[Show abstract][Hide abstract] ABSTRACT: Sr 2 IrO 4 is a magnetic insulator assisted by strong spin-orbit coupling (SOC) whereas the Sr 2 RuO 4 is a p-wave superconductor. The contrasting ground states have been shown to result from the critical role of the strong SOC in the iridate. Our investigation of structural, transport, and magnetic properties reveals that substituting 4d Ru 4+ (4d 4) ions for 5d Ir 4+ (5d 5) ions in Sr 2 IrO 4 directly adds holes to the t 2g bands, reduces the SOC and thus rebalances the competing energies in single-crystal Sr 2 Ir 1-x Ru x O 4. A profound effect of Ru doping driving a rich phase diagram is a structural phase transition from a distorted I4 1 /acd to a more ideal I4/mmm tetragonal structure near x=0.50 that accompanies a phase transition from an antiferromagnetic-insulating state to a paramagnetic-metal state. We also make a comparison drawn with Rh doped Sr 2 IrO 4 , highlighting important similarities and differences.
[Show abstract][Hide abstract] ABSTRACT: Sr2IrO4 is a magnetic insulator assisted by strong spin-orbit coupling (SOC)
whereas the Sr2RuO4 is a p-wave superconductor. The contrasting ground states
have been shown to result from the critical role of the strong SOC in the
iridate. Our investigation of structural, transport, and magnetic properties
reveals that substituting 4d Ru4+ (4d4) ions for 5d Ir4+(5d5) ions in Sr2IrO4
directly adds holes to the t2g bands, reduces the SOC and thus rebalances the
competing energies in single-crystal Sr2Ir1-xRuxO4. A profound effect of Ru
doping driving a rich phase diagram is a structural phase transition from a
distorted I41/acd to a more ideal I4/mmm tetragonal structure near x=0.50 that
accompanies a phase transition from an antiferromagnetic-insulating state to a
paramagnetic-metal state. We also make a comparison drawn with Rh doped
Sr2IrO4, highlighting important similarities and differences.
[Show abstract][Hide abstract] ABSTRACT: We consider the one-dimensional gas of fermions with spin S interacting via an attractive δ-function potential using the Bethe Ansatz solution. In zero magnetic field the atoms form bound states of fermions, i.e. generalized Cooper states with each atom having a different spin component. For low energy excitations the system is a Luttinger liquid and is properly described by a conformal field theory with conformal charge . The linear dispersion of a Luttinger liquid is asymptotically exact in the low-energy limit where the band curvature terms in the dispersion are irrelevant. For higher energy excitations, however, the spectral function displays deviations in the neighborhood of the single-particle (hole) energy, which can be described by an effective x-ray edge type model. Using the Bethe Ansatz solution we obtain expressions for the critical exponents for the single-particle (hole) Green's function. This model can be relevant in the context of ultracold atoms with effective total spin S confined to an elongated optical trap.
Nuclear Physics B 01/2015; 892. DOI:10.1016/j.nuclphysb.2015.01.009 · 3.93 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Here, we unveil evidence for a quantum phase-transition in CeCu_2Ge_2 which
displays both an incommensurate spin-density wave (SDW) ground-state, and a
strong renormalization of the quasiparticle effective masses (mu) due to the
Kondo-effect. For all angles theta between an external magnetic field (H) and
the crystallographic c-axis, the application of H leads to the suppression of
the SDW-state through a 2^nd-order phase-transition at a theta-dependent
critical-field H_p(theta) leading to the observation of small Fermi surfaces
(FSs) in the paramagnetic (PM) state. For H || c-axis, these FSs are
characterized by light mu's pointing also to the suppression of the
Kondo-effect at H_p with surprisingly, no experimental evidence for
quantum-criticality (QC). But as $H$ is rotated towards the a-axis, these mu's
increase considerably becoming undetectable for \theta > 56^0 between H and the
c-axis. Around H_p^a~ 30 T the resistivity becomes proportional T which,
coupled to the divergence of mu, indicates the existence of a field-induced
QC-point at H_p^a(T=0 K). This observation, suggesting FS hot-spots associated
with the SDW nesting-vector, is at odds with current QC scenarios for which the
continuous suppression of all relevant energy scales at H_p(theta,T) should
lead to a line of quantum-critical points in the H-theta plane. Finally, we
show that the complexity of its magnetic phase-diagram(s) makes CeCu_2Ge_2 an
ideal system to explore field-induced quantum tricritical and QC end-points.
