrm{Ag}}_{3}\mathrm{Li}{\mathrm{Ir}}_{2}{\mathrm{O}}_{6}$ is a Kitaev spin-liquid candidate material synthesized from $\ensuremath{\alpha}\text{\ensuremath{-}}{\mathrm{Li}}_{2}\mathrm{Ir}{\mathrm{O}}_{3}$ through a topotactic reaction. We investigate the structural and magnetic properties of two samples of ${\mathrm{Ag}}_{3}\mathrm{Li}{\mathrm{Ir}}_{2}{\mathrm{O}}_{6}$ based on the $^{7}\mathrm{Li}$ nuclear magnetic resonance line shape, Knight shift, and spin-lattice relaxation rate $1/{T}_{1}$. The first sample A shows signatures of magnetically ordered spins, and exhibits one sharp $^{7}\mathrm{Li}$ peak with the full width at half maximum increasing significantly below 14 K. $1/{T}_{1}^{\text{stretch}}$ of this sample displays a broad local maximum at 40 K, followed by a very sharp peak at ${T}_{N}=9\ifmmode\pm\else\textpm\fi{}1\phantom{\rule{0.16em}{0ex}}\mathrm{K}$ due to a critical slowing down of Ir spin fluctuations, a typical signature of magnetic long-range order. In order to shed light on the position-by-position variation of $1/{T}_{1}$ throughout the sample, we use a numerical inverse Laplace transform ${T}_{1}$ analysis based on Tikhonov regularization to deduce the density distribution function $P(1/{T}_{1})$. We demonstrate that $\ensuremath{\sim}60%$ of Ir spins are statically ordered at the NMR measurement timescale but the rest of the sample volume remains paramagnetic even at 4.2 K, presumably because of structural disorder induced primarily by stacking faults. In order to further investigate the influence of structural disorder, we compare these NMR results with those of a second sample B, which has been shown by transmission electron microscopy to have domains with unwanted Ag inclusion at the Li and Ir sites within the Ir honeycomb planes. Sample B displays an additional NMR peak with a relative intensity of $\ensuremath{\sim}17%$. The small Knight shift and $1/{T}_{1}$ of these defect-induced $^{7}\mathrm{Li}$ sites and the enhancement of bulk susceptibility at low temperatures suggest that these defects generate domains of only weakly magnetic Ir spins accompanied by free spins, leading to a lack of clear signatures of long-range order. The apparent lack of long-range order could be easily misinterpreted as evidence for the realization of a spin-liquid ground state in highly disordered Kitaev lattices.

The search for an ideal Kitaev spin liquid candidate with anyonic excitations and long-range entanglement has motivated the synthesis of a new family of intercalated Kitaev magnets such as ${\mathrm{H}}_{3}\mathrm{Li}{\mathrm{Ir}}_{2}{\mathrm{O}}_{6}$, ${\mathrm{Cu}}_{2}\mathrm{Ir}{\mathrm{O}}_{3}$, and ${\mathrm{Ag}}_{3}\mathrm{Li}{\mathrm{Ir}}_{2}{\mathrm{O}}_{6}$. The absence of a susceptibility peak and a two-step release of the magnetic entropy in these materials have been proposed as evidence of proximity to the Kitaev spin liquid. Here we present a comparative study of the magnetic susceptibility, heat capacity, and muon spin relaxation ($\ensuremath{\mu}\mathrm{SR}$) between two samples of ${\mathrm{Ag}}_{3}\mathrm{Li}{\mathrm{Ir}}_{2}{\mathrm{O}}_{6}$ in the clean and disordered limits. In the disordered limit, the absence of a peak in either susceptibility or heat capacity and the lack of zero-field muon precession in the $\ensuremath{\mu}\mathrm{SR}$ signal give the impression of a proximate spin liquid state. However, in the clean limit, peaks are resolved in both susceptibility and heat capacity, and spontaneous oscillations appear in the $\ensuremath{\mu}\mathrm{SR}$ signal, confirming long-range antiferromagnetic order in the ground state. The $\ensuremath{\mu}\mathrm{SR}$ oscillations fit to a Bessel function, characteristic of incommensurate order, as reported in the parent compound $\ensuremath{\alpha}\text{\ensuremath{-}}{\mathrm{Li}}_{2}\mathrm{Ir}{\mathrm{O}}_{3}$. Our results clarify the role of structural disorder in the intercalated Kitaev magnets.

