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Phase engineering strategies in two-dimensional transition metal dichalcogenides (2D-TMDs) have garnered significant attention due to their potential applications in electronics, optoelectronics, and energy storage. Various methods, including direct synthesis, pressure control, and chemical doping, have been employed to manipulate structural transitions in 2D-TMDs. Metal intercalation emerges as an effective technique to modulate phase transition dynamics by inserting external atoms or ions between the layers of 2D-TMDs, altering their electronic structure and physical properties. Here, we investigate the significant structural phase transitions in Pb(Ta1+xSe2)2 single crystals induced by Ta intercalation using a combination of Raman spectroscopy and first-principles calculations. The results highlight the pivotal role of Ta atoms in driving these transitions and elucidate the interplay between intercalation, phase transitions, and resulting electronic and vibrational properties in 2D-TMDs. By focusing on Pb(Ta1+xSe2)2 as an ideal case study and investigating like metal intercalation, this study advances understanding in the field and paves the way for the development of novel applications for 2D-TMDs, offering insights into the potential of these materials for future technological advancements.

The coexistence of superconductivity and ferromagnetism is an intrinsically interesting research focus in condensed matter physics but the study is limited by low superconducting ($T_c$) and magnetic ($T_m$) transition temperatures in related materials. Here, we used a scanning superconducting quantum interference device to image the in-situ diamagnetic and ferromagnetic responses of RbEuFe$_4$As$_4$ with high $T_c$ and $T_m$. We observed significant suppression of superfluid density in vicinity of the magnetic phase transition, signifying fluctuation-enhanced magnetic scatterings between Eu spins and Fe 3$d$ conduction electrons. Intriguingly, we observed multiple ferromagnetic domains which should be absent in an ideal magnetic helical phase. The formation of these domains demonstrates a weak $c$-axis ferromagnetic component probably arising from Eu spin-canting effect, indicative of possible superconductivity-driven domain Meissner and domain vortex-antivortex phases as revealed in EuFe$_2$(As$_{0.79}$P$_{0.21}$)$_2$. Our observations highlight RbEuFe$_4$As$_4$ is a unique system which includes multiple interplay channels between superconductivity and ferromagnetism., Comment: 11 pages, 5 figures

Strongly correlated electron systems with a kagome lattice can host abundant exotic quantum states such as superconductivity and spin/charge density waves (CDW) due to the complicated interactions between different degrees of freedoms in the framework of a unique two-dimensional geometrically frustrated lattice structure. Recently, successive orders of A-type antiferromagnetism (AFM), $2\times2\times2$ CDW and canted double-cone AFM have been manifested upon cooling in magnetic kagome FeGe. However, the mechanism of the CDW order and its interaction with magnetism are presently enigmatic at best. Here we investigate the evolution of CDW order with temperature across the spin canting transition in FeGe by single-crystal x-ray diffraction. Refinements of its modulated structure are presented using the superspace approach. Interestingly, the superlattice reflections originating from CDW-induced long-range structural modulation become extremely weak after the system enters the canted AFM while a $2\times2$ CDW in the $ab$ plane persists as a long-range order demonstrated by strong electronic modulation in the d$I$/d$V$ map of scanning tunneling spectroscopy. We discovered a novel CDW order without long-range structural modulation in FeGe probably because of the competition between CDW and canted AFM in determining the underlying crystal structure. In addition, occupational modulations of Ge1 atoms located in the kagome plane and displacive modulations of all the atoms were extracted from the refinements, confirming the existence of Ge atom dimerization along the $c$ axis as the major distortion and indicating a dynamic transformation between different CDW domains., Comment: 22 pages, 6 figures. Comments on the manuscript are welcome

We report the synthesis, crystal structure, and physical properties of a novel ternary compound, Th$_2$Cu$_4$As$_5$. The material crystallizes in a tetragonal structure with lattice parameters $a=4.0716(1)$ {\AA} and $c=24.8131(4)$ {\AA}. Its structure can be described as an alternating stacking of fluorite-type Th$_2$As$_2$ layers with antifluorite-type double-layered Cu$_4$As$_3$ slabs. The measurement of electrical resistivity, magnetic susceptibility and specific heat reveals that Th$_2$Cu$_4$As$_5$ undergoes bulk superconducting transition at 4.2 K. Moreover, all these physical quantities exhibit anomalies at 48 K, where the Hall coefficient change the sign. These findings suggest a charge-density-wave-like (CDW) transition, making Th$_2$Cu$_4$As$_5$ a rare example for studying the interplay between CDW and superconductivity., Comment: 11 pages, 6 figures, and 1 table

