Competition between magnetism and superconductivity in rare-earth nickel boride carbides

Iron-based magnetic superconductorsAEuFe4As4(ARb, Cs): natural superconductor-ferromagnet hybrids

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 atT_c= 35-37 K and magnetic ordering atT_m∼ 15 K in the sandwiched Eu²⁺-ion sheets. BelowT_m, 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.

Observing the Suppression of Superconductivity in RbEuFe_{4}As_{4} by Correlated Magnetic Fluctuations

In this Letter, we describe quantitative magnetic imaging of superconducting vortices in RbEuFe_{4}As_{4} in order to investigate the unique interplay between the magnetic and superconducting sublattices. Our scanning Hall microscopy data reveal a pronounced suppression of the superfluid density near the magnetic ordering temperature in good qualitative agreement with a recently developed model describing the suppression of superconductivity by correlated magnetic fluctuations. These results indicate a pronounced exchange interaction between the superconducting and magnetic subsystems in RbEuFe_{4}As_{4}, with important implications for future investigations of physical phenomena arising from the interplay between them.

A Novel Magnetic Material by Design: Observation of Yb3+ with Spin-1/2 in Yb x Pt5P

The localized f-electrons enrich the magnetic properties in rare-earth-based intermetallics. Among those, compounds with heavier 4d and 5d transition metals are even more fascinating because anomalous electronic properties may be induced by the hybridization of 4f and itinerant conduction electrons primarily from the d orbitals. Here, we describe the observation of trivalent Yb³⁺ with S = 1/2 at low temperatures in Yb _x Pt₅P, the first of a new family of materials. Yb _x Pt₅P (0.23 ≤ x ≤ 0.96) phases were synthesized and structurally characterized. They exhibit a large homogeneity width with the Yb ratio exclusively occupying the 1a site in the anti-CeCoIn₅ structure. Moreover, a sudden resistivity drop could be found in Yb _x Pt₅P below ∼0.6 K, which requires further investigation. First-principles electronic structure calculations substantiate the antiferromagnetic ground state and indicate that two-dimensional nesting around the Fermi level may give rise to exotic physical properties, such as superconductivity. Yb _x Pt₅P appears to be a unique case among materials.

Reentrants-wave superconductivity in the periodic Anderson model with attractive conduction band Hubbard interaction

Spin-flip scattering from magnetic impurities has a strong pair-breaking effect ins-wave superconductors where increasing the concentration of impurities rapidly destroys superconductivity. For small Kondo temperatureTKthe destruction of superconductivity is preceded by the reentrant superconductivity at finite temperature rangeTc2 Collapse

Key Words

• Anderson lattice model
• dynamical mean field theory
• superconductivity

Collapse

MESH Headings Collapse

Grants Collapse

Affiliation(s)

W V van Gerven Oei Institute of Physics Belgrade, University of Belgrade, Pregrevica 118, 11080 Belgrade, Serbia
D Tanasković Institute of Physics Belgrade, University of Belgrade, Pregrevica 118, 11080 Belgrade, Serbia

Collapse

Anomalous quantum Griffiths singularity in ultrathin crystalline lead films

Superconductor-insulator/metal transition (SIT/SMT) represents a prototype of quantum phase transition, where quantum fluctuation plays a dominant role and dramatically changes the physical properties of low-dimensional superconducting systems. Recent observation of quantum Griffiths singularity (QGS) offers an essential perspective to understand the subtleties of quantum phase transition in two-dimensional superconductors. Here we study the magnetic field induced SMT in ultrathin crystalline Pb films down to ultralow temperatures. The divergent critical exponent is observed when approaching zero temperature quantum critical point, indicating QGS. Distinctively, the anomalous phase boundary of SMT that the onset critical field decreases with decreasing temperatures in low temperature regime distinguishes our observation from previous reports of QGS in various two-dimensional superconductors. We demonstrate that the anomalous phase boundary originates from the superconducting fluctuations in ultrathin Pb films with pronounced spin-orbit interaction. Our findings reveal a novel aspect of QGS of SMT in two-dimensional superconductors with anomalous phase boundary.

