In the quasi-two-dimensional superconductor NbSe2, the superconducting transition temperature (Tc) is layer-dependent, decreasing by about 60% in the monolayer limit. However, for the extremely anisotropic copper-based high-Tc superconductor Bi2Sr2CaCu2O8+δ (Bi-2212), the Tc of the monolayer is almost identical with that of its bulk counterpart. To clarify the effect of dimensionality on superconductivity, here, we successfully fabricate ultrathin flakes of iron-based high-Tc superconductors CsCa2Fe4As4F2 and CaKFe4As4. It is found that the Tc of monolayer CsCa2Fe4As4F2 (after tuning to the optimal doping by ionic liquid gating) is about 20% lower than that of the bulk crystal, while the Tc of three-layer CaKFe4As4 decreases by 46%, showing a more pronounced dimensional effect than that of CsCa2Fe4As4F2. By carefully examining their anisotropy and the c-axis coherence length, we reveal the general trend and empirical law of the layer-dependent superconductivity in these quasi-two-dimensional superconductors.
We report the synthesis, crystal structure, and physical properties of a novel ternary compound, Th2Cu4As5. The material crystallizes in a tetragonal structure with lattice parameters a = 4.0639(3) Å and c = 24.8221(17) Å. Its structure can be described as an alternating stacking of fluorite-type Th2As2 layers with antifluorite-type double-layered Cu4As3 slabs. The measurement of electrical resistivity, magnetic susceptibility, and specific heat reveals that Th2Cu4As5 undergoes bulk superconducting transition at 4.2 K. Additionally, all these physical quantities exhibit anomalies at 48 K, accompanied by a sign change in the Hall coefficient, suggesting a charge-density-wave-like (CDW) phase transition. Drawing from both experimental data and band calculations, we propose that the superconducting and CDW-like phase transitions are, respectively, associated with the Cu4As3 slabs and the As plane in the Th2As2 layers.
The long-range magnetic ordering in frustrated magnetic systems is stabilized by coupling magnetic moments to various degrees of freedom, for example, by enhancing magnetic anisotropy via lattice distortion. Here, the unconventional spin-lattice coupled metamagnetic properties of atomically-thin CrOCl, a van der Waals antiferromagnet with inherent magnetic frustration rooted in the staggered square lattice, are reported. Using temperature- and angle-dependent tunneling magnetoconductance (TMC), in complementary with magnetic torque and first-principles calculations, the antiferromagnetic (AFM)-to-ferrimagnetic (FiM) metamagnetic transitions (MTs) of few-layer CrOCl are revealed to be triggered by collective magnetic moment flipping rather than the established spin-flop mechanism, when external magnetic field (H) enforces a lattice reconstruction interlocked with the five-fold periodicity of the FiM phase. The spin-lattice coupled MTs are manifested by drastic jumps in TMC, which show anomalous upshifts at the transition thresholds and persist much higher above the AFM Néel temperature. While the MTs exhibit distinctive triaxial anisotropy, reflecting divergent magnetocrystalline anisotropy of the c-axis AFM ground state, the resulting FiM phase has an a-c easy plane in which the magnetization axis is freely rotated by H. At the 2D limit, such a field-tunable FiM phase may provide unique opportunities to explore exotic emergent phenomena and novel spintronics devices.