Unconventional superconductivity (USC) in a highly correlated kagome system has been theoretically proposed for years, yet the experimental realization is hard to achieve. The recently discovered vanadium-based kagome materials, which exhibit both superconductivity and charge density wave (CDW) orders, are nonmagnetic and weakly correlated, thus unlikely host USC as theories proposed. Here we report the discovery of a chromium-based kagome metal, CsCr$_3$Sb$_5$, which is contrastingly characterised by strong electron correlations, frustrated magnetism, and characteristic flat bands close to the Fermi level. Under ambient pressure, it undergoes a concurrent structural and magnetic phase transition at 55 K, accompanying with a stripe-like $4a_0$ structural modulation. At high pressure, the phase transition evolves into two transitions, probably associated with CDW and antiferromagnetic spin-density-wave orderings, respectively. These density-wave (DW)-like orders are gradually suppressed with pressure and, remarkably, a superconducting dome emerges at 3.65-8.0 GPa. The maximum of the superconducting transition temperature, $T_\mathrm{c}^{\mathrm{max}}=$ 6.4 K, appears when the DW-like orders are completely suppressed at 4.2 GPa, and the normal state exhibits a non-Fermi-liquid behaviour, reminiscent of USC and quantum criticality in iron-based superconductors. Our work offers an unprecedented platform for investigating possible USC in a correlated kagome system., Comment: 32 pages, 14 figures

At ambient conditions SrAl4 adopts the BaAl4 structure type with space group I4/mmm. It undergoes a charge-density-wave (CDW) transition at TCDW = 243 K, followed by a structural transition at TS = 87 K. Temperature-dependent single-crystal X-ray diffraction (SXRD) leads to the observation of incommensurate superlattice reflections at q = \sigma c* with \sigma = 0.1116 at 200 K. The CDW has orthorhombic symmetry with the acentric superspace group F222(00sigma)00s, where F222 is a subgroup of Fmmm as well as of I4/mmm. Atomic displacements mainly represent a transverse wave, with displacements that are 90 deg out of phase between the two diagonal directions of the I-centered unit cell, resulting in a helical wave. Small longitudinal displacements are provided by the second harmonic modulation. The orthorhombic phase realized in SrAl4 is similar to that found in EuAl4. Electronic structure calculations and phonon calculations by density functional theory (DFT) have failed to reveal the mechanism of CDW formation. However, DFT reveals that Al atoms dominate the density of states near the Fermi level, thus, corroborating the SXRD measurements. SrAl4 remains incommensurately modulated at the structural transition, where the symmetry lowers from orthorhombic to b-unique monoclinic. We have identified a simple criterion, that correlates the presence of a phase transition with the interatomic distances. Only those compounds XAl4-xGax(X = Ba, Eu, Sr, Ca; 0 < x <4) undergo phase transitions, for which the ratio c/a falls within the narrow range 2.51 < c/a < 2.54.

Charge density wave (CDW) in kagome materials with the geometric frustration is able to carry unconventional characteristics. Recently, a CDW has been observed below the antiferromagnetic order in kagome FeGe, in which magnetism and CDW are intertwined to form an emergent quantum ground state. However, the CDW is only short-ranged and the structural modulation originating from it has yet to be determined experimentally. Here we realize a long-range CDW order by post-annealing process, and resolve the structure model through single crystal x-ray diffraction. Occupational disorder of Ge resulting from short-range CDW correlations above $T_\mathrm{CDW}$ is identified from structure refinements. The partial dimerization of Ge along the $c$ axis is unveiled to be the dominant distortion for the CDW. Occupational disorder of Ge is also proved to exist in the CDW phase due to the random selection of partially dimerized Ge sites. Our work provides useful insights for understanding the unconventional nature of the CDW in FeGe., Comment: 4 figures and 1 table in the main text. Supplementary materials included. To be published in SCIENCE CHINA Physics, Mechanics & Astronomy (SCPMA)

Ho2Ir3Si5 belongs to the family of three-dimensional (3D) R2Ir3Si5 (R = Lu, Er and Ho) compounds that exhibit a colossal first-order charge density wave (CDW) transition where there is a strong orthorhombic-to-triclinic distortion of the lattice accompanied by superlattice reflections. The analysis by single-crystal X-ray diffraction (SXRD) has revealed that the Ir-Ir zigzag chains along c are responsible for the CDW in all three compounds. The replacement of the rare earth element from non-magnetic Lu to magnetic Er or Ho lowers TCDW, where TCDWLu = 200 K, TCDWEr = 150 K and TCDWHo = 90 K. Out of the three compounds, Ho2Ir3Si5 is the only system where second-order superlattice reflections could be observed, indicative of an anharmonic shape of the modulation wave. The CDW transition is observed as anomalies in the temperature dependencies of the specific heat, electrical conductivity and magnetic susceptibility, which includes a large hysteresis of 90 to 130 K for all measured properties, thus corroborating the SXRD measurements. Similar to previously reported Er2Ir3Si5, there appears to be a coupling between CDW and magnetism such that the Ho3+ magnetic moments are influenced by the CDW transition, even in the paramagnetic state. Moreover, earlier investigations on polycrystalline material revealed antiferromagnetic (AFM) ordering at TN = 5.1 K, whereas AFM order is suppressed and only the CDW is present in our highly ordered single-crystal. First-principles calculations predict Ho2Ir3Si5 to be a metal with coexisting electron and hole pockets at the Fermi level. The Ho and Ir atoms have spherically symmetric metallic-type charge density distributions that are prone to CDW distortion. Phonon calculations affirm that the Ir atoms are primarily responsible for the CDW distortion, which is in agreement with the experiment.

