Critical Spin Research Articles

Properties of Odd-Frequency Superconductivity in Antiferromagnetic Ordered State

We investigate properties below T_c of odd-frequency pairing which is realized by antiferromagnetic critical spin fluctuations or spin wave modes. It is shown that \Delta(\epsilon_n) becomes maximum at finite \epsilon_n, and \Delta(\pi T) becomes maximum at finite T. Implications of the present results to the experimental results of CeCu_2Si_2 and CeRhIn_5 are given.

Phase transition in the Rényi-Shannon entropy of Luttinger liquids

The R\'enyi-Shannon entropy allows extraction of some universal information about many-body wave functions. For a critical spin chain with central charge $c=1$, we show that it exhibits a phase transition at some value ${n}_{c}$ of the R\'enyi parameter $n$ which depends on the Luttinger parameter $R$. A replica-free formulation establishes a connection to boundary entropies in conformal field theory and reveals that the transition is triggered by a vertex operator which becomes relevant at the boundary. Our numerical results (XXZ and ${J}_{1}\ensuremath{-}{J}_{2}$ spin chains) match the continuum limit prediction, confirming its universal character. The replica approach used in previous works turns out to be correct only for $n<{n}_{c}$. From the point of view of two-dimensional Rokhsar-Kivelson states, this transition reveals a singularity in the entanglement spectra.

On pulsar-driven mass ejection in low-mass X-ray binaries

There is accumulating evidence for mass ejection in low-mass X-ray binaries (LMXBs) driven by radio pulsar activity during X-ray quiescence. We consider the condition for mass ejection by comparing the radiation pressure from a millisecond pulsar, and the gas pressure at the inner Lagrange point or at the surrounding accretion disk. We calculate the critical spin period of the pulsar below which mass ejection is allowed. Combining with the evolution of the mass transfer rate, we present constraints on the orbital periods of the systems. We show that mass ejection could happen in both wide and compact LMXBs. It may be caused by transient accretion due to thermal instability in the accretion disks in the former, and irradiation-driven mass-transfer cycles in the latter.

Anisotropic In-Plane Resistivity in the Nematic Phase of the Iron Pnictides

We show that the interference between scattering by impurities and by critical spin fluctuations gives rise to anisotropic transport in the Ising-nematic state of the iron pnictides. The effect is closely related to the non-Fermi-liquid behavior of the resistivity near an antiferromagnetic quantum critical point. Our theory not only explains the observed sign of the resistivity anisotropy Δρ in electron-doped systems but also predicts a sign change of Δρ upon sufficient hole doping. Furthermore, our model naturally addresses the changes in Δρ upon sample annealing and alkaline-earth substitution.

Observing the origin of superconductivity in quantum critical metals

Despite intense efforts during the last 25 years, the physics of unconventional superconductors, including the cuprates with a very high transition temperature, is still a controversial subject. It is believed that superconductivity in many of these strongly correlated metallic systems originates in the physics of quantum phase transitions, but quite diverse perspectives have emerged on the fundamentals of the electron-pairing physics, ranging from Hertz style critical spin fluctuation glue to the holographic superconductivity of string theory. Here we demonstrate that the gross energy scaling differences that are behind these various pairing mechanisms are directly encoded in the frequency and temperature dependence of the dynamical pair susceptibility. This quantity can be measured directly via the second order Josephson effect and it should be possible employing modern experimental techniques to build a `pairing telescope' that gives a direct view on the origin of quantum critical superconductivity.

Field-induced quantum fluctuations in the heavy fermion superconductor CeCu2Ge2

Quantum-mechanical fluctuations in strongly correlated electron systems cause unconventional phenomena such as non-Fermi liquid behavior, and arguably high temperature superconductivity. Here we report the discovery of a field-tuned quantum critical phenomenon in stoichiometric CeCu2Ge2, a spin density wave ordered heavy fermion metal that exhibits unconventional superconductivity under ≃10 GPa of applied pressure. Our finding of the associated quantum critical spin fluctuations of the antiferromagnetic spin density wave order, dominating the local fluctuations due to single-site Kondo effect, provide new information about the underlying mechanism that can be important in understanding superconductivity in this novel compound.

Geometric entanglement from matrix product state representations

An efficient scheme for computing the geometric entanglement (GE) per lattice site for quantum many-body systems on a periodic finite-size chain is proposed in the context of a tensor network algorithm based on matrix product state representations. It has been systematically tested for three prototypical critical quantum spin chains, which belong to the same Ising universality class. The simulation results lend strong support to the previous claim (Shi et al 2010 New J. Phys.12 025008; Stéphan et al 2010 Phys. Rev. B 82 180406R) that the leading finite-size correction to the GE per lattice site is universal, with its remarkable connection to the celebrated Affleck–Ludwig boundary entropy corresponding to a conformally invariant boundary condition.