High-pressure studies with x-rays using diamond anvil cells

Pressure profoundly alters all states of matter. The symbiotic development of ultrahigh-pressure diamond anvil cells, to compress samples to sustainable multi-megabar pressures; and synchrotron x-ray techniques, to probe materials' properties in situ, has enabled the exploration of rich high-pressure (HP) science. In this article, we first introduce the essential concept of diamond anvil cell technology, together with recent developments and its integration with other extreme environments. We then provide an overview of the latest developments in HP synchrotron techniques, their applications, and current problems, followed by a discussion of HP scientific studies using x-rays in the key multidisciplinary fields. These HP studies include: HP x-ray emission spectroscopy, which provides information on the filled electronic states of HP samples; HP x-ray Raman spectroscopy, which probes the HP chemical bonding changes of light elements; HP electronic inelastic x-ray scattering spectroscopy, which accesses high energy electronic phenomena, including electronic band structure, Fermi surface, excitons, plasmons, and their dispersions; HP resonant inelastic x-ray scattering spectroscopy, which probes shallow core excitations, multiplet structures, and spin-resolved electronic structure; HP nuclear resonant x-ray spectroscopy, which provides phonon densities of state and time-resolved Mössbauer information; HP x-ray imaging, which provides information on hierarchical structures, dynamic processes, and internal strains; HP x-ray diffraction, which determines the fundamental structures and densities of single-crystal, polycrystalline, nanocrystalline, and non-crystalline materials; and HP radial x-ray diffraction, which yields deviatoric, elastic and rheological information. Integrating these tools with hydrostatic or uniaxial pressure media, laser and resistive heating, and cryogenic cooling, has enabled investigations of the structural, vibrational, electronic, and magnetic properties of materials over a wide range of pressure-temperature conditions.

Reentrant superconductivity in Eu(Fe(1-x)Ir(x))2As2

The interplay between superconductivity and Eu(2+) magnetic ordering in Eu(Fe1-xIrx)2As2 is studied by means of electrical transport and magnetic measurements. For the near optimally doped sample Eu(Fe0.86Ir0.14)2As2, we witnessed two distinct transitions: a superconducting transition below 22.6 K which is followed by a resistivity reentrance caused by the ordering of the Eu(2+) moments. Further, the low field magnetization measurements show a prominent diamagnetic signal due to superconductivity, which is remarkable in the presence of a large-moment magnetically ordered system. The electronic structure for 12.5% Ir doped EuFe1.75Ir0.25As2 is investigated along with the parent compound EuFe2As2. As compared to EuFe2As2, the doped compound has an effectively lower value of density of states throughout the energy scale with a more extended bandwidth and stronger hybridization involving Ir. Shifting of the Fermi energy and a change in band filling in EuFe1.75Ir0.25As2 with respect to the pure compound indicate electron doping in the system.

Superconductivity in LaT(M)BN and La3T(M2)B2N3 (T(M) = transition metal) synthesized under high pressure

Various layered boronitrides (LaN)(n)(T(M2)B(2)) (T(M) = transition metal; n = 2, 3) have been prepared using a high-pressure synthesis technique in which an inverse α-PbO-type T(M2)B(2) layer is separated by two or three rock salt-type LaN layers and these layers are connected through linear (BN) units. The electronic states of the distinguishing (BN) unit and intermediate rock salt-type LaN layer are discussed on the basis of density functional theory calculations. Bulk superconductivity has been found in LaNiBN (T(c) ≈ 4.1 K), CaNiBN (T(c) ≈ 2.2 K), and LaPtBN (T(c) ≈ 6.7 K), where the Fermi level E(F) is located in the bands composed of the T(M)(d)-B(2p) antibonding state and the main T(M)(d) band resides well below E(F). The non-superconductive T(M)-based compounds exhibit Pauli paramagnetic behavior, in which the highly itinerant nature of the electrons caused by strong T(M)(d)-B(2p) covalent bonding suppresses the long-range magnetic ordering.

THE RÖSSLER EQUATIONS REVISITED IN TERMS OF FEEDBACK CIRCUITS

We would like to show that complex dynamics can be deciphered and at least partly understood in terms of its underlying logical structure, more specifically in terms of the feedback circuits built in the differential equations. This approach has permitted to build a number of new three- and four-variable systems displaying chaotic dynamics.

Starting from the well-known Rössler equations for deterministic chaos, one asks how systems with the same types of steady states can be synthesized ab initio from appropriate feedback circuits (= appropriate logical structure). It is found that, granted an appropriate logical structure, the existence of a domain of chaotic dynamics is remarkably robust towards changes in the nature of the nonlinearity used, and towards those sign changes which respect the nature (positive vs negative) of the feedback circuits. Using logical arguments, it was also easy to find related systems with a single steady state. A variety of 3- and 4-d systems based on other combinations of feedback circuits and generating chaotic dynamics are described.