AMn6Bi5 materials (A = Na, K, Rb and Cs) consisting of unique Mn-cluster chains emerge as a new family of superconductors with the suppression of their antiferromagnetic (AFM) order under high pressures. Here, we report transverse incommensurate spin density waves (SDWs) for the Mn atoms with a propagating direction along the chain axes as a ground state for KMn6Bi5 by single crystal neutron diffraction. The SDWs have a refined amplitude of ~2.46 Bohr magnetons for the Mn atoms in the pentagons and ~0.29 Bohr magnetons with a large standard deviation for Mn atoms in the center between the pentagons. AFM dominate both the nearest-neighbor Mn-Mn interactions within the pentagon and next-nearest-neighbor Mn-Mn interactions out of the pentagon (along the propagating wave). The SDWs exhibit both local and itinerant characteristics probably formed by a cooperative interaction between local magnetic exchange and conduction electrons. A significant magnetoelastic effect during the AFM transition, especially along the chain direction, has been demonstrated by temperature-dependent x-ray powder diffraction. Single crystal x-ray diffraction below the AFM transition revealed satellite peaks originating from charge density waves along the chain direction with a q-vector twice as large as the SDW one, pointing to a strong real space coupling between them. Our work not only manifests a fascinating interplay among spin, charge, lattice and one dimensionality to trigger intertwined orders in KMn6Bi5 but also provides important piece of information for the magnetic structure of the parent compound to understand the mechanism of superconductivity in this new family.

Rhodium-containing compounds offer a fertile playground to explore novel materials with superconductivity and other fantastic electronic correlation effects. A new ternary rhodium-antimonide La$_2$Rh$_{3+\delta}$Sb$_4$ ($\delta\approx 1/8$) has been synthesized by a Bi-flux method. It crystallizes in the orthorhombic Pr$_2$Ir$_3$Sb$_4$-like structure, with the space group $Pnma$ (No. 62). The crystalline structure appears as stacking the two dimensional RhSb$_4$- and RhSb$_5$-polyhedra networks along $\mathbf{b}$ axis, and the La atoms embed in the cavities of these networks. Band structure calculations confirm it as a multi-band metal with a van-Hove singularity like feature at the Fermi level, whose density of states are mainly of Rh-4$d$ and Sb-5$p$ characters. The calculations also imply that the redundant Rh acts as charge dopant. Superconductivity is observed in this material with onset transition at $T_c^{on}\approx$ 0.8 K. Ultra-low temperature magnetic susceptibility and specific heat measurements suggest that it is an s-wave type-II superconductor. Our work may also imply that the broad $Ln_2$$Tm_{3+\delta}$Sb$_4$ ($Ln$=Rare earth, $Tm$=Rh, Ir...) family may host new material bases where new superconductors, quantum magnetism and other electronic correlation effects could be found., Comment: 15+1 pages, 7+1 figures, 2+1 tables

EuAl4 possesses the BaAl4 crystal structure type with tetragonal symmetry I4/mmm. It undergoes a charge-density-wave (CDW) transition at TCDW = 145 K and it features four consecutive antiferromagnetic phase transitions below 16 K. Here, we use single-crystal x-ray diffraction to determine incommensurately modulated crystal structure of EuAl4 in its CDW state. The CDW is shown to be incommensurate with modulation wave vector q = (0, 0, 0.1781(3)) at 70 K. The symmetry of the incommensurately modulated crystal structure is orthorhombic with superspace group Fmmm(00{\sigma})s00, where Fmmm is a subgroup of I4/mmm of index 2. Both the lattice and the atomic coordinates of the basic structure remain tetragonal. Symmetry breaking is entirely due to the modulation wave, where atoms Eu and Al1 have displacements exclusively along a, while the fourfold rotation would require equal displacement amplitudes along a and b. The calculated band structure of the basic structure and interatomic distances in the modulated crystal structure both indicate the aluminum atoms as location of the CDW. The temperature dependence of the specific heat reveals an anomaly at TCDW = 145 K of a magnitude similar to canonical CDW systems. The present discovery of orthorhombic symmetry for the CDW state of EuAl4 leads to the suggestion of monoclinic instead of orthorhombic symmetry for the third AFM state.