Physical Review B 09/2014; 90(15). DOI:10.1103/PhysRevB.90.155101 · 3.74 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: In the context of ultracold fermionic atoms with effective spin S confined to an elongated trap we study the one-dimensional gas interacting via an attractive δ-function potential using the Bethe ansatz solution. There are N = 2S + 1 fundamental states: The particles can either be unpaired or clustered in bound states of 2, 3, ..., 2S and 2S + 1 atoms. In a magnetic field, the rich ground state phase diagram consists of these N states and various mixed phases in which combinations of the fundamental states coexist. The phase diagram simplifies considerably in zero-field, where only bound states of N atoms can exist. Due to the harmonic confinement and within the local density approximation, the density profile of bound states decreases along the tube from the center of the trap to its boundaries. In an array of tubes with weak Josephson tunneling superfluid order may arise. In zero-field the response functions determining the superfluid and density wave order are calculated using conformal field theory and the exact Bethe ansatz solution. The response function for superfluidity consists of a power law with distance, while the correlation function for density waves is a power law of distance times a sinusoidal factor oscillating with distance with a period given by two times the Fermi momentum. For S = 1/2 superfluidity is a possibility for all densities and density waves can be excluded. For S ≥ 3/2 superfluidity may occur at low densities but at high densities it gives way to density waves. We discuss the scenario of phase separation where for S ≥ 3/2 the system has superfluid long-range order toward the trap boundaries and density waves at its center.
Journal of Physics Conference Series 08/2014; 529(1):012013. DOI:10.1088/1742-6596/529/1/012013
[Show abstract][Hide abstract] ABSTRACT: Motivated by recent experiments on ultracold fermions we study the effect of the spin-orbit interaction in an exactly solvable one-dimensional model with strong local attractive correlations between the fermions. We show that the asymptotes of correlation functions, calculated in the framework of the conformal field theory and finite size corrections of the Bethe ansatz exact solution, are strongly affected by the spin-orbit coupling. In the mixed phase, the correlation functions consist of terms that are the product of a power law of the distance and an oscillating function of the distance. We obtain the critical exponents for superfluidity and density waves. The leading exponents decrease as a function of spin-orbit coupling. In particular, the exponent for superfluidity is the smallest one signaling an instability to a phase where a weak interchain coupling can produce superfluidity of the Fulde-Ferrell-Larkin-Ovchinnikov (FFLO) type. The spatial oscillation of the order parameter is also modified by the spin-orbit parameter, i.e., its modulation is not just given by the spin imbalance.
Physical Review B 11/2013; 88(20). DOI:10.1103/PhysRevB.88.205127 · 3.74 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: We have observed a massive reconstruction of the Fermi surface of single
crystal chromium as a function of high pressure and high magnetic fields caused
by the spin-flip transition, with multiple new orbits appearing above 0.93 GPa.
Additionally, some orbits have field-induced effective masses of ~0.06-0.07 me,
seen only at high magnetic fields. Based on the temperature insensitivity
displayed by the oscillation amplitudes at these frequencies, we attribute the
orbits to quantum interference rather than to Landau quantization.
[Show abstract][Hide abstract] ABSTRACT: We observe an unusual combination of normal and superconducting state properties without any signature of strong spin fluctuations in single-crystal Ir3Te8. The electrical resistivity does not saturate by 700 K but exhibits a low-resistivity ratio, and it exhibits two extended linear regimes (approximately 20–330 and 370–700 K) with the same slope, separated by a small hysteretic interval marking a strong first-order phase transition from cubic to rhombohedral lattice symmetry at TS = 350 K. The electronic heat-capacity coefficient (11 mJ mol−1 K−2) is consistent with a net diamagnetic, rather than a Pauli paramagnetic, normal state that yields to superconductivity below a critical temperature TC = 1.8 K. The size of the heat-capacity jump near TC indicates bulk superconductivity.