The magnetic properties of iron-based superconductors $A{\mathrm{Fe}}_{2}{\mathrm{As}}_{2}$ ($A=\mathrm{K}$, Cs, and Rb), which are characterized by the V-shaped dependence of the critical temperature (${T}_{\mathrm{c}}$) on pressure ($P$), were studied by means of the muon spin rotation/relaxation technique. In all three systems studied the magnetism was found to appear for pressures slightly below the critical one (${P}_{\mathrm{c}}$), i.e., at pressure where ${T}_{\mathrm{c}}(P)$ changes the slope. Rather than competing, magnetism and superconductivity in $A{\mathrm{Fe}}_{2}{\mathrm{As}}_{2}$ are coexisting at the $P\ensuremath{\gtrsim}{P}_{\mathrm{c}}$ pressure region. Our results support the scenario of a transition from one pairing state to another, with different symmetries on either side of ${P}_{\mathrm{c}}$.

Kitaev magnets are materials with bond-dependent Ising interactions between localized spins on a honeycomb lattice. Such interactions could lead to a quantum spin-liquid (QSL) ground state at zero temperature. Recent theoretical studies suggest two potential signatures of a QSL at finite temperatures, namely, a scaling behavior of thermodynamic quantities in the presence of quenched disorder, and a two-step release of the magnetic entropy. Here, we present both signatures in ${\mathrm{Ag}}_{3}{\mathrm{LiIr}}_{2}{\mathrm{O}}_{6}$ which is synthesized from $\ensuremath{\alpha}\text{\ensuremath{-}}{\mathrm{Li}}_{2}{\mathrm{IrO}}_{3}$ by replacing the interlayer Li atoms with Ag atoms. In addition, the dc susceptibility data confirm the absence of a long-range order, and the ac susceptibility data rule out a spin-glass transition. These observations suggest a closer proximity to the QSL in ${\mathrm{Ag}}_{3}{\mathrm{LiIr}}_{2}{\mathrm{O}}_{6}$ compared to its parent compound $\ensuremath{\alpha}\text{\ensuremath{-}}{\mathrm{Li}}_{2}{\mathrm{IrO}}_{3}$ that orders at 15 K. We discuss an enhanced spin-orbit coupling due to a mixing between silver $d$ and oxygen $p$ orbitals as a potential underlying mechanism.

Nuclear magnetic resonance experiments reveal how electron spins fluctuate in Cu${}_{2}$IrO${}_{3}$, a test as to whether this material meets the criteria for an exotic---and still theoretical---magnetic state known as a Kitaev spin liquid.

Anyonic excitations emerging from a Kitaev spin liquid can form a basis for quantum computers. Searching for such excitations motivated intense research on the honeycomb iridate materials. However, access to a spin liquid ground state has been hindered by magnetic ordering. Cu2IrO3 is a new honeycomb iridate without thermodynamic signatures of a long-range order. Here, we use muon spin relaxation to uncover the magnetic ground state of Cu2IrO3. We find a two-component depolarization with slow and fast relaxation rates corresponding to distinct regions with dynamic and static magnetism, respectively. X-ray absorption spectroscopy and first principles calculations identify a mixed copper valence as the origin of this behavior. Our results suggest that a minority of Cu2+ ions nucleate regions of static magnetism whereas the majority of Cu+/Ir4+ on the honeycomb lattice give rise to a Kitaev spin liquid.

A quantum spin liquid is an exotic quantum state of matter that does not exhibit any classical magnetic ordered ground state. The realization of such a state in actual materials is of significant importance as it would provide a path to protected states for quantum information applications and also to Majorana fermions - both areas of vigorous current activity and interest in condensed matter physics. However, no solid evidence for the existence of such materials has been established in the laboratory, despite a decades-long search. Here, the authors report the pressure-induced melting of magnetic order and emergence of a new quantum state in \ensuremath{\alpha}-RuCl${}_{3}$, a candidate compound in the proximity of a quantum spin-liquid state. The high-pressure phase is found to be extremely robust against applied pressure up to ~140 GPa.