We report two novel titanium-based pnictide oxide compounds (EuF)2Ti2Pn2O (Pn = Sb, Bi), which are synthesized by replacing Sr2+ in (SrF)2Ti2Pn2O [Liu, R. H. Structure and Physical Properties of the Layered Pnictide-Oxides: (SrF)2Ti2Pn2O (Pn = As, Sb) and (SmO)2Ti2Sb2O. Chem. Mater. 2010, 22, 1503-1508] with Eu2+ using a solid-state reaction. (EuF)2Ti2Sb2O exhibits an obvious anomaly in resistivity and heat capacity at T â¼ 195 K, which may arise from the spin-density wave/charge-density wave instability. Similar features are also observed in BaTi2Pn2O, (SrF)2Ti2Pn2O, and Na2Ti2Pn2O (Pn = As and Sb) [Liu, R. H. Structure and Physical Properties of the Layered Pnictide-Oxides: (SrF)2Ti2Pn2O (Pn = As, Sb) and (SmO)2Ti2Sb2O. Chem. Mater. 2010, 22, 1503-1508, Ozawa, T. C. Chemistry of layered d-metal pnictide oxides and their potential as candidates for new superconductors. Sci. Technol. Adv. Mater. 2008, 9, 033003, Wang, X. F. Structure and physical properties for a new layered pnictide-oxide: BaTi2As2O. J. Phys.: Condens. Matter. 2010, 22, 075702, and Xu, H. C. Electronic structure of the BaTi2As2O parent compound of the titanium-based oxypnictide superconductor. Phys. Rev. B 2014, 89, 155108]. Magnetic susceptibility measurements indicate an antiferromagnetic transition at T â¼ 2.5 K for (EuF)2Ti2Sb2O. In particular, the electronic specific heat coefficients of both (EuF)2Ti2Sb2O and (EuF)2Ti2Bi2O are significantly enhanced compared to those of (SrF)2Ti2Pn2O, Na2Ti2Pn2O, and BaTi2Pn2O,1,5,6 which may be due to a strong electron correlation effect in this system. Thus, (EuF)2Ti2Pn2O (Pn = Sb, Bi) may provide new platforms for studying density wave, magnetic ordering, and electron correlation effects.
A quaternary compound with the composition Mo3ReRuC is obtained in a previously unexplored MoReRu-Mo2C system. According to X-ray structural analysis, Mo3ReRuC crystallizes in the noncentrosymmetric space group P4132 [cubic ß-Mn-type structure, a = 6.8107(1) Å]. Below 7.7 K, Mo3ReRuC becomes a bulk type-II superconductor with an upper critical field close to the Pauli paramagnetic limit. The specific heat data give a large normalized jump ΔCp/γTc = 2.3 at Tc, which points to a strongly coupled superconducting state. First-principles calculations show that its electronic states at the Fermi level are mainly contributed by Mo, Re, and Ru atoms and strongly increased by the spin-orbit coupling. Our finding suggests that the intermediate phase between alloys and carbides may be a good place to look for ß-Mn-type noncentrosymmetric superconductors.
Eu(Fe0.75Ru0.25)2As2is an intriguing system with unusual coexistence of superconductivity and ferromagnetism, providing a unique platform to study the nature of such coexistence. To establish a magnetic phase diagram, time-domain synchrotron Mössbauer experiments in151Eu have been performed on a single crystalline Eu(Fe0.75Ru0.25)2As2sample under hydrostatic pressures and at low temperatures. Upon compression the magnetic ordering temperature increases sharply from 20 K at ambient pressure, reaching â¼49 K at 10.1 GPa. With further compression, the magnetic order is suppressed and eventually collapses. Isomer shift values from Mössbauer measurements and x-ray absorption spectroscopy data at EuL3edge show that pressure drives Eu ions to a homogeneous intermediate valence state with mean valence of â¼2.4 at 27.4 GPa, possibly responsible for the suppression of magnetism. Synchrotron powder x-ray diffraction experiment reveals a tetragonal to collapsed-tetragonal structural transition around 5 GPa, a lower transition pressure than in the parent compound. These results provide guidance to further work investigating the interplay of superconductivity and magnetism.
We report synthesis, crystal structure, and physical properties of Sr2Cr2AsO3. The new compound crystallizes in a Sr2GaO3CuS-type structure with two distinct Cr sites, Cr(1) in the perovskite-like block layers of "Sr3Cr2O6" and Cr(2) in the ThCr2Si2-type layers of "SrCr2As2". An inter-block-layer charge transfer is explicitly evidenced, which dopes electrons in the CrO2 planes and simultaneously dopes holes into the CrAs layers. Measurements of electrical resistivity, magnetization, and specific heat, in combination with density-functional theoretical calculations, indicate that the title material is an antiferromagnetic metal. The Cr(2) magnetic moments in the CrAs layers order at 420 K, while the Cr(1) spins in the CrO2 planes show quasi-two-dimensional magnetism with long-range ordering below 80 K. Both Néel temperatures are significantly reduced, compared with those of the cousin material Sr2Cr3As2O2, probably due to the intrinsic charge-carrier doping. Complex re-entrant magnetic transitions with a huge magnetic hysteresis were observed at low temperatures.