Intercalation and stacking order modulation are two active ways in manipulating the interlayer interaction of transition metal dichalcogenides (TMDCs), which lead to a variety of emergent phases and allow for engineering material properties. Herein, the growth of Pb intercalated TMDCs--Pb(Ta$_{1+x}$Se$_2$)$_2$, the first 124-phase, is reported. Pb(Ta$_{1+x}$Se$_2$)$_2$ exhibits a unique two-step first-order structural phase transition at around 230 K. The transitions are solely associated with the stacking degree of freedom, evolving from a high temperature phase with ABC stacking and symmetry $R3m$ to an intermediate phase with AB stacking and $P3m1$, and finally to a low temperature phase with again symmetry $R3m$, but with ACB stacking. Each step involves a rigid slide of building blocks by a vector [1/3, 2/3, 0]. Intriguingly, gigantic lattice contractions occur at the transitions on warming. At low temperature, bulk superconductivity with $T_\textrm{c}\approx$ 1.8 K is observed. The underlying physics of the structural phase transitions are discussed from first-principle calculations. The symmetry analysis reveals topological nodal lines in the band structure. Our results demonstrate the possibility to realize higher order metal intercalated phases of TMDCs, advance our knowledge of polymorphic transitions and may inspire stacking order engineering in TMDCs and beyond.

Layered heterostructure materials with two different functional building blocks can teach us about emergent physical properties and phenomena arising from interactions between the layers. We report the intergrowth compounds KLa$M$$_{1-x}$Te$_{4}$ ($M$ = Mn, Zn; $x\approx$ 0.35) featuring two chemically distinct alternating layers [LaTe$_3$] and [K$M$$_{1-x}$Te]. Their crystal structures are incommensurate, determined by single X-ray diffraction for the Mn compound and transmission electron microscope (TEM) study for the Zn compound. KLaMn$_{1-x}$Te$_{4}$ crystallizes in the orthorhombic superspace group $Pmnm$(01/2${\gamma}$)$s$00 with lattice parameters $a$ = 4.4815(3) {\AA}, $b$ = 21.6649(16) {\AA} and $c$ = 4.5220(3) {\AA}. It exhibits charge density wave (CDW) order at room temperature with a modulation wave vector $\mathbf{q}$ = 1/2$\mathbf{b}$* + 0.3478$\mathbf{c}$* originating from electronic instability of Te-square nets in [LaTe$_{3}$] layers. The Mn analog exhibits a cluster spin glass behavior with spin freezing temperature $T_{\mathrm{f}}$ $\approx$ 5 K attributed to disordered Mn vacancies and competing magnetic interactions in the [Mn$_{1-x}$Te] layers. The Zn analog also has charge density wave order at room temperature with a similar $\mathbf{q}$-vector having the $\mathbf{c}$* component ~ 0.346 confirmed by selected-area electron diffraction (SAED). Electron transfer from [K$M_{1-x}$Te] to [LaTe$_{3}$] layers exists in KLa$M_{1-x}$Te$_{4}$, leading to an enhanced electronic specific heat coefficient. The resistivities of KLa$M_{1-x}$Te$_{4}$ ($M$ = Mn, Zn) exhibit metallic behavior at high temperatures and an upturn at low temperatures, suggesting partial localization of carriers in the [LaTe$_{3}$] layers with some degree of disorder associated with the $M$ atom vacancies in the [$M_{1-x}$Te] layers., Comment: 1 table, 10 figures, 35 pages

Magnetic topological semimetals, the latest member of topological quantum materials, are attracting extensive attention as they may lead to topologically-driven spintronics. Currently, magnetotransport investigations on these materials are focused on anomalous Hall effect. Here, we report on the magnetoresistance anisotropy of topological semimetal CeBi, which has tunable magnetic structures arising from localized Ce 4f electrons and exhibits both negative and positive magnetoresistances, depending on the temperature. We found that the angle dependence of the negative magnetoresistance, regardless of its large variation with the magnitude of the magnetic field and with temperature, is solely dictated by the field-induced magnetization that is orientated along a primary crystalline axis and flops under the influence of a rotating magnetic field. The results reveal the strong interaction between conduction electrons and magnetization in CeBi. They also indicate that magnetoresistance anisotropy can be used to uncover the magnetic behavior and the correlation between transport phenomena and magnetism in magnetic topological semimetals.