[Show abstract][Hide abstract] ABSTRACT: Sr3Ir2O7 exhibits a novel Jeff=1/2 insulating state that features a splitting
between Jeff=1/2 and 3/2 bands due to spin-orbit interaction. We report a
metal-insulator transition in Sr3Ir2O7 via either dilute electron doping (La3+
for Sr2+) or application of high pressure up to 35 GPa. Our study of
single-crystal Sr3Ir2O7 and (Sr1-xLax)3Ir2O7 reveals that application of high
hydrostatic pressure P leads to a drastic reduction in the electrical
resistivity by as much as six orders of magnitude at a critical pressure, PC =
13.2 GPa, manifesting a closing of the gap; but further increasing P up to 35
GPa produces no fully metallic state at low temperatures, possibly as a
consequence of localization due to a narrow distribution of bonding angles
{\theta}. In contrast, slight doping of La3+ ions for Sr2+ ions in Sr3Ir2O7
readily induces a robust metallic state in the resistivity at low temperatures;
the magnetic ordering temperature is significantly suppressed but remains
finite for (Sr0.95La0.05)3Ir2O7 where the metallic state occurs. The results
are discussed along with comparisons drawn with Sr2IrO4, a prototype of the
Jeff = 1/2 insulator.
[Show abstract][Hide abstract] ABSTRACT: We observe superconductivity below a critical temperature TC = 1.8 K in
single-crystal Ir3Te8, which also exhibits normal-state diamagnetism and a
linear temperature dependence of electrical resistivity for a wide temperature
interval, 20 K < T < 700 K. Single-crystal Ir3Te8 also undergoes a structural
phase transition at TS = 350 K from a cubic (above TS) to a rhombohedral
lattice below TS. Our first-principles electronic structure calculations reveal
two bands crossing the Fermi level; despite the three-dimensional lattice, one
band is quasi-two-dimensional, and is responsible for the observed diamagnetism
and structure transition. The strong non-Fermi-liquid behavior characterized by
the observed linearity in resistivity in such a nonmagnetic state suggests
novel physics in this newly discovered superconductor.
[Show abstract][Hide abstract] ABSTRACT: We report the magnetic and electric properties of Ba$_3$NiNb$_2$O$_9$, which
is a quasi-two-dimensional spin-1 triangular lattice antiferromagnet (TLAF)
with trigonal structure. At low $T$ and with increasing magnetic field, the
system evolves from a 120 degree magnetic ordering phase (A phase) to an
up-up-down ($uud$) phase (B phase) with a change of slope at 1/3 of the
saturation magnetization, and then to an "oblique" phase (C phase).
Accordingly, the ferroelectricity switches on at each phase boundary with
appearance of spontaneous polarization. Therefore, Ba$_3$NiNb$_2$O$_9$ is a
unique TLAF exhibiting both $uud$ phase and multiferroicity.
[Show abstract][Hide abstract] ABSTRACT: Sr2IrO4 is a magnetic insulator driven by spin-orbit interaction (SOI)
whereas the isoelectronic and isostructural Sr2RhO4 is a paramagnetic metal.
The contrasting ground states have been shown to result from the critical role
of the strong SOI in the iridate. Our investigation of structural, transport,
magnetic and thermal properties reveals that substituting 4d Rh4+ (4d5) ions
for 5d Ir4+(5d5) ions in Sr2IrO4 directly reduces the SOI and rebalances the
competing energies so profoundly that it generates a rich phase diagram for
Sr2Ir1-xRhxO4 featuring two major effects: (1) Light Rh doping (0\leqx\leq0.16)
prompts a simultaneous and precipitous drop in both the electrical resistivity
and the magnetic ordering temperature TC, which is suppressed to zero at x =
0.16 from 240 K at x=0. (2) However, with heavier Rh doping (0.24< x<0.85
(\pm0.05)) disorder scattering leads to localized states and a return to an
insulating state with spin frustration and exotic magnetic behavior that only
disappears near x=1. The intricacy of Sr2Ir1-xRhxO4 is further highlighted by
comparison with Sr2Ir1-xRuxO4 where Ru4+(4d4) drives a direct crossover from
the insulating to metallic states.