A microwave technique suitable for investigating the AC magnetic susceptibility of small samples in the GHz frequency range is presented. The method-which is based on the use of a coplanar waveguide resonator, within the resonator perturbation approach-allows one to obtain the absolute value of the complex susceptibility, from which the penetration depth and the superfluid density can be determined. We report on the characterization of several iron-based superconducting systems, belonging to the 11, 122, 1144, and 12442 families. In particular, we show the effect of different kinds of doping for the 122 family, and the effect of proton irradiation in a 122 compound. Finally, the paradigmatic case of the magnetic superconductor EuP-122 is discussed, since it shows the emergence of both superconducting and ferromagnetic transitions, marked by clear features in both the real and imaginary parts of the AC susceptibility.
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(Ta1+x Se2 )2 , the first 124-phase, is reported. Pb(Ta1+x Se2 )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 (high-T) phase with ABC stacking and R3m symmetry to an intermediate phase with AB stacking and P3m1, and finally to a low-temperature (low-T) phase again with R3msymmetry, 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-T, bulk superconductivity with Tc ≈ 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. The results demonstrate the possibility of realizing higher-order metal-intercalated phases of TMDCs and advance the knowledge of polymorphic transitions, and may inspire stacking-order engineering in TMDCs and beyond.
Superconductivity (SC) and ferromagnetism (FM) are normally antagonistic, and their coexistence in a single crystalline material appears to be very rare. Over a decade ago, the iron-based pnictides of doped EuFe2As2were found to render such a coexistence, primarily because of the Fe-3dmulti-orbitals which simultaneously satisfy the superconducting pairing and the ferromagnetic exchange interaction among Eu local spins. In 2016, the discovery of the iron-based superconductorsAEuFe4As4(A= Rb, Cs) provided an additional and complementary material basis for the study of the coexistence and the interplay between SC and FM. The two sibling compounds, which can be viewed as an intergrowth or a hybrid betweenAFe2As2and EuFe2As2, show SC in the FeAs bilayers atTc= 35-37 K and magnetic ordering atTmâ¼ 15 K in the sandwiched Eu2+-ion sheets. BelowTm, the Eu2+spins align ferromagnetically within each Eu plane, making the system as a natural atomic-thick superconductor-ferromagnet superlattice. This paper reviews the main research progress in the emerging topic during the past five years. An outlook for the future research opportunities is also presented.
In this study, we measure the in-plane transport properties of high-quality Ba(Fe0.914Co0.086)2As2 single crystals. Signatures of vortex unbinding Berezinskii-Kosterlitz-Thouless (BKT) transition are shown from both the conventional approach and the Fisher-Fisher-Huse dynamic scaling analysis, in which a characteristic Nelson-Kosterlitz jump is demonstrated. We also observe a non-Hall transverse signal exactly at the superconducting transition, which is explained in terms of guided motion of unbound vortices.
Iron-chalcogenide superconductors have emerged as a promising Majorana platform for topological quantum computation. By combining topological band and superconductivity in a single material, they provide significant advantage to realize isolated Majorana zero modes. However, iron-chalcogenide superconductors, especially Fe(Te,Se), suffer from strong inhomogeneity which may hamper their practical application. In addition, some iron-pnictide superconductors have been demonstrated to have topological surface states, yet no Majorana zero mode has been observed inside their vortices, raising a question of universality about this new Majorana platform. In this work, through angle-resolved photoemission spectroscopy and scanning tunneling microscopy/spectroscopy measurement, we identify Dirac surface states and Majorana zero modes, respectively, for the first time in an iron-pnictide superconductor, CaKFe4As4. More strikingly, the multiple vortex bound states with integer-quantization sequences can be accurately reproduced by our model calculation, firmly establishing Majorana nature of the zero mode.