Single crystals of a honeycomb lattice antiferromagnet, Tb$_2$Ir$_3$Ga$_9$ were synthesized, and the physical properties have been studied. From magnetometry, a long-range antiferromagnetic ordering at $\approx$12.5 K with highly anisotropic magnetic behavior was found. Neutron powder diffraction confirms that the Tb spins lie along the $\va $-axis, parallel to the shortest Tb-Tb contact. Two field-induced spin-flip transitions are observed when the field is applied parallel to this axis, separated by a plateau corresponding roughly to M$\approx$M$_{\rm{s}}$/2. Transport measurements show the resistivity to be metallic with a discontinuity at the onset of N\'eel order. Heat capacity shows a $\lambda$-like transition confirming the bulk nature of the magnetism. We propose a phenomenological spin-Hamiltonian that describes the magnetization plateau as a result of strong Ising character arising from a quasidoublet ground state of the Tb$^{3+}$ ion in a site of \textit{C$_s$} symmetry and expressing a significant bond dependent anisotropy., Comment: 11 pages, 9 figures

The charge and spin of the electrons in solids have been extensively exploited in electronic devices and in the development of spintronics. Another attribute of electrons - their orbital nature - is attracting growing interest for understanding exotic phenomena and in creating the next-generation of quantum devices such as orbital qubits. Here, we report on orbital-flop induced magnetoresistance anisotropy in CeSb. In the low temperature high magnetic-field driven ferromagnetic state, a series of additional minima appear in the angle-dependent magnetoresistance. These minima arise from the anisotropic magnetization originating from orbital-flops and from the enhanced electron scattering from magnetic multidomains formed around the first-order orbital-flop transition. The measured magnetization anisotropy can be accounted for with a phenomenological model involving orbital-flops and a spin-valve-like structure is used to demonstrate the viable utilization of orbital-flop phenomenon. Our results showcase a contribution of orbital behavior in the emergence of intriguing phenomena.

The pressure dependencies of the magnetic and superconducting transitions, as well as that of the superconducting upper critical field are reported for single crystalline EuRbFe$_4$As$_4$. Resistance measurements were performed under hydrostatic pressures up to 6.21 GPa and in magnetic fields up to 9 T. Zero-field-cool magnetization measurements were performed under hydrostatic pressures up to 1.24 GPa under 20 mT applied field. Superconducting transition temperature, $T_\text c$, up to 6.21 GPa and magnetic transition temperature, $T_\text M$, up to 1.24 GPa were obtained and a pressure-temperature phase diagram was constructed. Our results show that $T_\text c$ is monotonically suppressed upon increasing pressure. $T_\text M$ is linearly increased up to 1.24 GPa. For the studied pressure range, no signs of the crossing of $T_\text M$ and $T_\text c$ lines are observed. The normalized slope of the superconducting upper critical field is gradually suppressed with increasing pressure, which may be due to the continuous change of Fermi-velocity $v_F$ with pressure., Comment: 5 pages, 7 figures

We studied the crystal structure and spin state of MnP under high pressure with synchrotron X-ray diffraction and X-ray emission spectroscopy. MnP has an exceedingly strong anisotropy in compressibility, with the primary compressible direction along the b axis of the Pnma structure. X-ray emission spectroscopy reveals a pressure-driven quenching of the spin state in MnP. Firstprinciples calculations suggest that the strongly anisotropic compression behavior significantly enhances the dispersion of the Mn d-orbitals and the splitting of the d orbital levels compared to the hypothetical isotropic compression behavior. Thus, we propose spin quenching results mainly from the significant enhancement of the itinerancy of d electrons and partly from spin rearrangement occurring in the split d-orbital levels near the Fermi level. This explains the fast suppression of magnetic ordering in MnP under high pressure. The spin quenching lags behind the occurrence of superconductivity at ~8 GPa implying that spin fluctuations govern the electron pairing for superconductivity., Comment: 17 pages, 6 figures, accepted by New Journal of Physics

$A_2$Cr$_3$As$_3$ ($A$ = K, Rb, Cs) are the unique Cr-based ambient-pressure superconductors to date discovered by serendipity in 2015. The new superconducting family are structurally characterized by quasi one-dimensional [(Cr$_3$As$_3$)$^{2-}$]$_{\infty}$ double-walled subnanotubes, which exhibit peculiar properties that mostly point to unconventional superconductivity. In this conference paper, we first describe how the superconductors were discovered. Then we overview the recent progresses on crystal structure, electronic structures, theoretical models, and various physical properties in $A_2$Cr$_3$As$_3$. Some new experimental results are included. Finally we conclude by addressing the related open questions in this emerging subfield of superconductivity., Comment: 19 pages, 9 figures and 1 table, submitted to Philosophical Magzine

We report measurements of the upper critical field $H_{\mathrm{c2}}$ as functions of temperature $T$, polar angle $\theta$ (of the field direction with respect to the crystallographic $c$ axis), and azimuthal angle $\phi$ (of the field direction relative to the $a$ axis within the $ab$ plane) for the Cr-based superconductor K$_{2}$Cr$_{3}$As$_{3}$ with a quasi-one-dimensional and non-centrosymmetric crystal structure. We confirm that the anisotropy in $H_{\mathrm{c2}}(T)$ becomes inverse with decreasing temperature. At low temperatures, $H_{\mathrm{c2}}(\theta)$ data are featured by two maxima at $\theta$ = 0 ($\mathbf{H}\parallel c$) and $\pi/2$ ($\mathbf{H}\bot c$), which can be quantitatively understood only if uniaxial effective-mass anisotropy and absence of Pauli paramagnetic effect for $\mathbf{H}\bot c$ are taken simultaneously into consideration. The in-plane $H_{\mathrm{c2}}(\phi)$ profile shows a unique threefold modulation especially at low temperatures. Overall, the characteristic of the $H_{\mathrm{c2}}(\theta, \phi, T)$ data mostly resemble those of the heavy-fermion superconductor UPt$_3$, and we argue in favor of a dominant spin-triplet superconductivity with odd parity in K$_{2}$Cr$_{3}$As$_{3}$., Comment: Manuscript 7 pages, 3 figures.final version. Supplemental Material available upon request

