Guguchia

Tuning the Superconducting Gap Symmetry in Ba1-xRbxFe2As2

by Hydrostatic Pressure

Zurab Guguchia

Laboratory for Muon Spin Spectroscopy, Paul-Scherrer Institut, Switzerland

Hubertus Luetkens

Tbilisi

SokhumiIlia Vekua Sukhumi Institute of Physics and Technology (SIPT)

სოხუმის ილია ვეკუას სახელობის ფიზიკა-ტექნიკის ინსტიტუტი

Sokhumi Hubertus Luetkens

Thank you!

Laboratory for Muon Spin Spectroscopy (PSI) Hugo Keller Fabian von Rohr

Rustem Khasanov Biswas

Alex Amato

Pabitra

Elvezio Morenzoni

Hubertus Luetkens

Tbilisi State University Rafael Fernandes Jian Kang

Alexander Shengelaya Tornike Qimeridze Ana sliusareva

Giacomo Prando Ludwig-Maximilians-University of Munich

Dirk Johrendt Erwin Wiesenmeyer Hubertus Luetkens

Institute of Low Temperature and Structure Research

Zbigniew Bukowski

µSR

Bµ

Bµ internal or external field Hubertus Luetkens

aPx (t) ~ Muon Spin Polarization

μSR Spectra

Hubertus Luetkens

Frequency Frequency   Value Valueofoffield field atatmuon muonsite site (ω (ωL L==γγmmBBmm)) Damping Damping   Field Fielddistribution distribution and/or and/ordynamics dynamics

µ SR ⇒ local magnetic field distribution p(B) in the mixed state of a type II sc H Hc2

Hc1 Tc

Bext

Bext Since the muon is a local probe, the µSR relaxation function is given by the weighted sum of all oscillations:

P(t) Hubertus Luetkens

μSR technique

second moment of p(B) 2

-4

2 -4 ∆B ∆B ∝∝ λλ

Gaussian distribution p(B)

σ ∝ ∆B2

1/2

∝ 1 / λ2 ∝ ns / m*

σ : µSR relaxation rate Hubertus Luetkens

s-wave Superconducting Gap • BCS conventional pairing: isotropic s-wave pairing

B. MĂźhlschlegel, Z. Phys. 155, 313 (1959)

Hubertus Luetkens

s-wave gap

d-wave Pairing Symmetry

Ď•

Single crystal YBa2Cu3O6.95 J.E. Sonier et al, PRL 72, 744 (1994) Hubertus Luetkens

GPD spectrometer (Âľ E1 beamline)

Positive or negative muons: momentum ranges ~60-125MeV/c.

Muon range: ~1.5-20g/cm2, range width ~30mg/cm2-~4g/cm2 FWHM in CH2.

Polarization: Longitudinal (direction of the muon momentum; no spin rotation).

Hubertus Luetkens

Momentum acceptance (FWHM) 3% Pion momentum range [MeV/c] 200--125 Muon momentum range [MeV/c] 125-- 60 Rate of positive muon [mA-1s-1] 6e7--3e7 Spot size (FWHM) 39 X 28 mm

double-wall cells AC cryostat

Material: CuBe, MP35N, Al2O3 Pressure range: up to ~2.3 GPa (normal operation) Temperature range: 0.25K—300K

P ≈ 27 kbar Fraction of the muons in the sample ~ 50 % Hubertus Luetkens

Different families of Fe-HTS‘s

Hubertus Luetkens

Phase Diagrams of Fe-HTS’s

H. Luetkens et al., Nature Materials 8, 305 (2009)

E. Wiesenmayer et al., Phys. Rev. Lett. 107, 237001 (2011)

J.-H. Chu et al., Phys Rev B 79, 014506 (2009)

CaFe2As2

A. Kreyssig et al., Phys. Rev. B 78, 184517 (2008) M.S. Torikachvili et al., Phys. Rev. Lett. 101, 057006 (2008) Hubertus Luetkens

Z. Guguchia et. al., Phys. Rev. B 83, 144516 (2011).

Z. Guguchia et. al., JPCM 26, 285 (2013).

Central issue  What is the microscopic pairing mechanism in the Fe - HTS's?

 Identifying the SC gap symmetry/structure of the Fe-HTS’s?

Hubertus Luetkens

Multiband Superconductivity ARPES on Ba1-xKxFe2As2:

• Multiband superconductivity • Nodeless gaps

∆1 = 2.7(5) meV and ∆ 2 = 8.4(3) meV. V. B. Zabolotnyy et al., Nature 457, 569 (2009).