Mn-based ZrCuSiAs-type pnictides ThMnPnN (Pn = P, As) containing PbO-type Th2N2 layers were synthesized. The crystal and magnetic structures are determined using X-ray and neutron powder diffraction. While neutron diffraction indicates a C-type antiferromagnetic state at 300 K, the temperature dependence of the magnetic susceptibility shows cusps at 36 and 52 K respectively for ThMnPN and ThMnAsN. The susceptibility cusps are ascribed to a spontaneous antiferromagnetic-to-antiferromagnetic transition for Mn2+ moments, which is observed for the first time in Mn-based ZrCuSiAs-type compounds. In addition, measurements of the resistivity and specific heat suggest an abnormal increase in the density of states at the Fermi energy. The result is discussed in terms of the internal chemical pressure effect.
We studied the cobalt-doping effect on superconductivity and magnetism in a hole self-doped RbEuFe4As4 magnetic superconductor which shows superconductivity at [Formula: see text] 36.5 K and Eu2+ -spin ordering at [Formula: see text] 15 K. The Co solubility limit in RbEu(Fe1-x Co x )4As4 achieves x = 0.21 for the solid-state reaction at 880 °C. With increasing x, [Formula: see text] decreases gradually, and superconductivity eventually disappears at [Formula: see text]. A spin-density-wave transition at [Formula: see text] 35-40 K is recovered for [Formula: see text], which can be understood in terms of the hole-depletion and the disorder effects. On the other hand, [Formula: see text] remains unchanged despite the Co doping and, consequently, an intriguing superconducting ferromagnet without Meissner state is realized in the range of 0.125 [Formula: see text] 0.155. Our results indicate that the Eu2+ spins essentially decouple with superconductivity over a wide doping range, making the coexistence of superconductivity and ferromagnetism possible in the 1144-type system.
We examined the physical properties of the quasi-one-dimensional superconductor Ta4Pd3Te16 in the normal state by detailed measurements of susceptibility, in-plane anisotropic resistivity, magnetoresistance, Hall resistivity, and Seebeck coefficient. The large Wilson ratio, as inferred from normal-state susceptibility, indicates strong electron-electron interaction. The Hall and Seebeck coefficients show not only significant temperature-dependent behavior, indicating the multiband effect, but also an obvious anomaly around T 1 = 40 K. Analyses of both the Hall resistivity and thermopower using a two-band model indicate that the electrons dominate the electrical transport at low temperatures. Our results imply that it is the quantum fluctuations of the charge order taking place in the temperature range 30-50 K that may result in the abnormal normal-state properties of Ta4Pd3Te16.
A metastable vanadium oxytelluride V2Te2O is prepared via a topochemical deintercalation of interlayer Rb+ cations in Rb1-δV2Te2O. The new ternary mixed-anion compound crystallizes in a body-centered tetragonal lattice with a = 3.9282(1) Å and c = 13.277(5) Å, containing V2O square nets that are sandwiched by Te-atomic sheets. The charge-neutral [V2OTe2] block layers stack along the c axis with van der Waals forces, which shows a metallic behavior with a dominant T2 dependence for resistivity at low temperatures. The electronic specific-heat coefficient reaches 33.9 mJ K-2 mol-1, â¼4 times that of the electronic structure calculations, suggesting a significant electron-mass renormalization. The electron correlation effect is concurrently demonstrated by the Wilson and Kadowaki-Woods ratios. Neither charge/spin-density wave nor superconductivity was observed down to 0.03 K.
The interplay between superconductivity and magnetism is one of the oldest enigmas in physics. Usually, the strong exchange field of ferromagnet suppresses singlet superconductivity via the paramagnetic effect. In EuFe2(As0.79P0.21)2, a material that becomes not only superconducting at 24.2 K but also ferromagnetic below 19 K, the coexistence of the two antagonistic phenomena becomes possible because of the unusually weak exchange field produced by the Eu subsystem. We demonstrate experimentally and theoretically that when the ferromagnetism adds to superconductivity, the Meissner state becomes spontaneously inhomogeneous, characterized by a nanometer-scale striped domain structure. At yet lower temperature and without any externally applied magnetic field, the system locally generates quantum vortex-antivortex pairs and undergoes a phase transition into a domain vortex-antivortex state characterized by much larger domains and peculiar Turing-like patterns. We develop a quantitative theory of this phenomenon and put forth a new way to realize superconducting superlattices and control the vortex motion in ferromagnetic superconductors by tuning magnetic domains-unprecedented opportunity to consider for advanced superconducting hybrids.