Recently, new Cr-based superconductors, A$_2$Cr$_3$As$_3$ (A = K, Rb, Cs), have gained a strong interest because of their one-dimensional crystal structures and electron correlations. Here we report the crystal structure and physical properties of two related materials ACr$_3$As$_3$ (A = Rb, Cs) which are synthesized via a soft-chemical A+ deintercalation in A$_2$Cr$_3$As$_3$. The new compounds remain one-dimensional (Cr$_3$As$_3$)$_{\infty}$ linear chains, and the interchain distance can be tuned by the incorporation of the alkali-metal cations with different sizes. The physical-property measurements indicate a local-moment behavior at high temperatures, and the moments freeze into a cluster spin-glass state below 5$\sim$6 K. No superconductivity was observed in both materials. We also found that, with increasing the interchain distance, the Cr effective moments increase monotonically, accompanied with the enhancement of semi-conductivity. Our results shed light on the understanding of occurrence of superconductivity in A$_2$Cr$_3$As$_3$., Comment: 9 pages, 7 figures

We report the physical properties and electronic structure calculations of a layered chromium oxypnictide, Sr$_2$Cr$_3$As$_2$O$_2$, which crystallizes in a Sr$_2$Mn$_3$As$_2$O$_2$-type structure containing both CrO$_2$ planes and Cr$_2$As$_2$ layers. The newly synthesized material exhibits a metallic conduction with a dominant electron-magnon scattering. Magnetic and specific-heat measurements indicate at least two intrinsic magnetic transitions below room temperature. One is an antiferromagnetic transition at 291 K, probably associated with a spin ordering in the Cr$_2$As$_2$ layers. Another transition is broad, occurring at around 38 K, and possibly due to a short-range spin order in the CrO$_2$ planes. Our first-principles calculations indicate predominant two-dimensional antiferromagnetic exchange couplings, and suggest a KG-type (i.e. K$_2$NiF$_4$ type for CrO$_2$ planes and G type for Cr$_2$As$_2$ layers) magnetic structure, with reduced moments for both Cr sublattices. The corresponding electronic states near the Fermi energy are mostly contributed from Cr-3$d$ orbitals which weakly (modestly) hybridize with the O-2$p$ (As-4$p$) orbitals in the CrO$_2$ (Cr$_2$As$_2$) layers. The bare bandstructure density of states at the Fermi level is only $\sim$1/4 of the experimental value derived from the low-temperature specific-heat data, consistent with the remarkable electron-magnon coupling. The title compound is argued to be a possible candidate to host superconductivity., Comment: 10 pages, 9 figures

We report structural and physical properties of a new quasi-one-dimensional Cr-based compound, KCr$_{3}$As$_{3}$, which is prepared by potassium deintercalation from the superconductive K$_{2}$Cr$_{3}$As$_{3}$. KCr$_{3}$As$_{3}$ adopts the TlFe$_{3}$Te$_{3}$-type structure with space group $P6_{3}$/$m$ (No. 176). The high-temperature magnetic susceptibility obeys the Curie-Weiss law with an effective magnetic moment of 0.68 $\mu_{\mathrm{B}}$/Cr. Below 56 K the susceptibility deviates from the high-temperature Curie-Weiss behavior, coinciding with the rapid increase in resistivity, which suggests formation of spin clusters. The short-range spin correlations are also supported by the specific-heat data. The title material does not exhibit bulk superconductivity; instead, it shows a cluster spin-glass state below $\sim$ 5 K., Comment: 5 pages, 4 figures

EuBiS$_{2}$F is a self-doped superconductor due to the mixed valence of Eu. Here we report that, with the Ce substitution for Eu by 50 at.\%, the material exhibits ferromagnetic ordering at 8 K for the Ce-4$f$ moment, superconductivity at 2.2 K in the BiS$_2$ layers, and possibly antiferromagnetic ordering at 2.1 K for the Eu-4$f$ spins. The Eu valence is essentially divalent with the Ce incorporation. We tentatively interpret the coexistence of ferromagnetism and superconductivity by considering different Bi-6$p$ orbitals that are responsible for superconductivity itself and for mediating the ferromagnetic interaction, respectively. We argue that the antiferromagnetic ordering of the Eu-4$f$ spins is most likely due to a magnetic dipole-dipole interaction., Comment: 12 pages, 5 figures

