Iver Bakken Sperstad

Norwegian University of Science and Technology, Trondheim, Sor-Trondelag Fylke, Norway

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Publications (10)26.39 Total impact

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    Iver Bakken Sperstad, Einar B. Stiansen, Asle Sudbø
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    ABSTRACT: We investigate the critical behavior of a spin chain coupled to bosonic baths characterized by a spectral density proportional to $\omega^s$, with $s>1$. Varying $s$ changes the effective dimension $d_\text{eff} = d + z$ of the system, where $z$ is the dynamical critical exponent and the number of spatial dimensions $d$ is set to one. We consider two extreme cases of clock models, namely Ising-like and U(1)-symmetric ones, and find the critical exponents using Monte Carlo methods. The dynamical critical exponent and the anomalous scaling dimension $\eta$ are independent of the order parameter symmetry for all values of $s$. The dynamical critical exponent varies continuously from $z \approx 2$ for $s=1$ to $z=1$ for $s=2$, and the anomalous scaling dimension evolves correspondingly from $\eta \gtrsim 0$ to $\eta = 1/4$. The latter exponent values are readily understood from the effective dimensionality of the system being $d_\text{eff} \approx 3$ for $s=1$, while for $s=2$ the anomalous dimension takes the well-known exact value for the 2D Ising and XY models, since then $d_{\rm{eff}}=2$. A noteworthy feature is, however, that $z$ approaches unity and $\eta$ approaches 1/4 for values of $s < 2$, while naive scaling would predict the dissipation to become irrelevant for $s=2$. Instead, we find that $z=1,\eta=1/4$ for $s \approx 1.75$ for both Ising-like and U(1) order parameter symmetry. These results lead us to conjecture that for all site-dissipative $Z_q$ chains, these two exponents are related by the scaling relation $z = \text{max} {(2-\eta)/s, 1}$. We also connect our results to quantum criticality in nondissipative spin chains with long-range spatial interactions.
    Physical review. B, Condensed matter 04/2012; 85(21). · 3.77 Impact Factor
  • Einar Stiansen, Iver Bakken Sperstad, Asle Sudbø
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    ABSTRACT: We have performed large-scale Monte-Carlo simulations on a model describing a (2+1)D array of quantum dissipative Josephson junctions. With the superconducting phases as our fundamental degrees of freedom we are able to identify three distinct phases as function of Josephson coupling and dissipation strength. Apart from the fully superconducting state, where fluctuations in both space and time are at bay, and the normal phase, characterized by wild fluctuations, we find an additional phase featuring spatial phase coherence coinciding with temporal disorder.
    02/2012;
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    Iver Bakken Sperstad, Einar B. Stiansen, Asle Sudbø
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    ABSTRACT: We present large-scale Monte Carlo results for the dynamical critical exponent z and the spatio-temporal two-point correlation function of a (2+1)-dimensional quantum XY model with bond dissipation, proposed to describe a quantum critical point in high-Tc cuprates near optimal doping. The phase variables of the model, originating with a parametrization of circulating currents within the CuO_2 unit cells in cuprates, are compact, {\theta_{r,\tau}} \in [-\pi,\pi>. The dynamical critical exponent is found to be z \approx 1, and the spatio-temporal correlation functions are explicitly demonstrated to be isotropic in space-imaginary time. The model thus has a fluctuation spectrum where momentum and frequency enter on equal footing, rather than having the essentially momentum-independent marginal Fermi liquid-like fluctuation spectrum previously reported for the same model.
    Physical review. B, Condensed matter 11/2011; · 3.77 Impact Factor
  • Einar Stiansen, Iver Sperstad, Asle Sudbo
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    ABSTRACT: We study two versions of a (1+1)D Z4-symmetric model with Ohmic bond dissipation. In one version the phase variable is restricted to the interval [0,2pi>, while the domain is unrestricted in the other. The compact model features a completely ordered phase with a broken Z4-symmetry and a disordered phase, separated by a critical line. The non-compact model features three phases. In addition to the two phases exhibited by the compact model, there is also an intermediate phase, characterized by isotropic power-law phase correlations. We calculate the dynamical critical exponent z along the critical lines of both models to see if the compactness of the variable is relevant to the critical scaling between space and imaginary time. We find z 1 for the single phase transition in the compact model as well as for both transitions in the non- compact model.
    03/2011;
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    Einar B. Stiansen, Iver Bakken Sperstad, Asle Sudbø
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    ABSTRACT: Using large-scale Monte Carlo computations, we study two versions of a (1+1)D Z4-symmetric model with ohmic bond dissipation. In one of these versions, the variables are restricted to the interval [0,2pi>, while the domain is unrestricted in the other version. The compact model features a completely ordered phase with a broken Z4 symmetry and a disordered phase, separated by a critical line. The noncompact model features three phases. In addition to the two phases exhibited by the compact model, there is also an intermediate phase with isotropic quasi-long-range order. We calculate the dynamical critical exponent z along the critical lines of both models to see if the compactness of the variable is relevant to the critical scaling between space and imaginary time. There appears to be no difference between the two models in that respect, and we find z&ap;1 for the single phase transition in the compact model as well as for both transitions in the noncompact model.