The large size single crystals of (SnSe)1.16(NbSe2) misfit layered compound were grown and superconductivity with T c of 3.4 K was first discovered in this system. Powder x-ray diffraction and high resolution transmission electron microscopy clearly display the misfit feature between SnSe and NbSe2 subsystems. The Sommerfeld coefficient γ inferred from specific-heat measurements is 16.73 mJ mol-1 K-2, slightly larger than the usual misfit compounds. The normalized specific heat jump [Formula: see text] is about 0.98, and the electron-phonon coupling constant [Formula: see text] is estimated to be 0.80. The estimated value of the in-plane upper critical magnetic field, [Formula: see text](0), is about 7.82 T, exceeding the Pauli paramagnetic limit slightly. Both the specific-heat and H c2 data suggest that (SnSe)1.16(NbSe2) is a multi-band superconductor.
ThFeAsN1-x O x ([Formula: see text]) system with heavy electron doping has been studied by the measurements of x-ray diffraction, electrical resistivity, magnetic susceptibility and specific heat. The non-doped compound exhibits superconductivity at [Formula: see text] K, which is possibly due to an internal uniaxial chemical pressure that is manifested by the extremely small value of As height with respect to the Fe plane. With the oxygen substitution, the T c value decreases rapidly to below 2 K for [Formula: see text], and surprisingly, superconductivity re-appears in the range of [Formula: see text] with a maximum [Formula: see text] of 17.5 K at x = 0.3. For the normal-state resistivity, while the samples in intermediate non-superconducting interval exhibit Fermi liquid behavior, those in other regions show a non-Fermi-liquid behavior. The specific heat jump for the superconducting sample of x = 0.4 is [Formula: see text], which is discussed in terms of anisotropic superconducting gap. The peculiar phase diagram in ThFeAsN1-x O x presents additional ingredients for understanding the superconducting mechanism in iron-based superconductors.
We report a new quasi-one-dimensional compound KMn6Bi5 composed of parallel nanowires crystallizing in a monoclinic space group C2/ m with a = 22.994(2) Å, b = 4.6128(3) Å, c = 13.3830(13) Å and ß = 124.578(6)°. The nanowires are infinite [Mn6Bi5]- columns each of which is composed of a nanotube of Bi atoms acting as the cladding with a nanorod of Mn atoms located in the central axis of the nanotubes. The nanorods of Mn atoms inside the Bi cladding are stabilized by Mn-Mn bonding and are defined by distorted Mn-centered cluster icosahedra of Mn13 sharing their vertices along the b axis. The [Mn6Bi5]- nanowires are linked with weak internanowire Bi-Bi bonds and charge balanced with K+ ions. The [Mn6Bi5]- nanowires were directly imaged by high-resolution transmission electron microscopy and scanning transmission electron microscopy. Magnetic susceptibility studies show one-dimensional characteristics with an antiferromagnetic transition at â¼75 K and a small average effective magnetic moment (1.56 µB/Mn for H ⥠b and 1.37 µB/Mn for H ⥠b) of Mn from Curie-Weiss fits above 150 K. Specific heat measurements reveal an electronic specific heat coefficient γ of 6.5(2) mJ K-2(mol-Mn)-1 and a small magnetic entropy change Δ Smag ≈ 1.6 J K-1 (mol-Mn)-1 across the antiferromagnetic transition. In contrast to a metallic resistivity along the column, the resistivity perpendicular to the column shows a change from a semiconducting behavior at high temperatures to a metallic one at low temperatures, indicating an incoherent-to-coherent crossover of the intercolumn tunneling of electrons.