We carried out the measurements of magnetoresistance, magnetic susceptibility and specific heat on crystals of the low-dimensional transition metal telluride Ta$_4$Pd$_3$Te$_{16}$. Our results indicate that Ta$_4$Pd$_3$Te$_{16}$ is an anisotropic type-II superconductor with the extracted Ginzburg-Landau parameter $\kappa_{\text{GL}}=$ 84. The upper critical field $H_{c2}$($T$) shows a linear dependence at low temperature and the anisotropy of $H_{c2}$($T$) is strongly $T$-dependent, both of which indicate a multiband scenario. A detailed analysis reveals that the electronic specific heat $C_{\text{el}}$($T$) can be consistently described by a two-gap ($s$+$d$ waves) model from the base temperature $T/T_c\sim$ 0.12 up to $T_c$. Our data suggests multiband superconductivity in Ta$_4$Pd$_3$Te$_{16}$ with anisotropic gap structure., Comment: 6 pages, 4 figures

Since the discovery of high-temperature superconductivity (SC) in quasi-two-dimensional copper oxides, a few layered compounds, which bear similarities to the cuprates, have also been found to host unconventional SC. Our recent observation of SC at 6.1 K in correlated electron material K2Cr3As3 (J. K. Bao et al., arXiv: 1412.0067) represents an obviously different paradigm, primarily because of its quasi-one-dimensional (Q1D) nature. The new material is structurally featured by the (Cr3As3)2- double-walled subnano-tubes composed of face-sharing Cr6/2 (As6/2) octahedron linear chains, which are well separated by columns of K+ counterions. Later, an isostructural superconducting Rb2Cr3As3 was synthesized, thus forming a new superconducting family. Here we report the third member, Cs2Cr3As3, which possesses the largest interchain distance. SC appears below 2.2 K. Similar to the former two sister compounds, Cs2Cr3As3 exhibits a non-Fermi liquid behavior with a linear temperature dependence of resistivity in the normal state, and a high upper critical field beyond the Pauli limit as well, suggesting common unconventional SC in the Q1D Cr-based material., Comment: 8 pages, 4 figures, 1 table

We report $^{75}$As NMR measurements on the new quasi one-dimensional superconductor K$_{2}$Cr$_{3}$As$_{3}$ ($T_{c} \sim 6.1$~K) [J.\ K.\ Bao et al., Phys. Rev. X {\bf 5}, 011013 (2015)]. We found evidence for strong enhancement of Cr spin fluctuations above $T_c$ in the [Cr$_{3}$As$_{3}$]$_{\infty}$ double-walled subnano-tubes based on the nuclear spin-lattice relaxation rate $1/T_{1}$. The power law temperature dependence, $1/T_{1}T \sim T^{-\gamma}$ ($\gamma \sim 0.25$), is consistent with the Tomonaga-Luttinger liquid. Moreover, absence of the Hebel-Slichter coherence peak of $1/T_{1}$ just below $T_{c}$ suggests unconventional nature of superconductivity., Comment: Final version accepted for publication in PRL

Following the discovery of superconductivity in quasi-one-dimensional K$_2$Cr$_3$As$_3$ containing [(Cr$_3$As$_3$)$^{2-}$]$_{\infty}$ chains [J. K. Bao et al., arXiv: 1412.0067 (2014)], we succeeded in synthesizing an analogous compound, Rb$_2$Cr$_3$As$_3$, which also crystallizes in a hexagonal lattice. The replacement of K by Rb results in an expansion of $a$ axis by 3\%, indicating a weaker interchain coupling in Rb$_2$Cr$_3$As$_3$. Bulk superconductivity emerges at 4.8 K, above which the normal-state resistivity shows a linear temperature dependence up to 35 K. The estimated upper critical field at zero temperature exceeds the Pauli paramagnetic limit by a factor of two. Furthermore, the electronic specific-heat coefficient extrapolated to zero temperature in the mixed state increases with $\sqrt{H}$, suggesting existence of nodes in the superconducting energy gap. Hence Rb$_2$Cr$_3$As$_3$ manifests itself as another example of unconventional superconductor in the Cr$_3$As$_3$-chain based system., Comment: 5 pages, 4 figures, crystal structure data updated

We report the discovery of bulk superconductivity (SC) at 6.1 K in a quasi-one-dimensional (Q1D) chromium pnictide K$_2$Cr$_3$As$_3$ which contains [(Cr$_3$As$_3$)$^{2-}$]$_{\infty}$ double-walled subnano-tubes with face-sharing Cr$_{6/2}$ (As$_{6/2}$) octahedron linear chains in the inner (outer) wall. The material has a large electronic specific-heat coefficient of 70$\sim$75 mJ K$^{-2}$ mol$^{-1}$, indicating significantly strong electron correlations. Signature of non-Fermi liquid behavior is shown by the linear temperature dependence of resistivity in a broad temperature range from 7 to 300 K. Unconventional SC is preliminarily manifested by the estimated upper critical field exceeding the Pauli limit by a factor of three to four. The title compound represents a rare example that possibly unconventional SC emerges in a Q1D system with strong electron correlations., Comment: Specific-heat data down to 0.5 K were added