Extreme magnetoresistance (XMR) in topological semimetals is a recent discovery which attracts attention due to its robust appearance in a growing number of materials. To search for a relation between XMR and superconductivity, we study the effect of pressure on LaBi. By increasing pressure, we observe the disappearance of XMR followed by the appearance of superconductivity at $P\ensuremath{\approx}3.5$ GPa. We find a region of coexistence between superconductivity and XMR in LaBi in contrast to other superconducting XMR materials. The suppression of XMR is correlated with increasing zero-field resistance instead of decreasing in-field resistance. At higher pressures, $P\ensuremath{\approx}11$ GPa, we find a structural transition from the face-centered cubic lattice to a primitive tetragonal lattice, in agreement with theoretical predictions. The relationship between extreme magnetoresistance, superconductivity, and structural transition in LaBi is discussed.

We report a sudden reversal in the pressure dependence of ${T}_{\mathrm{c}}$ in the iron-based superconductor ${\text{RbFe}}_{2}{\text{As}}_{2}$, at a critical pressure ${P}_{\mathrm{c}}$ $=11$ kbar. Combined with our prior results on ${\text{KFe}}_{2}{\text{As}}_{2}$ and ${\text{CsFe}}_{2}{\text{As}}_{2}$, we find a universal V-shaped phase diagram for ${T}_{\mathrm{c}}$ versus $P$ in these fully hole-doped 122 materials, when measured relative to the critical point $({P}_{\mathrm{c}},{T}_{\mathrm{c}})$. From measurements of the upper critical field ${H}_{\mathrm{c}2}(T)$ under pressure in ${\text{KFe}}_{2}{\text{As}}_{2}$ and ${\text{RbFe}}_{2}{\text{As}}_{2}$, we observe the same twofold jump in $(1/{T}_{\mathrm{c}})\phantom{\rule{0.16em}{0ex}}{\left(\ensuremath{-}\ensuremath{\partial}{H}_{\text{c}2}/\ensuremath{\partial}T\right)}_{{T}_{\mathrm{c}}}$ across ${P}_{\mathrm{c}}$, compelling evidence for a sudden change in the structure of the superconducting gap. We argue that this change is due to a transition from one pairing state to another, with different symmetries on either side of ${P}_{\mathrm{c}}$. We discuss a possible link between scattering and pairing, and a scenario where a $d$-wave state favored by high-$Q$ scattering at low pressure changes to a state with ${s}_{\ifmmode\pm\else\textpm\fi{}}$ symmetry favored by low-$Q$ scattering at high pressure.

The Nernst effect was measured in the electron-doped cuprate superconductor ${\mathrm{Pr}}_{2\ensuremath{-}x}{\mathrm{Ce}}_{x}{\mathrm{CuO}}_{4}$ (PCCO) at four concentrations, from underdoped $(x=0.13)$ to overdoped $(x=0.17)$, for a wide range of temperatures above the critical temperature ${T}_{c}$. A magnetic field $H$ up to 15 T was used to reliably access the normal-state quasiparticle contribution to the Nernst signal ${N}_{\mathrm{qp}}$, which is subtracted from the total signal $N$, to obtain the superconducting contribution ${N}_{\mathrm{sc}}$. As a function of $H$, ${N}_{\mathrm{sc}}$ peaks at a field ${H}^{\ensuremath{\star}}$ whose temperature dependence obeys ${H}_{c2}^{\ensuremath{\star}}ln(T/{T}_{c}$), as it does in a conventional superconductor such as ${\mathrm{Nb}}_{x}{\mathrm{Si}}_{1\ensuremath{-}x}$. The doping dependence of the characteristic field scale ${H}_{c2}^{\ensuremath{\star}}$, shown to be closely related to the upper critical field ${H}_{c2}$, tracks the domelike dependence of ${T}_{c}$, showing that superconductivity is weakened below the quantum critical point where the Fermi surface is reconstructed, presumably by the onset of antiferromagnetic order. Our data at all dopings are quantitatively consistent with the theory of Gaussian superconducting fluctuations, eliminating the need to invoke unusual vortexlike excitations above ${T}_{c}$, and ruling out phase fluctuations as the mechanism for the fall of ${T}_{c}$ with underdoping. We compare the properties of PCCO with those of hole-doped cuprates and conclude that the domes of ${T}_{c}$ and ${H}_{c2}$ versus doping in the latter materials are also controlled predominantly by phase competition rather than phase fluctuations.