Physical Review B 09/2012; 86(12):125105. DOI:10.1103/PhysRevB.86.125105 · 3.74 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: We report an experimental/theoretical study of single-crystal Bi(2)Ir(2)O(7) that possesses a metallic state with strongly exchange-enhanced paramagnetism. The ground state of Bi(2)Ir(2)O(7) is characterized by the following features: (1) a divergent low-temperature magnetic susceptibility that indicates no long-range order down to 50 mK; (2) strongly field-dependent coefficients of the low-temperature T and T(3) terms of the specific heat; (3) a conspicuously large Wilson ratio R(W) ≈ 53.5; and (4) unusual temperature and field dependences of the Hall resistivity that abruptly change below 80 K, without any clear correlation with the magnetic behavior. All these unconventional properties suggest the existence of an exotic ground state in Bi(2)Ir(2)O(7).
[Show abstract][Hide abstract] ABSTRACT: In the context of ultracold atoms with effective spin S = 5/2 confined
to an elongated trap we study the one-dimensional Fermi gas interacting
via an attractive δ-function potential using the Bethe ansatz
solution. There are N = 2S + 1 = 6 fundamental states: The particles can
either be unpaired or clustered in bound states of 2, 3, …, 2S and
2S + 1 fermions. The rich ground state phase diagram consists of these
six states and various mixed phases in which combinations of the
fundamental states coexist. Possible scenarios for phase separation due
to the harmonic confinement along the tube are explored within the local
density approximation. In an array of tubes with weak Josephson
tunneling superfluid order may arise. The response functions determining
the type of superfluid order are calculated using conformal field theory
and the exact Bethe ansatz solution. They consist of a power law with
distance times a sinusoidal term oscillating with distance. The
wavelength of the oscillations is related to the periodicity of a
generalized Fulde-Ferrell-Larkin-Ovchinnikov (FFLO) state.
Modern Physics Letters B 06/2012; 26(16):30009-. DOI:10.1142/S0217984912300098 · 0.75 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: We investigated the magnetic phase diagram of single crystals of SrNdFeO4 by measuring the magnetic properties, the specific heat and the dielectric permittivity. The system has two magnetically active ions, Fe3+ and Nd3+. The Fe3+ spins are antiferromagnetically ordered below 360 K with the moments lying in the ab plane, and undergo a reorientation transition at about 35–37 K to an antiferromagnetic order with the moments along the c axis. A short-range, antiferromagnetic ordering of Nd3+ along the c axis was attributed to the reorientation of Fe3+ followed by a long-range ordering at lower temperature [ Oyama et al. J. Phys.: Condens. Matter. 16 1823 (2004)]. At low temperatures and magnetic fields above 8 T, the Nd3+ moments are completely spin polarized. The dielectric permittivity also shows anomalies associated with spin configuration changes, indicating that this compound has considerable coupling between spin and lattice. A possible magnetic structure is proposed to explain the results.
[Show abstract][Hide abstract] ABSTRACT: In the context of a gas of ultracold atoms with effective spin S=3/2 confined to an elongated trap, we study the one-dimensional Fermi gas interacting via an attractive δ-function potential within the grand-canonical ensemble. The particles can be either unbound or clustered in bound states of two, three, and four fermions. The rich μ versus H ground-state phase diagram (μ is the chemical potential and H the external magnetic field) consists of the four basic states and the various possible mixed phases in which some these states coexist. Extending the analysis of K. Yang [ Phys. Rev. B 63 140511 (2001)] for S=1/2, we study the correlation functions of the generalized Cooper clusters of bound states of two, three, and four particles using conformal field theory and the exact Bethe Ansatz solution. The correlation functions consist of a power law with distance times a sinusoidal term oscillating with distance. In an array of tubes with weak Josephson tunneling, the type of superfluid order is determined by these correlation functions. The wavelength of the oscillations is related to the periodicity of a generalized Fulde-Ferrell-Larkin-Ovchinnikov (FFLO) state for higher spin particles. All the relevant states are analyzed for S=3/2.