We have synthesized a novel europium bismuth sulfofluoride, Eu3Bi2S4F4, by solid-state reactions in sealed evacuated quartz ampoules. The compound crystallizes in a tetragonal lattice (space group I4/mmm, a = 4.0771(1) A, c = 32.4330(6) A, and Z = 2), in which CaF2-type Eu3F4 layers and NaCl-like BiS2 bilayers stack alternately along the crystallographic c axis. There are two crystallographically distinct Eu sites, Eu(1) and Eu(2) at the Wyckoff positions 4e and 2a, respectively. Our bond-valence-sum calculation, based on the refined structural data, indicates that Eu(1) is essentially divalent, whilst Eu(2) has an average valence of +2.64(5). This anomalous Eu valence state is further confirmed and supported, respectively, by Mossbauer and magnetization measurements. The Eu3+ components donate electrons into the conduction bands that are mainly composed of Bi- 6px and 6py states. Consequently, the material itself shows metallic conduction, and superconducts at 1.5 K without extrinsic chemical doping., Comment: 24 pages, 5 figures

Superconductivity (SC) and charge-density wave (CDW) are two contrasting yet relevant collective electronic states which have received sustained interest for decades. Here we report that, in a layered europium bismuth sulfofluoride, EuBiS$_2$F, a CDW-like transition occurs at 280 K, below which SC emerges at 0.3 K, without any extrinsic doping. The Eu ions were found to exhibit an anomalously temperature-independent mixed valence of about +2.2, associated with the formation of CDW. The mixed valence of Eu gives rise to self electron doping into the conduction bands mainly consisting of the in-plane Bi-6$p$ states, which in turn brings about the CDW and SC. In particular, the electronic specific-heat coefficient is enhanced by ~ 50 times, owing to the significant hybridizations between Eu-4$f$ and Bi-6$p$ electrons, as verified by band-structure calculations. Thus, EuBiS$_2$F manifests itself as an unprecedented material that simultaneously accommodates SC, CDW and $f$-electron valence instability., Comment: 10 pages, 6 figures

We have performed an isovalent substitution study in a layered titanium oxypnictide system BaTi$_{2}$(Sb$_{1-x}$Bi$_{x}$)$_{2}$O (0$\leq x\leq$ 0.40) by the measurements of x-ray diffraction, electrical resistivity and magnetic susceptibility. The parent compound BaTi$_{2}$Sb$_{2}$O is confirmed to exhibit superconductivity at 1.5 K as well as charge- or spin-density wave (CDW/SDW) ordering below 55 K. With the partial substitution of Sb by Bi, the lattice parameters $a$, $c$ and $c/a$ all increase monotonically, indicating negative chemical pressure and lattice distortion on the (super)conducting Ti$_2$Sb$_2$O-layers. The Bi doping elevates the superconducting transition temperature to its maximum $T_c$=3.7 K at $x=$0.17, and then $T_c$ decreases gradually with additional Bi doping. A metal-to-nonmetal transition takes place around $x$=0.3, and superconductivity at $\sim$1K exists at the nonmetal side. The CDW/SDW anomaly, in comparison, is rapidly suppressed by the Bi doping, and vanishes for $x\geq$0.17. The results are discussed in terms of negative chemical pressure and disorder effect., Comment: 5 pages, 5 figures. Minor changes were made

Heavily doped Ba$_{1-x}$K$_{x}$Mn$_{2}$As$_{2}$ ($x$=0.19 and 0.26) single crystals were successfully grown, and investigated by the measurements of resistivity and anisotropic magnetic susceptibility. In contrast to the antiferromagnetic insulating ground state of the undoped BaMn$_{2}$As$_{2}$, the K-doped crystals show metallic conduction with weak ferromagnetism below $\sim$50 K and Curie-Weiss-like in-plane magnetic susceptibility above $\sim$50 K. Under high pressures up to 6 GPa, the low-temperature metallicity changes into a state characterized by a Kondo-like resistivity minimum without any signature of superconductivity above 2.5 K. Electronic structure calculations for $x$=0.25 using $2\times2\times1$ supercell reproduce the hole-doped metallic state. The density of states at Fermi energy have significant As 4$p$ components, suggesting that the 4$p$ holes are mainly responsible for the metallic conduction. Our results suggest that the interplay between itinerant 4$p$ holes and local 3$d$ moments is mostly responsible for the novel metallic state., Comment: Some data were re-arranged and/or added, 8 figures