    Physical review. B, Condensed matter 01/2011; 83. · 3.77 Impact Factor
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    Iver B. Sperstad, Einar B. Stiansen, Asle Sudbo
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    ABSTRACT: The dynamical critical exponent $z$ is a fundamental quantity in characterizing quantum criticality, and it is well known that the presence of dissipation in a quantum model has significant impact on the value of $z$. Studying quantum Ising spin models using Monte Carlo methods, we estimate the dynamical critical exponent $z$ and the correlation length exponent $\nu$ for different forms of dissipation. For a two-dimensional quantum Ising model with Ohmic site dissipation, we find $z \approx 2$ as for the corresponding one-dimensional case, whereas for a one-dimensional quantum Ising model with Ohmic bond dissipation we obtain the estimate $z \approx 1$. Comment: 9 pages, 8 figures. Submitted to Physical Review B
    Physical review. B, Condensed matter 02/2010; · 3.77 Impact Factor
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    Iver Bakken Sperstad, Jacob Linder, Asle Sudbø
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    ABSTRACT: We study quantum transport in ballistic $s_\pm$-wave superconductors where coupling between the two bands is included, and apply our model to three possible probes for detecting the internal phase shift of such a pairing state: tunneling spectroscopy in a N$|s_\pm$-wave junction, crossed Andreev reflection in a two-lead N$|s_\pm$-wave$|$N system, and Josephson current in a s-wave$|$I$|s_\pm$-wave Josephson junction. Whereas the first two probes are insensitive to the superconducting phase in the absence of interband coupling, the Josephson effect is intrinsically phase-dependent, and is moreover shown to be relatively insensitive to the strength of the interband coupling. Focusing on the Josephson current, we find a 0-$\pi$ transition as a function of the ratio of effective barrier transparency for the two bands, as well as a similar phase-shift effect as a function of temperature. An essential feature of this $s_\pm$-wave model is non-sinusoidality of the current-phase relation, and we compute the dependence of the critical current on an external magnetic field, showing how this feature may be experimentally observable for this system. We also comment on the possible experimental detection of the phase shift effects in $s_\pm$-wave superconductors. Comment: 14 pages, 14 figures
    Physical review. B, Condensed matter 08/2009; · 3.77 Impact Factor
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    Jacob Linder, Iver B. Sperstad, Asle Sudbø
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    ABSTRACT: We investigate Josephson junctions with superconducting ferropnictides, both in the diffusive and ballistic limit. We focus on the proposed $s_\pm$-wave state, and find that the relative phase shift intrinsic to the $s_\pm$-wave state may provide 0-$\pi$ oscillations in the Josephson current. This feature can be used to discriminate this pairing state from the conventional s-wave symmetry. The 0-$\pi$ oscillations appear both as a function of the ratio of the interface resistances for each band and, more importantly, as a function of temperature, which greatly aids in their detection. Comment: 4 pages, 4 figures. Revised version with added results for ballistic limit. Accepted for publication in Phys. Rev. B - Rapid Communications
    Physical review. B, Condensed matter 01/2009; · 3.77 Impact Factor
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    Iver B. Sperstad, Jacob Linder, Asle Sudbø
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    ABSTRACT: We calculate the Josephson current in a diffusive superconductor/ferromagnet/superconductor junction, where the ferromagnetic region contains multiple layers (or domains). In particular, we study a configuration where there are two layers with an arbitrary relative in-plane magnetization orientation, and also include non-ideal interfaces and arbitrary spin-flip scattering. We study the 0-$\pi$ oscillations of the critical current for varying junction width $d$, and find that the $\pi$ state vanishes entirely when the magnetic misorientation angle of the two layers exceeds a critical angle $\phi_c$. While $\phi_c \to \pi/2$ in the limit of high temperatures, we find that $\phi_c$ becomes smaller than $\pi/2$ at low temperatures compared to $T_c$. 0-$\pi$ oscillations are also found when varying the temperature or the misorientation angle for fixed values of $d$, and we present phase diagrams that show qualitatively the conditions for the appearance of such oscillations. We also point out how one may obtain significant enhancement of the critical current in such a system by switching the magnetization for selected values of the junction width $d$, and comment on the necessary conditions for establishing a long range triplet Josephson effect. Comment: 12 pages, 11 figures. Accepted for publication in Phys. Rev. B. High-quality figures will be available in the published version
    Physical review. B, Condensed matter 08/2008; · 3.77 Impact Factor
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    ABSTRACT: We construct a mean-field theory for itinerant ferromagnetism coexisting with a nonunitary superconducting state, where only the majority-spin band is gapped and contains line nodes, while the minority-spin band is gapless at the Fermi level. Our study is motivated by recent experimental results, which indicate that this may be the physical situation realized in the heavy-fermion compound UGe2. We investigate the stability of the mean-field solution of the magnetic and superconducting order parameters. Also, we provide theoretical predictions for experimentally measurable properties of such a nonunitary superconductor: the specific heat capacity, the Knight shift, and the tunneling conductance spectra. Our study should be useful for direct comparison with experimental results and also for further predictions of the physics that may be expected in ferromagnetic superconductors.
    Phys. Rev. B. 01/2008; 77(18).