Entanglement Entropy of Critical Spin Liquids

Quantum spin liquids are phases of matter whose internal structure is not captured by a local order parameter. Particularly intriguing are critical spin liquids, where strongly interacting excitations control low energy properties. Here we calculate their bipartite entanglement entropy that characterizes their quantum structure. In particular we calculate the Renyi entropy S(2) on model wave functions obtained by Gutzwiller projection of a Fermi sea. Although the wave functions are not sign positive, S(2) can be calculated on relatively large systems (>324 spins) using the variational Monte Carlo technique. On the triangular lattice we find that entanglement entropy of the projected Fermi sea state violates the boundary law, with S(2) enhanced by a logarithmic factor. This is an unusual result for a bosonic wave function reflecting the presence of emergent fermions. These techniques can be extended to study a wide class of other phases.

Thermodynamic properties of the kagome lattice in herbertsmithite

Strongly correlated Fermi systems are among the most intriguing and fundamental systems in physics, whose realization in some compounds is still to be discovered. We show that herbertsmithite ZnCu_3(OH)_6Cl_2 can be viewed as a strongly correlated Fermi system whose low temperature thermodynamic in magnetic fields is defined by a quantum critical spin liquid. Our calculations of its thermodynamic properties are in good agreement with recent experimental facts and allow us to reveal their scaling behavior which strongly resembles that observed in HF metals and 2D 3He.

Low current density spin-transfer torque effect assisted by in-plane microwave field

A strategy is presented to greatly reduce both the critical spin polarized current density and the magnetization switching time in elliptical magnetic spin valve. This method is a combination of microwave field and spin polarized current. The numerical simulation at zero temperature shows that the critical current density and the magnetization switching time are sensitive to the frequency and the amplitude of microwave magnetic field. A 75% reduction in critical current density is observed when the frequency of microwave coincides with the natural ferromagnetic resonance frequency of free layer.

ON DISCOVERY OF THE FERROMAGNETIC SUPERCONDUCTOR UCoGe

We report the coexistence of ferromagnetic order and superconductivity in UCoGe at ambient pressure. The data obtained from the basic thermal, magnetic and transport properties on the macro and microscopic scale show that UCoGe is a weak ferromagnet with a Curie temperature TC = 3 K, and also, is a superconductor with a resistive transition temperature Ts = 0.8 K. Those present evidence that UCoGe is an unconventional superconductor and argue that superconductivity is mediated by critical ferromagnetic spin fluctuations.

Chiral criticality in the doped helimagnets Mn1−yFeySi

The critical spin fluctuations in doped compounds Mn$_{1-y}$Fe$_y$Si have been studied by means of ac-susceptibility measurements, polarized neutron small angle scattering and spin echo spectroscopy. It is shown that these compounds undergo the transition from the paramagnetic to helimagnetic phase through continuous, yet well distinguishable crossovers: (i) from paramagnetic to partially chiral, (ii) from partially chiral to highly chiral fluctuating state. The crossover points are identified on the basis of combined analysis of the temperature dependence of ac-susceptibility and polarized SANS data. The whole transition is marked by two inflection point of the temperature dependence of ac-susceptibility: the upper one corresponds to the crossover to partially chiral state at $T^*$, where the inverse correlation length $\kappa \approx 2 k$, the lower one corresponds to the transition to the spin helix structure. The intermediate crossover to the highly chiral phase is observed at the inflection point $T_k$ of the first derivative of ac-susceptibility, where $\kappa \approx k$. The temperature crossovers to the highly chiral fluctuating state is associated with the enhancing influence of the Dzyaloshinskii-Moria interaction close to $T_c$.

On the Meissner Effect of the Odd-Frequency Superconductivity with Critical Spin Fluctuations: Possibility of Zero Field FFLO Pairing

We investigate the influence of critical spin fluctuations on electromagnetic responses in the odd-frequency superconductivity. It is shown that the Meissner kernel of the odd-frequency superconductivity is strongly reduced by the critical spin fluctuation or the massless spin wave mode in the antiferromagnetic phase. These results imply that the superfluid density is reduced, and the London penetration depth is lengthened for the odd-frequency pairing. It is also shown that the zero field Flude-Ferrell-Larkin-Ovchinnikov pairing is spontaneously realized both for even- and odd-frequency in the case of sufficiently strong coupling with low lying spin-modes.

Entanglement entropy from charge statistics: Exact relations for noninteracting many-body systems

We present an exact expression for the entanglement entropy generated at a quantum point contact between noninteracting electronic leads in terms of the full counting statistics of charge fluctuations, which we illustrate with examples from both equilibrium and nonequilibrium transport. The formula is also applicable to ground-state entanglement entropy in systems described by noninteracting fermions in any dimension, which in one dimension include the critical spin-1/2 $\mathit{XX}$ and Ising models where conformal field theory predictions for the entanglement entropy are reproduced from the full counting statistics. These results may play an important role in experimental measurements of entanglement entropy in mesoscopic structures and cold atoms in optical lattices.