D. V. Evtushinsky et al.,Phys. Rev. B 79, 054517 (2009).

D. V. Evtushinsky et al., New J. Phys. 11, 055069 (2009).

Hubertus Luetkens

Z. Guguchia et al., Phys. Rev. B 84, 094513 (2011).

Non universal SC gap structure in Fe-based high-temperature superconductors Ba1-xKxFe2As2, Ba1-xRbxFe2As2 BaFe2-xNixAs2, BaFe2-xCoxAs2 KxFe2-ySe2, FeTe1-xSex

LaOFeP, LiFeP, KFe2As2, BaFe2(As1-xPx)2 BaFe2-xRuxAs2, Over-doped Ba1-xKxFe2As2

J-Ph. Rheid et. al., Supercond. Sci. Technol. 25, 084013 (2012).

M. Abdel-Hafiez et. al., arXiv: 1502.07130v1 (2015).

Y. Zhang et. al., Nature Physics 8, 371-375 (2012).

M.S. LaadLuetkens et. al.,, Phys. Rev. Lett. 103, 017002 (2009).

Hubertus

Phase diagram of Ba1-xRbxFe2As2

Z. Guguchia et al., manuscript under preparation (2015).

Hubertus Luetkens

S. Peschke et al., JIGC 640, 830-835 (2014).

ÂľSR time spectra and Fourier transforms for optimally doped Ba 1-xRbxFe2As2 (x = 0.35)

Z. Guguchia et al., Nature Comm. (2015).

Hubertus Luetkens

Diamagnetic shift and the muon spin relaxation rate for optimally doped Ba1-xRbxFe2As2 (x = 0.35)

∆Bdia = µ 0 [ H int, SC − H int, NS ] T / Tc ≈ 0.4

Gaussian distribution p(B)

σ ∝ ∆B2

1/2

∝ 1 / λ2 ∝ ns / m*

σ : µSR relaxation rate

Z. Guguchia et al., Nature Comm. (2015).

Hubertus Luetkens

Temperature and pressure dependent magnetic penetration depth in optimally doped Ba1-xRbxFe2As2 (x = 0.35)

No nodes in the SC-gap Nodes in the SC-gap

Nodes: Are they accidental or imposed by symmetry?

λ−2 (T , ∆ 0,1 ) λ−2 (T , ∆ 0, 2 ) λ−2 (T ) = ω1 − 2 + ω2 −2 λ− 2 (0) λ (0, ∆ 0,1 ) λ (0, ∆ 0, 2 )

Constant gap - ∆ 0,i = ∆ i .

Angle dependent gap - ∆ 0,i = ∆ i cos(2ϕ ).

∆1 = 2.7(5) meV and ∆ 2 = 8.4(3) meV. Z. Guguchia et al., Nature Comm. (2015).

Hubertus Luetkens

Z. Guguchia et al., Phys. Rev. B 84, 094513 (2011).

Three pocket model used in calculations

Z. Guguchia et al., Nature Comm. (2015).

Hubertus Luetkens

Temperature and pressure dependent magnetic penetration depth in optimally doped Ba1-xRbxFe2As2 (x = 0.35)

Z. Guguchia et al., Nature Comm. (2015).

Hubertus Luetkens

Density plot of the gap function for the d − wave state

Z. Guguchia et al., Nature Comm. (2015).

Hubertus Luetkens

Possible explanations for the pressure induced nodes in optimally doped Ba1-xRbxFe2As2 (x = 0.35)

• •

In optimally doped Ba0.6K0.4Fe2As2 a sub-dominant d-wave state close in energy to the +− dominant s state was found.

A systematic comparison of quasiparticle excitations in the 1111, 122, and 111 families of iron-pnictide superconductors implies that the nodal state may be induced when the pnictogen height from the iron plane decreases below a threshold value of 1.33 Å.

S. Maiti et. al., Phys. Rev. Lett. 107, 147002 (2011).

R. Thomale et. al., Phys. Rev. Lett. 107, 117001 (2011).

T. Bohm et. al., Phys. Rev. X 4, 041046 (2014).

Hubertus Luetkens

Pressure dependence of various quantities of optimally doped Ba1-xRbxFe2As2 (x = 0.35)

Z. Guguchia et al., Nature Comm. (2015).

Hubertus Luetkens

K. Hashimoto et al., Science 336, 1554 (2012).

P. Walmsley et. al., Phys. Rev. Lett. 110, 257002 (2013).

Temperature and pressure dependent magnetic penetration depth in over-doped Ba1-xRbxFe2As2 (x = 0.65)

Z. Guguchia et al., manuscript under preparation (2015).

Hubertus Luetkens

Conclusions

s-wave d-wave

 The SC transition temperature stays nearly constant under pressure, whereas a strong reduction of λ(0) is observed in Ba1-xRbxFe2As2 (x = 0.35).  We demonstrated that in the Fe-based superconductor Ba1-xRbxFe2As2 (x = 0.35) a nodal SC gap is promoted by hydrostatic pressure.  The SC gap symmetry as well as the pressure effects on the quantities characterizing the SC state strongly depends on the hole doping level x.

Hubertus Luetkens

Thank you very much for your attention!

Hubertus Luetkens