[Show abstract][Hide abstract] ABSTRACT: Measuring surface conductivity we have observed the evolution of Shubnikov de Haas oscillations under quasihydrostatic pressure for the pnictide parent compound BaFe2As2. Prior results in the reconstructed state have observed small pockets which emerge from zone folding as a result of structural changes with cooling. For pressures below 20 kbar, both Fermi surface orbits grow in size. The effective masses increase with pressure suggesting enhanced correlation in the system, and a series of magnetic breakdown orbits are observed confirming that band structure calculations setting them in close proximity are correct.
[Show abstract][Hide abstract] ABSTRACT: Ca2RuO4 is intimately associated with both
negative volume thermal expansion (NVTE) and negative linear thermal
expansion (NLTV) when doped by a 3d transition metal ion M for Ru. The
NVTE and NLTE observed in this system constitutes a compelling and
extraordinary example in that (1) the coefficient of NVTE and NLTE
reaches -213 x 10-6 K-1 and -148 x 10-6
K-1, respectively, constituting colossal negative thermal
expansion (NTE); (2) the NTE anomalies closely track the onset
temperatures of orbital and magnetic orders, in sharp contrast to
classic NTE that shows no relevance to physical properties; (3) the NTE
and physical properties can be effectively tuned via varying M and x in
Ca2Ru1-xMxO4; (4) the NTE
occurs near room temperature and extends over a wide temperature
interval ranging from 100 K to 350 K. Moreover, NTE and Invar effect
commonly exist in these 4d-based ruthenates and 5d-based iridates, e.g.
Srn+1IrnO3n+1 and BaIrO3.
These novel NTE materials provide a much-needed paradigm for functional
materials with anomalous thermal expansion and electronic
characteristics.
[Show abstract][Hide abstract] ABSTRACT: A gas of ultracold 6Li atoms (effective spin 1/2) confined to an elongated trap with one-dimensional properties is a candidate to display three different phases: (i) fermions bound in Cooper-pair-like states, (ii) unbound spin-polarized particles, and (iii) a mixed phase in which Cooper bound states and unpaired particles coexist. It is of great interest to extend these studies to fermionic atoms with higher spin, e.g., for neutral 40K, 43Ca, 87Sr, or 173Yb atoms. Within the grand-canonical ensemble, we investigated the μ versus H phase diagram (μ is the chemical potential and H the external magnetic field) for S=3/2,...,9/2 for the ground state using the exact Bethe ansatz solution of the one-dimensional Fermi gas with an attractive δ-function interaction potential. There are N=2S+1 fundamental states: the particles can be either unpaired or clustered in bound states of 2, 3, ⋯, 2S, and 2S+1 fermions. The rich phase diagram consists of these N states and various mixed phases in which combinations of the fundamental states coexist. Bound states of N fermions are not favorable in high magnetic fields, but always present if the field is low. For S=3/2, possible scenarios for phase separation are explored within the local density approximation. For S=3/2, the phase diagram for the superposition of a Zeeman and a quadrupolar splitting is also discussed.
[Show abstract][Hide abstract] ABSTRACT: The interplay of spin-orbit interactions and electronic correlations
dominates the physical properties of pyrochlore iridates, R2Ir2O7 (R = Y, rare
earth element), which are typically magnetic insulators. We report an
experimental/theoretical study of single-crystal Bi2Ir2O7 where substitutions
of Bi for R sensitively tips the balance between competing interactions so as
to favor a metallic state with a strongly exchange enhanced paramagnetism. The
ground state is characterized by the following features: (1) A divergent
low-temperature magnetic susceptibility that indicates no long-range order down
to 50 mK; (2) strongly field-dependent coefficients of the low-temperature T-
and T3-terms of the specific heat; (3) a conspicuously large Wilson ratio R_W
\approx 53.5; and (4) unusual temperature and field dependences of the Hall
resistivity that abruptly change below 80 K, without any clear correlation with
the magnetic behavior. All these unconventional properties suggest the
existence of an exotic ground state in Bi2Ir2O7.