Ferromagnetic resonance, magnetic susceptibility, and transformation of domain structure in CoFeB film with growth induced anisotropy

Field dependence of magnetic susceptibility in nanocrystalline CoFeB film was studied in a wide frequency range from 500 kHz to 15 GHz. Anomalies of the susceptibility were detected exciting CoFeB film with a solenoidal coil, microwave strip line, in the tunable microwave cavity as well as employing magneto-optical domains imaging. Critical spin fluctuations in the form of “soft” modes were observed in a whole range of orientations of magnetic field perpendicular to the “easy” magnetic axis. A sequence of domain structure transformations was extensively examined in a “hard” direction in in-plane magnetic field reduced below the field of uniaxial anisotropy Hp = 535 Oe. At first, uniformly magnetized state in CoFeB film transforms to stripe domains separated by low angle Néel domain walls (DWs) parallel to the “hard”-axis. Then, at critical field Hcr = 232 Oe, Néel DWs gradually convert to the“easy”-axis oriented Bloch DWs loaded with vertical Bloch lines (VBLs). After field reversal at H = −Hcr, backward conversion of VBL-loaded Bloch DWs to Néel DWs results in instantaneous energy release and sharp anomaly of magnetic susceptibility. Appearance of critical spin fluctuations accomplishes domains transformation to the uniformly magnetized state at H = −535 Oe.

Connecting distant ends of one-dimensional critical systems by a sine-square deformation

We study one-dimensional quantum critical spin systems with sine-square deformation, in which the energy scale in the Hamiltonian at position $x$ is modified by the function ${f}_{x}=\mathrm{sin}{}^{2}[\frac{\ensuremath{\pi}}{L}(x\ensuremath{-}\frac{1}{2})]$, where $L$ is the length of the system. By investigating the entanglement entropy, spin correlation functions, and wave-function overlap, we show that the sine-square deformation changes the topology of the geometrical connection of the ground state drastically: Although the system apparently has open edges, the sine-square deformation links those ends and realizes the periodic ground state at the level of the wave function. Our results propose a method to control the topology of quantum states by energy-scale deformation.

Critical behavior of XY spin chain with Dzyaloshinsky–Moriya interaction studied in terms of Loschmidt echo

In this paper, the critical behavior of the general XY spin chain with the Dzyaloshinsky–Moriya (DM) interaction is studied by means of a Loschmidt Echo (LE) calculation. LE presents a Gauss decay in the region of magnetic field intensity |λ|<1 and an exponential decay in the region of |λ|>1. There exists a critical spin chain size NC. When spin chain size is larger than NC, the value of λ corresponding to the minimum value of LE (λm) is independent of the spin chain size and keeps a stable value. In the region of λ<0, the stable value is same for different DM interactions. In the region of λ>0, the stable value varies with changing DM interaction.

The spin correlation function in 2D statistical mechanics models with inhomogeneous linedefects

We consider the critical spin–spin correlation function of the 2D Ising model with a linedefect whose strength is an arbitrary function of the position. By using path integraltechniques in the continuum description of this model in terms of fermion fields, we obtainan analytical expression for the correlator as a functional of the position-dependentcoupling. Thus, our result provides one of the few analytical examples that allows one toillustrate the transit of a magnetic system from scaling to non-scaling behavior in a criticalregime. We also show that the non-scaling behavior obtained for the spin correlator along anonuniformly altered line of an Ising model remains unchanged in the Ashkin–Teller model.

Critical spin dynamics in the antiferromagnetLa4Ni3O8fromLa139nuclear magnetic resonance

We report $^{139}\mathrm{La}$ nuclear magnetic resonance data in ${\mathrm{La}}_{4}{\mathrm{Ni}}_{3}{\mathrm{O}}_{8}$ that reveal the presence of antiferromagnetic order below ${T}_{N}~105$ K. This remarkable compound contains two-dimensional (2D) layers of ${\mathrm{NiO}}_{2}$ with unusual Ni valences. ${\mathrm{La}}_{4}{\mathrm{Ni}}_{3}{\mathrm{O}}_{8}$, with antiferromagnetic order of $S=1/2 {\mathrm{Ni}}^{1+}$ spins, is analogous to the parent compounds of the cuprate superconductors. Our data clearly reveal dramatic spectral changes and low-energy antiferromagnetic correlations associated with the onset of long-range order below ${T}_{N}$. These critical spin fluctuations, however, do not diverge at the second-order phase transition but rather appear to be associated with a $T=0$ transition characteristic of 2D spin fluctuations.

Self-Consistent Fluctuation Approach to Strongly Correlated Electron Systems

A microscopic self-consistent fluctuation theory is proposed for strongly correlated electron systems which covers a region in the vicinity of the quantum critical point. We construct the fully antisymmetric full vertices self-consistently in terms of the two-particle fluctuations with the renormalized vertex corrections. The single-particle self-energy is computed by the way similar to the paramagnon theory. We demonstrate the application of the present theory to the Hubbard models in one and two dimensions at half filling. The characteristic behaviors expected from the critical spin fluctuations such as a development of the pseudogap in the quasiparticle bands are reproduced by the present theory. In one dimension, the tendency of the spin-charge separation is obtained in the single-particle excitation spectrum.