We report a sudden reversal in the pressure dependence of Tc in the iron-based superconductor CsFe2As2, similar to that discovered recently in KFe2As2 [Tafti et al., Nat. Phys. 9, 349 (2013)]. As in KFe2As2, we observe no change in the Hall coefficient at the zero temperature limit, again ruling out a Lifshitz transition across the critical pressure Pc. We interpret the Tc reversal in the two materials as a phase transition from one pairing state to another, tuned by pressure, and investigate what parameters control this transition. Comparing samples of different residual resistivity, we find that a 6-fold increase in impurity scattering does not shift Pc. From a study of X-ray diffraction on KFe2As2 under pressure, we report the pressure dependence of lattice constants and As-Fe-As bond angle. The pressure dependence of these lattice parameters suggests that Pc should be significantly higher in CsFe2As2 than in KFe2As2, but we find on the contrary that Pc is lower in CsFe2As2. Resistivity measurements under pressure reveal a change of regime across Pc, suggesting a possible link between inelastic scattering and pairing symmetry.

We report superconductivity in the ternary half-Heusler compound LuPtBi, with ${T}_{c}=1.0$ K and ${H}_{c2}=1.6$ T. The crystal structure of LuPtBi lacks inversion symmetry, hence the material is a noncentrosymmetric superconductor. Magnetotransport data show semimetallic behavior in the normal state, which is evidence for the importance of spin-orbit interaction. The combination of strong spin-orbit coupling and noncentrosymmetric crystal structure make LuPtBi a strong candidate for 3D topological superconductivity.

We have studied the effect of pressure on the pyrochlore iridate ${\mathrm{Eu}}_{2}{\mathrm{Ir}}_{2}{\mathrm{O}}_{7}$, which, at ambient pressure, has a thermally driven insulator to metal transition at ${T}_{\mathrm{MI}}\ensuremath{\sim}120\phantom{\rule{0.28em}{0ex}}\mathrm{K}$. As a function of pressure, the insulating gap closes, apparently continuously near $P\ensuremath{\sim}6\phantom{\rule{0.28em}{0ex}}\mathrm{GPa}$. However, rather than ${T}_{\mathrm{MI}}$ going to zero as expected, the insulating ground state crosses over to a metallic state with a negative temperature coefficient of resistivity, suggesting that these ground states have a novel character. The high-temperature state also crosses over near 6 GPa from an incoherent to a conventional metal, implying that there is a connection between the high- and the low-temperature states.

Recently, high-transition-temperature (high-T(c)) superconductivity was discovered in the iron pnictide RFeAsO(1-x)F(x) (R, rare-earth metal) family of materials. We use neutron scattering to study the structural and magnetic phase transitions in CeFeAsO(1-x)F(x) as the system is tuned from a semimetal to a high-T(c) superconductor through fluorine (F) doping, x. In the undoped state, CeFeAsO develops a structural lattice distortion followed by a collinear antiferromagnetic order with decreasing temperature. With increasing fluorine doping, the structural phase transition decreases gradually and vanishes within the superconductivity dome near x D 0 : 10, whereas the antiferromagnetic order is suppressed before the appearance of superconductivity for x > 0.06, resulting in an electronic phase diagram remarkably similar to that of the high-T(c) copper oxides. Comparison of the structural evolution of CeFeAsO(1-x)F(x) with other Fe-based superconductors suggests that the structural perfection of the Fe-As tetrahedron is important for the high-T(c) superconductivity in these Fe pnictides.