The aim of this work is to contribute to a better understanding of the respective roles of feedback circuits and nonlinearity — both essential — in so-called "non trivial behavior", including deterministic chaos. Special emphasis is put on the interest of using the Jacobian matrix and its by-products (characteristic equation, eigenvalues, …) not only close to steady states (where linear stability analysis can be performed) but also elsewhere in phase space, where precious indications about the global behavior can be collected.

Evidence for competing magnetic and superconducting phases in superconducting Eu 1-x Sr x Fe 2-y Co y As 2 single crystals

In single crystals of Eu(1-x)Sr(x)Fe(2-y)Co(y)As(2), Co doping suppresses spin-density wave (SDW) ordering and induces a superconducting transition. A resistivity reentrance due to the antiferromagnetic ordering of Eu(2+) spins is observed, indicating the competition between antiferromagnetism (AFM) and superconductivity (SC). It is striking that the resistivity reentrance can be completely suppressed by a small magnetic field due to a field-induced metamagnetic transition from AFM to ferromagnetism (FM). The resistivity reentrance can also be suppressed by the substitution of Eu(2+) ions with nonmagnetic Ba(2+)/Sr(2+) to completely destroy the AFM ordering. These results indicate that the AFM order appears destructive to SC, while FM can coexist with the superconductivity. Further we find that magnon excitation exists in AFM ordering and can be suppressed by an applied field. Coexistence of SC from the FeAs layer and the inner field produced by the ferromagnetic Eu(2+) layer suggest a possible p-wave component in the superconducting order parameter.

Superconductivity induced by phosphorus doping and its coexistence with ferromagnetism in EuFe2(As0.7P0.3)(2)

We have studied EuFe2(As0.7P0.3)(2) by the measurements of x-ray diffraction, electrical resistivity, thermopower, magnetic susceptibility, magnetoresistance, and specific heat. Partial substitution of As with P results in the shrinkage of lattice, which generates chemical pressure to the system. It is found that EuFe2(As0.7P0.3)(2) undergoes a superconducting transition at 26 K, followed by ferromagnetic ordering of Eu2+ moments at 20 K. This finding is the first observation of superconductivity stabilized by internal chemical pressure, and supplies a rare example showing the coexistence of superconductivity and ferromagnetism in the ferroarsenide family.

Bulk electronic structure of the antiferromagnetic superconducting phase in ErNi2B2C

We have performed temperature- (T-)dependent laser-photoemission spectroscopy of the antiferromagnetic (AF) superconductor ErNi2B2C to study the electronic-structure evolution reflecting the interplay between antiferromagnetism and superconductivity. The spectra at the superconducting (SC) phase show a very broad spectral shape. A T-dependent SC gap shows a sudden deviation from the BCS prediction just below TN. This observation can be explained well by the theoretical model and thus represents the characteristic bulk electronic structure of the AF SC phase for the first time.

Universality and scaling for the breakup of phase synchronization at the onset of chaos in a periodically driven Rössler oscillator

Universal behavior discovered earlier in two-dimensional noninvertible maps is found numerically in a periodically driven Rössler system. The critical behavior is associated with the limit of a period-doubling cascade at the edge of the Arnold tongue, and may be reached by variation of two control parameters. The corresponding scaling regularities, distinct from those of the Feigenbaum cascade, are demonstrated. Presence of a critical quasiattractor, an infinite set of stable periodic orbits of quadrupled periods, is outlined. As argued, this type of critical behavior may occur in a wide class of periodically driven period-doubling systems.

Theoretical model for the superconducting and magnetically ordered borocarbides

We present a theory of superconductivity in the presence of a general magnetic structure in a form suitable for the description of complex magnetic phases encountered in borocarbides. The theory, complemented with some details of the band structure and with the magnetic phase diagram, may explain the nearly reentrant behavior and the anisotropy of the upper critical field of HoNi2B2C. The onset of the helical magnetic order depresses superconductivity via the reduction of the interaction between phonons and electrons caused by the formation of magnetic Bloch states. At mean field level, no additional suppression of superconductivity is introduced by the incommensurability of the helical phase.